Summary

CVE-2026-47067 is an atom table exhaustion vulnerability (CWE-770) in hackney's URL parser (src/hackneyurl.erl). hackneyurl:parseurl/1 converts every URL scheme it encounters into a BEAM atom via binarytoatom/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 systemlimit.

Details

1. Scheme extraction and conversion

In src/hackneyurl.erl, parseurl/1 extracts the scheme binary (the part before ://), validates it with isvalidscheme/1 (RFC 3986 alphabet: alpha-led, <=19 bytes, alphanumeric/+/-/. body), lowercases it, then calls:

erlang binarytoatom(SchemeLower, utf8)

The resulting atom is stored on the #hackneyurl{} 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, {unsupportedscheme, }}, 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 systemlimit and the node terminates; recovery requires a full restart.

PoC

1. Call hackneyurl:parseurl/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:systeminfo(:atomcount) climbing by one per unique scheme. 3. At 1,048,576 atoms the VM crashes with systemlimit.

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

Summary

CRLF injection in hackney's WebSocket upgrade request builder (src/hackneyws.erl). init/1 copies the host, path, headers, and protocols options from the caller-supplied opts map verbatim into #wsdata{}, and dohandshake/1 splices them directly into the raw HTTP/1.1 upgrade request by binary concatenation with no \r\n or \0 stripping. A caller that passes any of these fields from untrusted input can inject arbitrary header lines into the outbound upgrade request.

Details

dohandshake/1 builds the upgrade request at several concatenation sites:

- Host header (lines 583–590): the host binary is written straight into Host: <host>:<port>\r\n. - Sec-WebSocket-Protocol (lines 601–602): protocol tokens are joined with , and appended as a header line. - Extra headers (line 606): caller-supplied {Name, Value} tuples are concatenated as Name: Value\r\n with no sanitization of either component. - Request path (line 611): the path is interpolated into the GET <path> HTTP/1.1\r\n request line.

None of these sites reject \r, \n, or \0. A header value like <<"benign\r\nAuthorization: Bearer token">> produces two distinct header lines on the wire. A path with an embedded \r\n rewrites the request line itself.

PoC

1. Call :hackneyws.startlink/1 with headers: [{"X-User", "v\r\nAuthorization: Bearer attacker"}]. 2. Connect to a raw TCP listener and capture the bytes hackney writes. 3. The request contains a standalone Authorization: Bearer attacker line that the upstream WebSocket server parses as a legitimate header.

Impact

Header injection / request smuggling in outbound WebSocket upgrades. Affects hackney 2.0.0 through 4.0.0 wherever host, path, headers, or protocols options are populated from network or user input. Consequences include forging authentication headers toward the upstream server, log and cache poisoning, and request smuggling through intermediary proxies. CVSS v4.0: 6.9 (MEDIUM).

Resources

Introduction commit: https://github.com/benoitc/hackney/commit/690cecaf236fba49526da404a5bc889a24367a3e Patch commit: https://github.com/benoitc/hackney/commit/52310ca807e7b48441ba0e9129171f535313fdd1