CVE-2026-47066

### Summary [CVE-2026-47066](https://nvd.nist.gov/vuln/detail/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: ```erlang 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: ```erlang 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: ```erlang 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

## Summary Loop with Unreachable Exit Condition ('Infinite Loop') vulnerability in benoitc hackney allows Excessive Allocation. The Alt-Svc response header parser in src/hackney\_altsvc.erl does not guarantee forward progress. When parse\_token/2 receives a non-token, non-whitespace, non-comma byte (e.g. !, @, =, ;), it returns the input unchanged. skip\_comma/1 also returns the buffer unchanged when the first byte is not a comma. parse\_entries/2 then recurses with identical data, creating a tight infinite tail-recursive loop that pins a scheduler at 100% CPU. The calling process never returns. The entry point parse\_and\_cache/3 is called synchronously in the connection process on every HTTP response. A single-byte Alt-Svc: ! response header is sufficient to trigger the hang; the header is fully controlled by any HTTP origin the client connects to. This issue affects hackney: from 2.0.0-beta.1 before 4.0.1.