GHSA-23VG-4CJM-66Q5
Vulnerability from github – Published: 2026-10-15 20:19 – Updated: 2026-10-15 20:19Summary
openapi3filter.ValidationHandler.Middleware validates requests with kin-openapi's
legacy router, which selects an OpenAPI operation from the decoded URL.Path. The
method is documented as implementing gorilla/mux.MiddlewareFunc, but a downstream
gorilla/mux router configured with UseEncodedPath() selects its handler from the
encoded representation (URL.RawPath).
An unauthenticated request such as /public%2Fstatus can therefore be validated as an
intentionally public operation (/public/status) and then dispatched to a protected
operation (/{id}). The configured, rejecting AuthenticationFunc is never invoked,
and the protected downstream handler runs.
This is distinct from GHSA-r277-6w6q-xmqw. The behavior below uses an explicit authentication callback that rejects missing credentials and remains reproducible after the v0.144.0 fix for the unsafe default callback.
Affected products
- Product:
github.com/getkin/kin-openapi - Component:
openapi3filter.ValidationHandler.Middleware - Confirmed post-GHSA affected releases: v0.144.0, v0.145.0, and v0.146.0
- Reporter attestation: each listed release was checked out separately, the attached
single-file test was copied into
openapi3filter/, and that test passed; maintainers should independently confirm the version range - Confirmed affected development revision:
d33e44e67734dcde81b576582185e1343fb3985d - v0.143.0 also reproduces this mechanism, but GHSA-r277-6w6q-xmqw already provides a simpler bypass on that release; the distinct post-fix claim starts at v0.144.0
- Earlier releases: not tested; please treat the introduction boundary as unknown
Security impact
An unauthenticated remote requester can reach a handler whose OpenAPI operation requires authentication when all of the following are true:
- the application uses
ValidationHandler.Middlewareas the security gate; - the downstream router preserves and interprets an encoded path, such as
gorilla/mux with
UseEncodedPath(); - a public exact route and a protected parameterized route overlap after decoding;
- the public and protected operations expose the same HTTP method;
- the protected handler relies on the validation layer's security decision.
The protected operation's path-parameter value is constrained: it is an encoded
representation that decodes to a public literal path (public%2Fstatus in this PoC),
not an arbitrary attacker-selected identifier. A protected operation without a
compatible parameterized path is not reachable through this specific mechanism.
- Primitive: the security decision is made for a different OpenAPI operation than the operation dispatched by the application router.
- Demonstrated impact: a protected handler executes while the explicitly rejecting
AuthenticationFuncis never invoked. - Impact ceiling: confidentiality or integrity loss when the reachable protected handler has that authority; the PoC does not demonstrate data disclosure or state modification.
- Availability impact: not demonstrated.
Root cause
At the confirmed development revision:
openapi3filter/validation_handler.go:ValidationHandler.Loadconstructsrouters/legacy.NewRouterunconditionally.routers/legacy/router.go: route selection uses the decoded request path.routers/gorillamux/router.go: the kin-openapi gorilla router constructs a gorilla/mux router withUseEncodedPath().ValidationHandler.Middlewareforwards the unchanged request after validation.
The component making the authentication decision and the component dispatching the handler can therefore disagree about the selected operation.
Reproduction
The attached PoC uses a real net/http server, an explicit API-key callback that
rejects missing credentials, and a downstream gorilla/mux router with
UseEncodedPath().
From a kin-openapi checkout, copy the self-contained attachment
encoded_path_route_confusion_test.go.txt into openapi3filter/ as
encoded_path_route_confusion_test.go, then run:
go test -v ./openapi3filter -run '^TestEncodedPathAuthorizationBypass$' -count=1
Expected attack observation:
status=202 downstream_route=/{id} protected_id=public%2Fstatus authentication_calls=0
wiring=Router.Use status=202 downstream_route=/{id} protected_id=public%2Fstatus authentication_calls=0
Controls included in the test show that:
/ordinaryselects the protected operation, invokes authentication once, and is rejected with HTTP 401;/public/statusreaches only the intentionally public handler;- the encoded request reaches the protected
/{id}handler with HTTP 202 while the authentication callback remains uncalled; - both external wrapping and the documented
router.Use(validationHandler.Middleware)wiring reproduce the same result.
Reverse-proxy reachability
The same request was reproduced through Nginx 1.31.3 using the official
nginx:alpine image digest
sha256:4a73073bd557c65b759505da037898b61f1be6cbcc3c2c3aeac22d2a470c1752.
