EEF-CVE-2026-82672

## Summary

Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') vulnerability in elixir-mint mint allows a malicious HTTP/1 server to desynchronize a strict intermediary and the Mint client on a pooled connection, enabling response-queue poisoning against subsequent requests that share the connection.

`Mint.HTTP1.Parse.chunk_size/1` in `lib/mint/http1/parse.ex` stops at the first non-hexadecimal byte of a chunked response's chunk-size line and returns the remainder unexamined. `Mint.HTTP1.decode_body/5` in `lib/mint/http1.ex` then discards every byte up to the CRLF with `Parse.ignore_until_crlf/1`, so the accepted grammar is a run of hex digits followed by arbitrary bytes, where RFC 9112 permits only a `;`-introduced chunk extension. Lines such as `5ZZZZZ` and `5 9` are accepted as chunk size 5, and `0ZZZZ` is accepted as the terminating chunk that ends the message body. An RFC-strict intermediary rejects such a line while Mint accepts it, so the two disagree on chunk boundaries and on where the response ends.

This issue affects mint: from 0.1.0 before 1.10.1.

## Details

**1. Chunk-size parsing.** `Mint.HTTP1.Parse.chunk_size/1` in `lib/mint/http1/parse.ex` folds leading hexadecimal digits into an accumulator through `parse_hex_prefix/3` and, on the first byte that is not a hex digit, returns `{:ok, size, rest}` with `rest` unexamined. The sign and digit-count checks added by earlier fixes constrain only the digits.

**2. Tail skipping.** The caller, `Mint.HTTP1.decode_body/5` in `lib/mint/http1.ex`, hands `rest` to `Parse.ignore_until_crlf/1`, which advances over any byte until it finds CRLF. Nothing between the last hex digit and the CRLF is validated, so the accepted grammar is `1*HEXDIG *OCTET CRLF`, where RFC 9112 section 7.1 allows only an optional `;`-introduced `chunk-ext`. The same tolerance applies to the terminating zero-length chunk, which is the token that ends the message body.

**3. Parser disagreement.** The sibling `Content-Length` parser, `Mint.HTTP1.Parse.content_length_header/1`, trims trailing whitespace and requires the whole remaining value to be digits, rejecting anything else. An RFC-strict intermediary that rejects or reframes a chunk-size line with a non-extension tail, on a connection where Mint accepts it, yields a framing disagreement about chunk length and, through the terminating chunk, about where the message ends.

## Proof of concept

1. Start a loopback TCP server that serves one `HTTP/1.1 200 OK` response with `transfer-encoding: chunked` and controls the chunk-size line byte for byte. 2. Connect with `Mint.HTTP1` (mint 1.10.0 from Hex), send a request and stream the response. 3. Positive controls: chunk-size lines `+5`, `Z5` and `00000000000000005` are refused with `:invalid_chunk_size`, confirming the build carries the earlier chunk-size fixes. 4. Baseline: `5` and `5;name=value` are accepted with body `hello`. 5. Finding: `5ZZZZZ`, `5 anything at all`, `5<TAB>foo`, `5 9` and `5}~!` are each accepted as chunk size 5 with body `hello`. 6. Terminator: `0ZZZZ` and `0 9` in place of the final `0` chunk are accepted and end the body. 7. Contrast: `Content-Length: +5`, `Content-Length: 5ZZZ` and `Content-Length: 5 9` are refused with `:invalid_content_length_header` in the same run.

The reporter ran this on Elixir 1.18 / OTP 27 and Elixir 1.18.4 / OTP 28 with identical results.

## Impact

A malicious or attacker-influenced HTTP/1 origin behind an RFC-strict intermediary can make the intermediary and the Mint client disagree on chunk boundaries and on where the response body ends. On a pooled keep-alive connection that disagreement lets bytes from one response be attributed to the next, poisoning the responses returned to unrelated requests that share the connection.

## Configurations

Exploitation requires a deployment topology in which an RFC-strict HTTP/1 intermediary (proxy, load balancer, or WAF) sits between the Mint client and the attacker-influenced origin, and HTTP/1 connections between the client and the intermediary are reused across requests (keep-alive with connection pooling). Mint clients that talk directly to an origin without an intermediary, or that do not reuse connections, are not exploitable for response-queue poisoning even if the vulnerable parsing behavior is present.