GHSA-9653-rcfr-5c62 Hackney vulnerable to atom-table exhaustion via unrecognized URL schemes

• Hex hackney: >= 2.0.0, < 4.0.1 → fixed in 4.0.1

Summary CVE-2026-47067 is an atom table exhaustion vulnerability (CWE-770) in hackney's URL parser (`src/hackney_url.erl`). `hackney_url:parse_url/1` converts every URL scheme it encounters into a BEAM atom via `binary_to_atom/2`. Because BEAM atoms are never garbage-collected and the atom table has a hard limit of 1,048,576 entries, an attacker who can feed URLs with attacker-chosen scheme prefixes — directly as request targets, as webhook/callback URLs, or via `Location` headers in redirect chains — can exhaust the atom table and crash the entire BEAM VM with `system_limit`. Details 1. Scheme extraction and conversion In `src/hackney_url.erl`, `parse_url/1` extracts the scheme binary (the part before `://`), validates it with `is_valid_scheme/1` (RFC 3986 alphabet: alpha-led, `<=19` bytes, alphanumeric/`+`/`-`/`.` body), lowercases it, then calls: ```erlang binary_to_atom(SchemeLower, utf8) ``` The resulting atom is stored on the `#hackney_url{}` record and returned to the caller. 2. Permanent atom accumulation The validation constrains the alphabet but not uniqueness. The allowed scheme space is enormous (≈52·65¹⁸ values), far exceeding the default atom limit of 1,048,576. Each distinct scheme mints a new permanent atom. Even when hackney subsequently rejects an unsupported scheme with `{error, {unsupported_scheme, _}}`, the atom has already been interned and is never reclaimed. 3. Crash vector The most dangerous path is redirect following: when hackney follows a `Location` header, the redirect target URL is re-parsed by the same function. An attacker-controlled server can serve a sequence of redirects — or a batch of URLs from an upstream feed — each with a fresh unique scheme, driving the atom count monotonically upward. At the limit the BEAM emits `system_limit` and the node terminates; recovery requires a full restart. PoC 1. Call `hackney_url:parse_url/1` (or `:hackney.request/5`) repeatedly with URLs whose scheme prefixes are unique on each call: `aaaa://x`, `aaab://x`, `aaac://x`, … 2. After enough iterations, observe `erlang:system_info(:atom_count)` climbing by one per unique scheme. 3. At 1,048,576 atoms the VM crashes with `system_limit`. Alternatively, point hackney at a server that replies with a feed of ~1M URLs with distinct schemes (or uses redirect chains with rotating schemes); the atom table is exhausted and the node crashes without the client being able to intervene. Impact Unauthenticated remote denial of service via permanent resource exhaustion leading to VM termination. Any application using hackney 2.0.0 through 4.0.0 that processes attacker-influenced URLs — direct request targets, webhook URLs, or `Location` headers in followed redirects — is affected. No authentication or special configuration is required. CVSS v4.0 score: 8.7 (HIGH). References • Introduction commit: https://github.com/benoitc/hackney/commit/d9713695c0d99855d12c73fd8a0b4be0543950c4 • Patch commit: https://github.com/benoitc/hackney/commit/31f6f0e27e096ad88743dfded4f030a3ee74972e