The exact container invocation was:
docker run --rm --name kin-openapi-nginx \
--add-host=host.docker.internal:host-gateway \
-p 18081:8080 -p 18082:8081 \
-v "$PWD/poc/nginx.conf:/etc/nginx/nginx.conf:ro" \
nginx:alpine
Container port 8080 uses proxy_pass http://host.docker.internal:18080; without a
replacement URI. Host port 18081 therefore preserved /public%2Fstatus; the protected
handler returned HTTP 202 and the
authentication callback remained at zero calls. With
proxy_pass http://host.docker.internal:18080/; on container port 8081 / host port
18082, Nginx canonicalized the upstream URI; both layers selected /public/status and
the bypass did not occur.
Nginx access-log observations:
server_port=8080 "GET /public%2Fstatus HTTP/1.1" 202 0 "curl/8.7.1"
server_port=8081 "GET /public%2Fstatus HTTP/1.1" 204 0 "curl/8.7.1"
server_port=8080 "GET /ordinary HTTP/1.1" 401 0 "curl/8.7.1"
The response route headers were X-Poc-Route: /{id} on the preserving listener and
X-Poc-Route: /public/status on the canonicalizing listener. Backend observations for
the two requests were:
request_uri="/public%2Fstatus" path="/public/status" raw_path="/public%2Fstatus" auth_before=0
completed request_uri="/public%2Fstatus" auth_after=0
request_uri="/public/status" path="/public/status" raw_path="" auth_before=0
completed request_uri="/public/status" auth_after=0
This control shows that upstream path handling is a deployment precondition rather than claiming that every proxy configuration is affected.
The canonical protected control produced:
request_uri="/ordinary" path="/ordinary" raw_path="" auth_before=0
completed request_uri="/ordinary" auth_after=1
Suggested remediation
The authentication layer and downstream dispatcher must select the same OpenAPI operation from the same request representation.
Potential approaches:
- make
ValidationHandleraccept a caller-suppliedrouters.Routerand require it to match downstream routing semantics; - document that
NewValidatoralready accepts a router but is safe only when the supplied router selects operations using the same semantics as the dispatcher; callers must also supply anAuthenticationFuncthroughValidationOptions, or protected requests fail with the existing missing-authentication-service error; - as an interim hardening option, make
ValidationHandlerfail closed on an encoded path separator whenURL.PathandURL.RawPathcan select different operations, and verify this behavior against the full suite; - document the route-identity requirement and add encoded-separator regression tests.
Replacing the legacy router with the kin-openapi gorilla router closes this PoC, but a local compatibility check changed two existing missing-path-parameter tests from HTTP 400 semantics to route-not-found HTTP 404 semantics. A direct router swap is therefore not a fully validated drop-in fix.
Public-reference triage
The public advisory list and related path-encoding reports reviewed on 2026-08-18 did not describe this authentication-boundary crossing. Relevant nearby reports include issues #397, #1089, and #1105 and PR #400. This search is not proof of novelty; please deduplicate against private reports.
Credit
Preferred credit: Satoru Kanno (GMO CONNECT Inc., GMO Internet Group, Inc.)
