Summary
Fabio - Incomplete fix for CVE-2025-48865: operator-configured trust headers (clientip/tls/requestid) still strippable via the Connection header
Affected: github.com/fabiolb/fabio >= 1.6.6 through 1.7.1 and master HEAD (c75f8a6).
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:73r.Header.Set(cfg.ClientIPHeader, remoteIP) - TLSHeader (
proxy.header.tls) - http_headers.go:153r.Header.Set(cfg.TLSHeader, cfg.TLSHeaderValue)(TLS connections) - RequestID (
proxy.header.requestid) - http_proxy.go:90r.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.
CVE-2026-62987 has a CVSS score of 5.8 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.7.2); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Frequently Asked Questions
- What is CVE-2026-62987? CVE-2026-62987 is a medium-severity security vulnerability in github.com/fabiolb/fabio (go), affecting versions >= 1.6.6, <= 1.7.1. It is fixed in 1.7.2.
- How severe is CVE-2026-62987? CVE-2026-62987 has a CVSS score of 5.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/fabiolb/fabio are affected by CVE-2026-62987? github.com/fabiolb/fabio (go) versions >= 1.6.6, <= 1.7.1 is affected.
- Is there a fix for CVE-2026-62987? Yes. CVE-2026-62987 is fixed in 1.7.2. Upgrade to this version or later.
- Is CVE-2026-62987 exploitable, and should I be worried? Whether CVE-2026-62987 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-62987 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-62987? Upgrade
github.com/fabiolb/fabioto 1.7.2 or later.