{"id":"GHSA-9653-rcfr-5c62","summary":"Hackney vulnerable to atom-table exhaustion via unrecognized URL schemes","details":"### Summary\n\n[CVE-2026-47067](https://nvd.nist.gov/vuln/detail/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`.\n\n### Details\n\n**1. Scheme extraction and conversion**\n\nIn `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, `\u003c=19` bytes, alphanumeric/`+`/`-`/`.` body), lowercases it, then calls:\n\n```erlang\nbinary_to_atom(SchemeLower, utf8)\n```\n\nThe resulting atom is stored on the `#hackney_url{}` record and returned to the caller.\n\n**2. Permanent atom accumulation**\n\nThe 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.\n\n**3. Crash vector**\n\nThe 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.\n\n### PoC\n\n1. 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`, …\n2. After enough iterations, observe `erlang:system_info(:atom_count)` climbing by one per unique scheme.\n3. At 1,048,576 atoms the VM crashes with `system_limit`.\n\nAlternatively, 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.\n\n### Impact\n\nUnauthenticated 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)**.\n\n## References\n\n* Introduction commit: https://github.com/benoitc/hackney/commit/d9713695c0d99855d12c73fd8a0b4be0543950c4\n* Patch commit: https://github.com/benoitc/hackney/commit/31f6f0e27e096ad88743dfded4f030a3ee74972e","aliases":["CVE-2026-47067","EEF-CVE-2026-47067"],"modified":"2026-06-26T22:15:08.679887417Z","published":"2026-06-26T22:01:36Z","database_specific":{"nvd_published_at":"2026-05-25T15:16:21Z","cwe_ids":["CWE-770"],"severity":"HIGH","github_reviewed":true,"github_reviewed_at":"2026-06-26T22:01:36Z"},"references":[{"type":"WEB","url":"https://github.com/benoitc/hackney/security/advisories/GHSA-9653-rcfr-5c62"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-47067"},{"type":"WEB","url":"https://github.com/benoitc/hackney/commit/31f6f0e27e096ad88743dfded4f030a3ee74972e"},{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-47067.html"},{"type":"PACKAGE","url":"https://github.com/benoitc/hackney"},{"type":"WEB","url":"https://osv.dev/vulnerability/EEF-CVE-2026-47067"}],"affected":[{"package":{"name":"hackney","ecosystem":"Hex","purl":"pkg:hex/hackney"},"ranges":[{"type":"SEMVER","events":[{"introduced":"2.0.0"},{"fixed":"4.0.1"}]}],"versions":["2.0.0","2.0.1","3.0.0","3.0.1","3.0.2","3.0.3","3.1.0","3.1.1","3.1.2","3.2.0","3.2.1","4.0.0"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/06/GHSA-9653-rcfr-5c62/GHSA-9653-rcfr-5c62.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"}]}