follow-redirects leaks Custom Authentication Headers to Cross-Domain Redirect Targets

Summary

## Summary When an HTTP request follows a cross-domain redirect (301/302/307/308), `follow-redirects` only strips `authorization`, `proxy-authorization`, and `cookie` headers (matched by regex at index.js:469-476). Any custom authentication header (e.g., `X-API-Key`, `X-Auth-Token`, `Api-Key`, `Token`) is forwarded verbatim to the redirect target. Since `follow-redirects` is the redirect-handling dependency for **axios** (105K+ stars), this vulnerability affects the entire axios ecosystem. ## Affected Code `index.js`, lines 469-476: ```javascript if (redirectUrl.protocol !== currentUrlParts.protocol && redirectUrl.protocol !== "https:" || redirectUrl.host !== currentHost && !isSubdomain(redirectUrl.host, currentHost)) { removeMatchingHeaders(/^(?:(?:proxy-)?authorization|cookie)$/i, this._options.headers); } ``` The regex only matches `authorization`, `proxy-authorization`, and `cookie`. Custom headers like `X-API-Key` are not matched. ## Attack Scenario 1. App uses axios with custom auth header: `headers: { 'X-API-Key': 'sk-live-secret123' }` 2. Server returns `302 Location: https://evil.com/steal` 3. follow-redirects sends `X-API-Key: sk-live-secret123` to `evil.com` 4. Attacker captures the API key ## Impact Any custom auth header set via axios leaks on cross-domain redirect. Extremely common pattern. Affects all axios users in Node.js. ## Suggested Fix Add a `sensitiveHeaders` option that users can extend, or strip ALL non-standard headers on cross-domain redirect. ## Disclosure Source code review, manually verified. Found 2026-03-20.