follow-redirects sensitiveHeaders regex escaping can expose credential headers

Summary

follow-redirects 1.16.0 introduced the sensitiveHeaders option (the fix for CVE-2026-40895 / GHSA-r4q5-vmmm-2653), which lets an application declare extra header names that must be removed when a redirect crosses to a different host or downgrades protocol. The filter is a single case-insensitive regular expression built by concatenating the declared header names into an alternation after passing each through escapeRegex. escapeRegex escapes only a partial set of metacharacters and leaves ^, |, [, {, and } unescaped. Header names may legitimately contain ^ and | (both are valid RFC 9110 token characters that Node accepts and sends), and for a declared name containing either, the generated pattern can never match the intended header. As a result, an application that adopts this option to protect a custom credential header, then follows an attacker-controlled cross-origin redirect, forwards that credential to the attacker with no error or warning, while believing it is protected. The failure is silent: the protection simply does not exist for the affected names. The built-in list (Authorization, Proxy-Authorization, Cookie) contains no metacharacters and is not affected by this defect.

Details

The filter is constructed once, in the RedirectableRequest constructor at index.js:118-120:

this._headerFilter = new RegExp("^(?:" +
    sensitiveHeaders.concat(options.sensitiveHeaders).map(escapeRegex).join("|") +
  ")$", "i");

escapeRegex at index.js:703-705:

function escapeRegex(regex) {
  return regex.replace(/[\]\\/()*+?.$]/g, "\\$&");   // escapes ] \ / ( ) * + ? . $  -- NOT ^ | [ { }
}

The character class escapes ], \, /, (, ), *, +, ?, ., and $. It omits ^, |, [, {, and }. Of these, Node's http.ClientRequest accepts ^ and | in header names (they are valid tchar values under RFC 9110) and rejects [, {, with , so and are the reachable vectors for a silent credential leak. Separately, although , , cannot appear as live header names, declaring a entry that contains one of them injects an unbalanced token into the alternation (for example an unterminated character class), so the filter throws a in the constructor and the request fails to start. That case is a loud request-time crash from the same incomplete-escaping root cause rather than a silent bypass, and the canonical escaper below fixes it as well.

For a declared sensitive header name containing an unescaped metacharacter, the alternation is broken in a fail-open way:

• A name containing ^, for example X-Api^Key, yields the alternative X-Api^Key inside the pattern. The ^ becomes a mid-pattern start-of-string anchor, so the alternative can never match any real header name. test("X-Api^Key") returns false, and the header is not stripped.
• A name containing |, for example X-Token|Secret, is split into two separate alternatives X-Token and Secret. The real header X-Token|Secret is never matched (not stripped), while unrelated headers literally named X-Token or Secret would be stripped instead.

The generated filter source, reproduced from the exact library logic, is:

^(?:Authorization|Proxy-Authorization|Cookie|X-Api^Key|X-Token|Secret|X-Control-Key)$

Direct matching results against this filter: X-Api^Key -> false, X-Token|Secret -> false, X-Control-Key (normal name) -> true, and the split-off X-Token -> true and Secret -> true (collateral wrong-stripping). The built-ins Authorization, Proxy-Authorization, Cookie all -> true.

Secondary, related weakness (filter snapshot not refreshed after beforeRedirect): the filter is built once at construction and applied at index.js:490, which runs before the beforeRedirect(this._options, ...) hook and the subsequent _sanitizeOptions at index.js:494-506. Because the strip happens before the hook and this._headerFilter is never rebuilt, any entry a caller adds to options.sensitiveHeaders inside beforeRedirect is silently ignored. This facet is lower severity: mutating sensitiveHeaders inside beforeRedirect is not a documented usage of the option (the documented path is the request option), so it is best treated as a hardening item bundled with the escaping fix rather than an independent control bypass. It is included here because both issues undermine the same control and share a remediation.

Suggested remediation: two changes close both facets.

1. Escape the complete set of regular-expression metacharacters:

function escapeRegex(str) { return str.replace(/[.*+?^${}()|[\]\\]/g, "\\$&"); }

This was verified to restore correct matching for all affected names while preserving the built-in matches.

1. Build the filter at strip time, or rebuild it after the beforeRedirect hook re-sanitizes options, so entries added inside the hook take effect. For example, replace the cached this._headerFilter with a this._getHeaderFilter() that constructs the RegExp from the current this._options.sensitiveHeaders on each redirect.

Additionally, since header names have a strict grammar, consider validating each entry of options.sensitiveHeaders against the RFC 9110 token character set and throwing an explicit error on invalid names, so misconfiguration fails loudly. Add a unit test asserting that every declared name is actually matched by the generated filter; the current suite only exercises metacharacter-free names (Sensitive-A, x-sensitive-b), which is why the defect passes CI.

References:

• https://github.com/follow-redirects/follow-redirects/security/advisories/GHSA-r4q5-vmmm-2653
• https://github.com/follow-redirects/follow-redirects/pull/284
• https://github.com/follow-redirects/follow-redirects/commit/844c4d302ac963d29bdb5dc1754ec7df3d70d7f9

PoC

Environment: Node.js v24.18.0 on Windows 11. Run each script from a checkout of follow-redirects at 1.16.0 so that require("./index.js") resolves to the library under test.

Facet 1, metacharacter header names, leak.js:

var followRedirects = require("./index.js");
var http = require("http");
var redirector = http.createServer(function (req, res) {
  req.resume();
  res.writeHead(302, { Location: "http://127.0.0.1:45902/" });   // cross-origin (different port)
  res.end();
});
var collector = http.createServer(function (req, res) {
  res.end(JSON.stringify(req.headers));                          // attacker-controlled target
});
redirector.listen(45901, "127.0.0.1", function () {
  collector.listen(45902, "127.0.0.1", function () {
    followRedirects.http.request({
      hostname: "127.0.0.1", port: 45901, path: "/",
      headers: {
        "X-Api^Key":      "SECRET-LEAKED-1",     // declared sensitive, name contains ^
        : ,     
        :  ,  
        :     
      },
      : [, , ]
    },  () {
       body = ; res.(,  () { body += c; });
      res.(,  () {
         h = .(body);
        .(, h[] || );
        .(, h[] || );
        .(, h[] || );
        .(, h[] || );
        redirector.(); collector.();
      });
    }).();
  });
});

Observed output (1.16.0):

X-Api^Key      : SECRET-LEAKED-1          <- credential LEAKED to redirect target
X-Token|Secret : SECRET-LEAKED-2          <- credential LEAKED to redirect target
X-Control-Key  : (strippe