Hackney has an infinite loop on non-token byte at start of an Alt-Svc entry

Summary

CVE-2026-47066 is an infinite loop (CWE-835) in hackney's Alt-Svc response header parser (src/hackney_altsvc.erl). When an HTTP server returns an Alt-Svc header whose value begins with a non-token byte (e.g. !, @, =, ;), the parser enters a tight tail-recursive loop that pins an Erlang scheduler at 100% CPU and permanently hangs the calling connection process. Because the parser is invoked synchronously on every HTTP response, any attacker-controlled origin can trigger the hang with a single-byte header value.

Details

1. Parser dispatch

parse_and_cache/3 is called inside the hackney connection process on each HTTP response. It collects all Alt-Svc header values via collect_altsvc_headers/1, concatenates them, and passes the result to parse/1, which calls parse_entries(Header, []).

2. Failed token consumption

parse_entries/2 → parse_entry/1 → parse_protocol/1 → parse_token(Data, <<>>). The function parse_token/2 pattern-matches leading bytes: alphanumeric, -, _, whitespace, and comma all have explicit clauses. Any other byte (e.g. !) falls through to the catch-all:

parse_token(Rest, <<>>) -> {undefined, Rest}.

This returns the input unchanged — no byte is consumed.

3. No-progress loop

parse_entry propagates {undefined, Rest} back to parse_entries/2, which calls skip_comma(Rest). Because the first byte is not ,, skip_comma also returns Rest unchanged. parse_entries then recurses with the identical buffer:

parse_entries(Data, Acc)  % Data identical to previous iteration

Erlang tail recursion never preempts on a pure CPU loop, so the scheduler is pinned and the process never yields or returns.

4. Root cause

parse_entries/2 has no guard that detects zero-byte progress after a failed parse_entry call and no fallback to advance past the offending byte.

PoC

1. Start an HTTP server that responds with the header Alt-Svc: ! (any single non-token byte suffices).
2. Issue any HTTP GET request via hackney to that server:

   hackney:request(get, "http://attacker.example/", [], <<>>, [])
   

3. Observe that the call never returns; the Erlang scheduler hosting the connection process is pinned at 100% CPU indefinitely.

Alternatively, call the parser directly: hackney_altsvc:parse(<<"!">>) — the process hangs immediately.

Impact

Denial of service via unbounded CPU consumption. Any application using hackney 2.0.0-beta.1 through 4.0.0 that connects to attacker-controlled HTTP endpoints is affected. No authentication is required; a single response header byte is sufficient to hang the connection process. Fixed in hackney 4.0.1. CVSS v4.0 score: 8.7 (HIGH).

Resources

• Introduction commit: https://github.com/benoitc/hackney/commit/408e5fe20302226ea8c74dde2bcbd452d712b5b2
• Patch commit: https://github.com/benoitc/hackney/commit/e548aba1f97ffa3f4750da7b772998fb78c01894