mint: Content-Length header accepts non-RFC "+" sign prefix

Summary

Mint's HTTP/1 client accepts Content-Length header values with a leading + sign (e.g. +0, +123), which RFC 7230 forbids (Content-Length = 1*DIGIT). On a connection shared with a strict fronting proxy or load balancer, this parser disagreement is a response-smuggling primitive: the proxy frames the body one way, Mint frames it another, and bytes meant for one response leak into the next consumer's response stream.

Details

'Elixir.Mint.HTTP1.Parse':content_length_header/1 in lib/mint/http1/parse.ex parses the header value with Integer.parse/1. By design, Integer.parse/1 accepts an optional + or - sign prefix. The length >= 0 guard rules out negatives, but inputs such as "+0", "+123", or "+1" pass through and are returned as valid lengths.

A strict proxy or load balancer rejects or reframes Content-Length: +0\r\n, while Mint silently treats it as 0. When Mint reuses the socket (keep-alive, pipelining, or any pooled connection) and the connection is shared with a proxy that frames the same bytes differently, trailing bytes the proxy attributes to response N are attributed by Mint to response N+1. Across trust boundaries (shared pools, multi-tenant fronting) this enables response smuggling.

PoC

1. Stand up a raw TCP server that returns HTTP/1.1 200 OK\r\nContent-Length: +0\r\nConnection: keep-alive\r\n\r\n<smuggled bytes>.
2. Connect a Mint HTTP/1 client to the server and issue a request.
3. Observe that Mint reports the response as status 200 with Content-Length: "+0" and an empty body, leaving the smuggled bytes sitting in the socket buffer for the next response.

Impact

Response-smuggling / request-response desync primitive in Mint's HTTP/1 client parser. Anyone using Mint (directly or via Finch, Tesla's Mint adapter, Req, etc.) to talk through a shared or pooled connection where a fronting proxy enforces RFC 7230 strictly while Mint does not is exposed. The attacker is the response producer (a malicious or compromised upstream, or anything that can inject bytes into a shared origin response); exploitation into a cross-request data leak additionally requires the deployment to share a Mint connection across trust boundaries.