Summary
Hackney vulnerable to atom-table exhaustion via unrecognized URL schemes
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:
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
- 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, … - After enough iterations, observe
erlang:system_info(:atom_count)climbing by one per unique scheme. - 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.
References
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).
The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-47067? CVE-2026-47067 is a high-severity allocation of resources without limits or throttling vulnerability in hackney (erlang), affecting versions >= 2.0.0, < 4.0.1. It is fixed in 4.0.1. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
- Which versions of hackney are affected by CVE-2026-47067? hackney (erlang) versions >= 2.0.0, < 4.0.1 is affected.
- Is there a fix for CVE-2026-47067? Yes. CVE-2026-47067 is fixed in 4.0.1. Upgrade to this version or later.
- Is CVE-2026-47067 exploitable, and should I be worried? Whether CVE-2026-47067 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-47067 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-47067? Upgrade
hackneyto 4.0.1 or later.