follow-redirects array-form headers bypass cross-origin credential stripping

### Summary follow-redirects removes sensitive credential headers (Authorization, Proxy-Authorization, Cookie, and any caller-declared `sensitiveHeaders`) before following a redirect that crosses to a different host or downgrades to a less secure protocol. The removal routine iterates the caller's headers container with `for...in` and matches header names against a filter. This assumes the headers container is a plain object keyed by header names. Node's underlying `http.request`, which follow-redirects is a drop-in replacement for, also accepts headers as an alternating array of names and values. For an array, `for...in` yields the numeric indices `"0"`, `"1"`, `"2"` and so on, which never match a header-name filter, so nothing is removed. An application that sends array-form headers and follows an attacker-controlled cross-origin redirect therefore forwards its Authorization, Cookie, and Proxy-Authorization headers verbatim to the attacker-controlled destination, with no error or warning. The `sensitiveHeaders` option added to fix CVE-2026-40895 / GHSA-r4q5-vmmm-2653 is equally ineffective, because it relies on the same helper. This is an unauthenticated confidentiality vulnerability: any remote server the victim application contacts can harvest the victim's credential headers by responding with a cross-origin redirect. ### Details The cross-origin credential-stripping control is implemented by `removeMatchingHeaders(regex, headers)` at `index.js:630-640`: ```js function removeMatchingHeaders(regex, headers) { var lastValue; for (var header in headers) { // for...in over an ARRAY yields "0","1","2"... if (regex.test(header)) { // the header-name filter never matches a numeric index lastValue = headers[header]; delete headers[header]; } } return (lastValue === null || typeof lastValue === "undefined") ? undefined : String(lastValue).trim(); } ``` It is invoked from `_processResponse` for three purposes, all with the caller-supplied `this._options.headers`: - `index.js:490` - strip the credential filter (`this._headerFilter`, which covers the built-in list Authorization/Proxy-Authorization/Cookie plus any `sensitiveHeaders`) when the redirect crosses origin or downgrades protocol. - `index.js:470` - strip the `Host` header (`/^host$/i`). - `index.js:466` - strip `content-*` headers when a 301/302/303 converts POST to GET. The caller-supplied headers container is passed straight through to each redirected request via `nativeProtocol.request(this._options)` at `index.js:352`. `_sanitizeOptions` (index.js:300-331) only replaces `options.headers` when it is falsy, so an array is left untouched. Node's `http.request` accepts `options.headers` as an alternating array of names and values (for example `["Authorization", "Bearer ...", "Cookie", "session=..."]`). This is a supported way to control header ordering and duplication. For such an array, `for (var header in headers)` iterates the numeric indices `"0"`, `"1"`, `"2"`, and the header-name regex never matches any of them, so `removeMatchingHeaders` deletes nothing. Every one of the three strip operations above silently no-ops: - The built-in credential headers (Authorization, Cookie, Proxy-Authorization) are forwarded to the cross-origin redirect target. - Any `sensitiveHeaders` the caller declared are forwarded too. - The stale `Host` of the previous origin is re-sent to the new origin. - On a POST-to-GET redirect the `content-*` headers are retained, leaving `Content-Length` inconsistent with the now-empty body. The primary and highest-impact consequence is disclosure of credential headers. The stale `Host` re-send and the `Content-Length` inconsistency are secondary correctness defects arising from the same root cause. Root-cause history: the `for...in`-based `removeMatchingHeaders` used for the built-in credential strip has been present since the cross-host Authorization drop was introduced (commit `724087d`, first released in v1.11.0) and is unchanged through 1.16.0. The `sensitiveHeaders` layer was introduced in 1.16.0 (commit `844c4d3`). The behavior was verified directly on both 1.15.11 and 1.16.0. Suggested remediation: handle or explicitly reject Node's array-form headers. The strictest fix is to detect `Array.isArray(options.headers)` in `_sanitizeOptions` and throw a clear `TypeError` instructing callers to pass an object, so misuse fails loudly instead of silently downgrading security. If array support is desired, extend `removeMatchingHeaders` to walk the array in name/value pairs: ```js function removeMatchingHeaders(regex, headers) { if (Array.isArray(headers)) { var lastValue; for (var i = 0; i < headers.length; i += 2) { if (regex.test(headers[i])) { lastValue = headers[i + 1]; headers.splice(i, 2); i -= 2; } } return (lastValue === null || typeof lastValue === "undefined") ? undefined : String(lastValue).trim(); } // existing object logic } ``` Apply the same guard to the `Host` and `content-*` removal paths. Add regression tests that exercise array-form headers across a cross-origin redirect; the current suite only covers object-form headers. References: - https://github.com/follow-redirects/follow-redirects/security/advisories/GHSA-r4q5-vmmm-2653 - https://github.com/follow-redirects/follow-redirects/commit/724087d696c0bcd55c13ee0f3560c4558138e422 - https://github.com/follow-redirects/follow-redirects/commit/844c4d3 (sensitiveHeaders option) ### PoC Environment: Node.js v24.18.0 on Windows 11. The underlying Node array-header behavior is long-standing and not version-specific. Run each script from a checkout of follow-redirects at 1.16.0 so that `require("./index.js")` resolves to the library under test. Precondition check (Node accepts and serializes array-form headers): ``` node -e 'var http=require("http");var s=http.createServer((q,r)=>r.end(JSON.stringify(q.headers)));s.listen(0,"127.0.0.1",()=>{var p=s.address().port;htt…