Mechanize sends credential headers to another host after an HTTP redirect

Summary

mechanize leaked credentials to the redirect target when an HTTP redirect crossed to another host. Credentials set through Mechanize#request_headers= leaked even when they were Authorization.

Details

Two defects, both in lib/mechanize/http/agent.rb.

**1. Mechanize#request_headers= bypassed the redirect strip entirely.** #request_add_headers copied @request_headers onto every request unconditionally, with no host check, including the request issued after a redirect. The strip in #response_redirect mutated only the per-request headers hash and never touched agent state. Because request_headers= is the documented way to set a default credential for every request, the header the code explicitly protected — Authorization — was the one most likely to leak.

**2. The strip list omitted Proxy-Authorization and Cookie2.** Only CREDENTIAL_HEADERS = ['Authorization'] and COOKIE_HEADERS = ['Cookie'] were removed from the per-request headers hash on a cross-host redirect.

Cookies held in Mechanize#cookie_jar and credentials held in Mechanize::HTTP::AuthStore are not affected. Both are looked up per-URI, so they never follow a redirect to a foreign host. The exposure was limited to headers the caller set by hand.

PoC

agent = Mechanize.new
agent.request_headers = { 'Authorization' => 'Bearer secret' }
agent.get('https://example.test/redirects-to-attacker')
# the request to the attacker's host carries "Authorization: Bearer secret"

Impact

An attacker who controls a redirect target — through an open redirect on the site being fetched, an attacker-supplied fetch URL, DNS rebinding, or MITM — captures bearer tokens and session cookies from any mechanize agent that sets credentials through request_headers= or the per-request headers argument. Disclosure only; no integrity or availability impact.

Patches

Fixed in mechanize v2.14.1.

• Credentials and cookies are withheld from a request that follows a redirect across an origin, from both header sources: the per-request headers argument and Mechanize#request_headers=.
• CREDENTIAL_HEADERS gains Proxy-Authorization; COOKIE_HEADERS gains Cookie2.

Proxy-Authorization is withheld here although curl does not withhold it, because Net::HTTP tunnels https: requests with CONNECT, so a caller-supplied Proxy-Authorization travels inside the tunnel to the origin server rather than to the proxy.

Headers set through Mechanize#request_headers= no longer reach a redirect target on another origin when they are credentials. Callers that relied on the previous behavior will see those headers withheld.

What this fix does not cover

Only the headers in CREDENTIAL_HEADERS and COOKIE_HEADERS are withheld. A caller-supplied header that carries a credential under some other name — X-API-Key, X-Vault-Token, or any bespoke token header — still follows a redirect to another origin, matching the behavior of curl's CURLOPT_HTTPHEADER. If you set such a header, do not enable redirect following for requests that carry it, or scope it to a single request whose destination you control.

Workarounds

Set Mechanize#redirect_ok = false and handle redirects explicitly, or avoid request_headers= for credentials and pass them per-request only to hosts you intend to authenticate to.

Credit

Reported by @SnailSploit.