{
"affected": [
{
"package": {
"ecosystem": "Go",
"name": "github.com/getkin/kin-openapi"
},
"ranges": [
{
"events": [
{
"introduced": "0.144.0"
},
{
"fixed": "0.147.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-100351"
],
"database_specific": {
"cwe_ids": [
"CWE-289",
"CWE-436"
],
"github_reviewed": true,
"github_reviewed_at": "2026-10-15T20:19:20Z",
"nvd_published_at": "2026-10-15T16:17:31Z",
"severity": "HIGH"
},
"details": "## Summary\n\n`openapi3filter.ValidationHandler.Middleware` validates requests with kin-openapi\u0027s\nlegacy router, which selects an OpenAPI operation from the decoded `URL.Path`. The\nmethod is documented as implementing `gorilla/mux.MiddlewareFunc`, but a downstream\ngorilla/mux router configured with `UseEncodedPath()` selects its handler from the\nencoded representation (`URL.RawPath`).\n\nAn unauthenticated request such as `/public%2Fstatus` can therefore be validated as an\nintentionally public operation (`/public/status`) and then dispatched to a protected\noperation (`/{id}`). The configured, rejecting `AuthenticationFunc` is never invoked,\nand the protected downstream handler runs.\n\nThis is distinct from GHSA-r277-6w6q-xmqw. The behavior below uses an explicit\nauthentication callback that rejects missing credentials and remains reproducible\nafter the v0.144.0 fix for the unsafe default callback.\n\n## Affected products\n\n- Product: `github.com/getkin/kin-openapi`\n- Component: `openapi3filter.ValidationHandler.Middleware`\n- Confirmed post-GHSA affected releases: v0.144.0, v0.145.0, and v0.146.0\n- Reporter attestation: each listed release was checked out separately, the attached\n single-file test was copied into `openapi3filter/`, and that test passed; maintainers\n should independently confirm the version range\n- Confirmed affected development revision: `d33e44e67734dcde81b576582185e1343fb3985d`\n- v0.143.0 also reproduces this mechanism, but GHSA-r277-6w6q-xmqw already provides a\n simpler bypass on that release; the distinct post-fix claim starts at v0.144.0\n- Earlier releases: not tested; please treat the introduction boundary as unknown\n\n## Security impact\n\nAn unauthenticated remote requester can reach a handler whose OpenAPI operation\nrequires authentication when all of the following are true:\n\n1. the application uses `ValidationHandler.Middleware` as the security gate;\n2. the downstream router preserves and interprets an encoded path, such as\n gorilla/mux with `UseEncodedPath()`;\n3. a public exact route and a protected parameterized route overlap after decoding;\n4. the public and protected operations expose the same HTTP method;\n5. the protected handler relies on the validation layer\u0027s security decision.\n\nThe protected operation\u0027s path-parameter value is constrained: it is an encoded\nrepresentation that decodes to a public literal path (`public%2Fstatus` in this PoC),\nnot an arbitrary attacker-selected identifier. A protected operation without a\ncompatible parameterized path is not reachable through this specific mechanism.\n\n- Primitive: the security decision is made for a different OpenAPI operation than the\n operation dispatched by the application router.\n- Demonstrated impact: a protected handler executes while the explicitly rejecting\n `AuthenticationFunc` is never invoked.\n- Impact ceiling: confidentiality or integrity loss when the reachable protected\n handler has that authority; the PoC does not demonstrate data disclosure or state\n modification.\n- Availability impact: not demonstrated.\n\n## Root cause\n\nAt the confirmed development revision:\n\n- `openapi3filter/validation_handler.go`: `ValidationHandler.Load` constructs\n `routers/legacy.NewRouter` unconditionally.\n- `routers/legacy/router.go`: route selection uses the decoded request path.\n- `routers/gorillamux/router.go`: the kin-openapi gorilla router constructs a\n gorilla/mux router with `UseEncodedPath()`.\n- `ValidationHandler.Middleware` forwards the unchanged request after validation.\n\nThe component making the authentication decision and the component dispatching the\nhandler can therefore disagree about the selected operation.\n\n## Reproduction\n\nThe attached PoC uses a real `net/http` server, an explicit API-key callback that\nrejects missing credentials, and a downstream gorilla/mux router with\n`UseEncodedPath()`.\n\nFrom a kin-openapi checkout, copy the self-contained attachment\n`encoded_path_route_confusion_test.go.txt` into `openapi3filter/` as\n`encoded_path_route_confusion_test.go`, then run:\n\n```sh\ngo test -v ./openapi3filter -run \u0027^TestEncodedPathAuthorizationBypass$\u0027 -count=1\n```\n\nExpected attack observation:\n\n```text\nstatus=202 downstream_route=/{id} protected_id=public%2Fstatus authentication_calls=0\nwiring=Router.Use status=202 downstream_route=/{id} protected_id=public%2Fstatus authentication_calls=0\n```\n\nControls included in the test show that:\n\n- `/ordinary` selects the protected operation, invokes authentication once, and is\n rejected with HTTP 401;\n- `/public/status` reaches only the intentionally public handler;\n- the encoded request reaches the protected `/{id}` handler with HTTP 202 while the\n authentication callback remains uncalled;\n- both external wrapping and the documented\n `router.Use(validationHandler.Middleware)` wiring reproduce the same result.