CVE-2026-62987: Github.com/fabiolb/fabio Operator-configured Trust Headers (clientip/tls/requestid) Still Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-fq95-v8xc-jm3v",
  "modified" : "2026-09-22T20:34:22Z",
  "published" : "2026-09-22T20:34:22Z",
  "aliases" : [ "CVE-2026-62987" ],
  "summary" : "Fabio - Incomplete fix for CVE-2025-48865: operator-configured trust headers (clientip/tls/requestid) still strippable via the Connection header",
  "details" : "**Affected:** github.com/fabiolb/fabio >= 1.6.6 through 1.7.1 and master HEAD (c75f8a6).\n\n### Summary\nThe 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.\n\n### Details\nThe three unprotected trust headers and where fabio sets them:\n- ClientIPHeader (`proxy.header.clientip`) - http_headers.go:73 `r.Header.Set(cfg.ClientIPHeader, remoteIP)`\n- TLSHeader (`proxy.header.tls`) - http_headers.go:153 `r.Header.Set(cfg.TLSHeader, cfg.TLSHeaderValue)` (TLS connections)\n- RequestID (`proxy.header.requestid`) - http_proxy.go:90 `r.Header.Set(p.Config.RequestID, id())`\n\nOrder 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.Del`s every listed header. The three trust headers fabio just set are therefore deleted before the request reaches the backend.\n\n### PoC\nConfigure `proxy.header.clientip=X-Client-IP`, `proxy.header.requestid=X-Request-ID`. Raw request:\n```\nGET / HTTP/1.1\nHost: foo.com\nConnection: close, X-Client-IP, X-Request-ID\n```\nBackend 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.\n\n### Impact\nWhere 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.\n\nParent: 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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/fabiolb/fabio"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.6.6"
      }, {
        "fixed" : "1.7.2"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.7.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/fabiolb/fabio/security/advisories/GHSA-fq95-v8xc-jm3v"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-62987"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fabiolb/fabio/commit/240526a8004077edad4fb96d25b382bfc3901357"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/fabiolb/fabio"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/fabiolb/fabio/releases/tag/v1.7.2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-290", "CWE-348" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-22T20:34:22Z",
    "nvd_published_at" : "2026-09-21T17:17:38Z"
  }
}