CVE-2026-62987

Fabio is an HTTP(S) and TCP router for deploying applications managed by consul. From 1.6.6 until 1.7.2, the CVE-2025-48865 fix in proxy/http_headers.go uses protectHeaders for a hardcoded set of forwarded headers but omits the operator-configured ClientIPHeader, TLSHeader, and RequestID names. In proxy/http_proxy.go, HTTPProxy.ServeHTTP calls addHeaders to set these trust headers before Go ReverseProxy processes the inbound Connection header, allowing an unauthenticated client to name and remove the configured headers before the request reaches the backend. Deployments that enable the corresponding proxy.header options can therefore lose client-IP, TLS-termination, or request-correlation signals used by backend authorization and auditing; the options are empty by default, and the hardcoded protected forwarded headers are unaffected. This issue is fixed in version 1.7.2.

Fabio is an HTTP(S) and TCP router for deploying applications managed by consul. From 1.6.6 until 1.7.2, the CVE-2025-48865 fix in proxy/http_headers.go uses protectHeaders for a hardcoded set of forwarded headers but omits the operator-configured ClientIPHeader, TLSHeader, and RequestID names. In proxy/http_proxy.go, HTTPProxy.ServeHTTP calls addHeaders to set these trust headers before Go ReverseProxy processes the inbound Connection header, allowing an unauthenticated client to name and remove the configured headers before the request reaches the backend. Deployments that enable the corresponding proxy.header options can therefore lose client-IP, TLS-termination, or request-correlation signals used by backend authorization and auditing; the options are empty by default, and the hardcoded protected forwarded headers are unaffected. This issue is fixed in version 1.7.2.

Affected: github.com/fabiolb/fabio >= 1.6.6 through 1.7.1 and master HEAD (c75f8a6).

Summary

The v1.6.6 fix for CVE-2025-48865 sweeps the client Connection header against a hardcoded allowlist protectHeaders (proxy/http_headers.go:28-36) containing only the 7 X-Forwarded family headers. Fabio also injects three operator-configured server-side trust headers that are NOT in that allowlist, so the original hop-by-hop stripping attack still works against them.

Details

The three unprotected trust headers and where fabio sets them:

• ClientIPHeader (proxy.header.clientip) - http_headers.go:73 r.Header.Set(cfg.ClientIPHeader, remoteIP)
• TLSHeader (proxy.header.tls) - http_headers.go:153 r.Header.Set(cfg.TLSHeader, cfg.TLSHeaderValue) (TLS connections)
• RequestID (proxy.header.requestid) - http_proxy.go:90 r.Header.Set(p.Config.RequestID, id())

Order of operations in HTTPProxy.ServeHTTP: (1) line 90 sets RequestID; (2) line 175 calls addHeaders, which sweeps the Connection header (keeping the three configured names because they are absent from protectHeaders) and then sets ClientIPHeader/TLSHeader; (3) line 223 runs the Go httputil.ReverseProxy, whose removeHopByHopHeaders (Go stdlib net/http/httputil/reverseproxy.go) iterates the inbound Connection header and h.Dels every listed header. The three trust headers fabio just set are therefore deleted before the request reaches the backend.

PoC

Configure proxy.header.clientip=X-Client-IP, proxy.header.requestid=X-Request-ID. Raw request:

GET / HTTP/1.1
Host: foo.com
Connection: close, X-Client-IP, X-Request-ID

Backend sees X-Client-IP: <empty> and X-Request-ID: <empty>. Baseline (Connection: close) => backend sees the real client IP and a request id. The same works for proxy.header.tls over TLS (Connection: close, X-Secure strips the TLS assertion). Reproduced by the two PASS-ing tests in proxy/poc_cve48865_incomplete_test.go on master HEAD (Go 1.26.4). Control: X-Forwarded-For (in protectHeaders) survives, confirming the gap is specific to the configured headers.

Impact

Where a backend trusts these fabio-set headers, an external unauthenticated client can strip/downgrade the trust signal: bypass/poison IP-based ACL or audit that reads the clientip header, downgrade a TLS-terminated request to appear non-TLS to a backend keying off the TLS header, or drop request-id correlation. Scope: deployments that enable the relevant proxy.header.* option (all empty by default). Fix: add the configured ClientIPHeader/TLSHeader/RequestID names to the protectHeaders set (or strip their token from the inbound Connection header) inside addHeaders.

Parent: CVE-2025-48865 / GHSA-q7p4-7xjv-j3wf (fixed in v1.6.6). This is an incomplete-fix sibling - the same hop-by-hop stripping primitive applies to the operator-configured trust headers the allowlist does not cover.

Fabio is an HTTP(S) and TCP router for deploying applications managed by consul. From 1.6.6 until 1.7.2, the CVE-2025-48865 fix in proxy/http_headers.go uses protectHeaders for a hardcoded set of forwarded headers but omits the operator-configured ClientIPHeader, TLSHeader, and RequestID names. In proxy/http_proxy.go, HTTPProxy.ServeHTTP calls addHeaders to set these trust headers before Go ReverseProxy processes the inbound Connection header, allowing an unauthenticated client to name and remove the configured headers before the request reaches the backend. Deployments that enable the corresponding proxy.header options can therefore lose client-IP, TLS-termination, or request-correlation signals used by backend authorization and auditing; the options are empty by default, and the hardcoded protected forwarded headers are unaffected. This issue is fixed in version 1.7.2.