\n\n## Reverse-proxy reachability\n\nThe same request was reproduced through Nginx 1.31.3 using the official\n`nginx:alpine` image digest\n`sha256:4a73073bd557c65b759505da037898b61f1be6cbcc3c2c3aeac22d2a470c1752`.\n\nThe exact container invocation was:\n\n```sh\ndocker run --rm --name kin-openapi-nginx \\\n --add-host=host.docker.internal:host-gateway \\\n -p 18081:8080 -p 18082:8081 \\\n -v \"$PWD/poc/nginx.conf:/etc/nginx/nginx.conf:ro\" \\\n nginx:alpine\n```\n\nContainer port 8080 uses `proxy_pass http://host.docker.internal:18080;` without a\nreplacement URI. Host port 18081 therefore preserved `/public%2Fstatus`; the protected\nhandler returned HTTP 202 and the\nauthentication callback remained at zero calls. With\n`proxy_pass http://host.docker.internal:18080/;` on container port 8081 / host port\n18082, Nginx canonicalized the upstream URI; both layers selected `/public/status` and\nthe bypass did not occur.\n\nNginx access-log observations:\n\n```text\nserver_port=8080 \"GET /public%2Fstatus HTTP/1.1\" 202 0 \"curl/8.7.1\"\nserver_port=8081 \"GET /public%2Fstatus HTTP/1.1\" 204 0 \"curl/8.7.1\"\nserver_port=8080 \"GET /ordinary HTTP/1.1\" 401 0 \"curl/8.7.1\"\n```\n\nThe response route headers were `X-Poc-Route: /{id}` on the preserving listener and\n`X-Poc-Route: /public/status` on the canonicalizing listener. Backend observations for\nthe two requests were:\n\n```text\nrequest_uri=\"/public%2Fstatus\" path=\"/public/status\" raw_path=\"/public%2Fstatus\" auth_before=0\ncompleted request_uri=\"/public%2Fstatus\" auth_after=0\nrequest_uri=\"/public/status\" path=\"/public/status\" raw_path=\"\" auth_before=0\ncompleted request_uri=\"/public/status\" auth_after=0\n```\n\nThis control shows that upstream path handling is a deployment precondition rather\nthan claiming that every proxy configuration is affected.\n\nThe canonical protected control produced:\n\n```text\nrequest_uri=\"/ordinary\" path=\"/ordinary\" raw_path=\"\" auth_before=0\ncompleted request_uri=\"/ordinary\" auth_after=1\n```\n\n## Suggested remediation\n\nThe authentication layer and downstream dispatcher must select the same OpenAPI\noperation from the same request representation.\n\nPotential approaches:\n\n1. make `ValidationHandler` accept a caller-supplied `routers.Router` and require it to\n match downstream routing semantics;\n2. document that `NewValidator` already accepts a router but is safe only when the\n supplied router selects operations using the same semantics as the dispatcher;\n callers must also supply an `AuthenticationFunc` through `ValidationOptions`, or\n protected requests fail with the existing missing-authentication-service error;\n3. as an interim hardening option, make `ValidationHandler` fail closed on an encoded\n path separator when `URL.Path` and `URL.RawPath` can select different operations,\n and verify this behavior against the full suite;\n4. document the route-identity requirement and add encoded-separator regression tests.\n\nReplacing the legacy router with the kin-openapi gorilla router closes this PoC, but a\nlocal compatibility check changed two existing missing-path-parameter tests from HTTP\n400 semantics to route-not-found HTTP 404 semantics. A direct router swap is therefore\nnot a fully validated drop-in fix.\n\n## Public-reference triage\n\nThe public advisory list and related path-encoding reports reviewed on 2026-08-18 did\nnot describe this authentication-boundary crossing. Relevant nearby reports include\nissues #397, #1089, and #1105 and PR #400. This search is not proof of novelty; please\ndeduplicate against private reports.\n\n## Credit\n\nPreferred credit: Satoru Kanno (GMO CONNECT Inc., GMO Internet Group, Inc.)",
"id": "GHSA-23vg-4cjm-66q5",
"modified": "2026-10-15T20:19:20Z",
"published": "2026-10-15T20:19:20Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/getkin/kin-openapi/security/advisories/GHSA-23vg-4cjm-66q5"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-100351"
},
{
"type": "WEB",
"url": "https://github.com/getkin/kin-openapi/commit/eda80e2676e9f577ceed2dd80e64f16083edb041"
},
{
"type": "PACKAGE",
"url": "https://github.com/getkin/kin-openapi"
},
{
"type": "WEB",
"url": "https://github.com/getkin/kin-openapi/releases/tag/v0.147.0"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N",
"type": "CVSS_V3"
}
],
"summary": "kin-openapi: ValidationHandler security-requirement bypass via encoded-path operation confusion"
}
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