mint has potential CRLF injection in its HTTP request line via unvalidated `method`/`target`

Summary

Mint's HTTP/1 request encoder splices the caller-supplied method and target directly into the request line without character validation. An application that forwards attacker-controlled input as the HTTP method or the target to Mint.HTTP.request/5 is exposed to request-line CRLF injection, allowing the attacker to terminate the request line early, inject arbitrary headers, and pipeline a fully attacker-chosen second request onto the same TCP connection.

Details

encode_request_line/2 in lib/mint/http1/request.ex writes method and target to the wire verbatim. encode_headers/1 validates header names and values, but there is no equivalent validate_method!/1.

Mint 1.7.0 added validate_request_target/2, which rejects CRLF and other control characters in target by default and closes the path/query vector. The method field remains unvalidated, so a CRLF-bearing method such as "GET / HTTP/1.1\r\nX-Smuggled: 1\r\nGET /admin" is accepted and written to the socket as-is. Bytes after the first \r\n are interpreted by the peer as an injected header, or, with a second \r\n, as an additional pipelined request.

PoC

1. Stand up a Mint-using gateway/proxy that calls Mint.HTTP.request(conn, method, "/", [], nil) with method taken from caller input.
2. Send a request whose forwarded method is "GET / HTTP/1.1\r\nX-Smuggled-Header: pwned\r\nGET /admin/delete-everything".
3. Observe the bytes received by the upstream server: the smuggled header line and the second request line appear verbatim in the outbound stream.

Impact

CRLF injection / HTTP request smuggling in the HTTP/1 client encoder, exploitable under default configuration whenever an application passes caller-influenced input as the HTTP method. An attacker who controls the method can inject arbitrary outbound headers (forged Host, Authorization, cache-poisoning headers) and smuggle additional, fully attacker-chosen requests to the upstream server over the same connection, potentially reaching endpoints the legitimate caller never intended to invoke.

Resources

• Introduction commit: https://github.com/elixir-mint/mint/commit/8db1acff30b6a9433762c18b1e1f891b8c1f74f7
• Patch commit: https://github.com/elixir-mint/mint/commit/fad091454cbb7449b19edb8e1fee12ca7cf28c3a