Fabio is an HTTP(S) and TCP router for deploying applications managed by consul. From 1.6.6 until 1.7.2, the CVE-2025-48865 fix in proxy/http_headers.go uses protectHeaders for a hardcoded set of forwarded headers but omits the operator-configured ClientIPHeader, TLSHeader, and RequestID names. In proxy/http_proxy.go, HTTPProxy.ServeHTTP calls addHeaders to set these trust headers before Go ReverseProxy processes the inbound Connection header, allowing an unauthenticated client to name and remove the configured headers before the request reaches the backend. Deployments that enable the corresponding proxy.header options can therefore lose client-IP, TLS-termination, or request-correlation signals used by backend authorization and auditing; the options are empty by default, and the hardcoded protected forwarded headers are unaffected. This issue is fixed in version 1.7.2.

Affected: github.com/fabiolb/fabio >= 1.6.6 through 1.7.1 and master HEAD (c75f8a6).

Summary

The v1.6.6 fix for CVE-2025-48865 sweeps the client Connection header against a hardcoded allowlist protectHeaders (proxy/http_headers.go:28-36) containing only the 7 X-Forwarded family headers. Fabio also injects three operator-configured server-side trust headers that are NOT in that allowlist, so the original hop-by-hop stripping attack still works against them.

Details

The three unprotected trust headers and where fabio sets them:

• ClientIPHeader (proxy.header.clientip) - http_headers.go:73 r.Header.Set(cfg.ClientIPHeader, remoteIP)
• TLSHeader (proxy.header.tls) - http_headers.go:153 r.Header.Set(cfg.TLSHeader, cfg.TLSHeaderValue) (TLS connections)
• RequestID (proxy.header.requestid) - http_proxy.go:90 r.Header.Set(p.Config.RequestID, id())

Order of operations in HTTPProxy.ServeHTTP: (1) line 90 sets RequestID; (2) line 175 calls addHeaders, which sweeps the Connection header (keeping the three configured names because they are absent from protectHeaders) and then sets ClientIPHeader/TLSHeader; (3) line 223 runs the Go httputil.ReverseProxy, whose removeHopByHopHeaders (Go stdlib net/http/httputil/reverseproxy.go) iterates the inbound Connection header and h.Dels every listed header. The three trust headers fabio just set are therefore deleted before the request reaches the backend.

PoC

Configure proxy.header.clientip=X-Client-IP, proxy.header.requestid=X-Request-ID. Raw request:

GET / HTTP/1.1
Host: foo.com
Connection: close, X-Client-IP, X-Request-ID

Backend sees X-Client-IP: <empty> and X-Request-ID: <empty>. Baseline (Connection: close) => backend sees the real client IP and a request id. The same works for proxy.header.tls over TLS (Connection: close, X-Secure strips the TLS assertion). Reproduced by the two PASS-ing tests in proxy/poc_cve48865_incomplete_test.go on master HEAD (Go 1.26.4). Control: X-Forwarded-For (in protectHeaders) survives, confirming the gap is specific to the configured headers.

Impact

Where a backend trusts these fabio-set headers, an external unauthenticated client can strip/downgrade the trust signal: bypass/poison IP-based ACL or audit that reads the clientip header, downgrade a TLS-terminated request to appear non-TLS to a backend keying off the TLS header, or drop request-id correlation. Scope: deployments that enable the relevant proxy.header.* option (all empty by default). Fix: add the configured ClientIPHeader/TLSHeader/RequestID names to the protectHeaders set (or strip their token from the inbound Connection header) inside addHeaders.

Parent: CVE-2025-48865 / GHSA-q7p4-7xjv-j3wf (fixed in v1.6.6). This is an incomplete-fix sibling - the same hop-by-hop stripping primitive applies to the operator-configured trust headers the allowlist does not cover.

Fabio is an HTTP(S) and TCP router for deploying applications managed by consul. From 1.6.6 until 1.7.2, the CVE-2025-48865 fix in proxy/http_headers.go uses protectHeaders for a hardcoded set of forwarded headers but omits the operator-configured ClientIPHeader, TLSHeader, and RequestID names. In proxy/http_proxy.go, HTTPProxy.ServeHTTP calls addHeaders to set these trust headers before Go ReverseProxy processes the inbound Connection header, allowing an unauthenticated client to name and remove the configured headers before the request reaches the backend. Deployments that enable the corresponding proxy.header options can therefore lose client-IP, TLS-termination, or request-correlation signals used by backend authorization and auditing; the options are empty by default, and the hardcoded protected forwarded headers are unaffected. This issue is fixed in version 1.7.2.

Fabio - Incomplete fix for CVE-2025-48865: operator-configured trust headers (clientip/tls/requestid) still strippable via the Connection header in github.com/fabiolb/fabio