CVE-2026-96618: Hex exhausts memory and disk on oversized or compressed responses from an untrusted mirror
Improper Handling of Highly Compressed Data (Data Amplification) vulnerability in hexpm hex allows an untrusted package mirror to exhaust the memory or disk space of the machine running mix deps.get, mix deps.update, mix hex.package fetch or mix hex.docs fetch.
Hex treats a mirror configured with HEXMIRRORURL, HEXMIRROR or mirrorurl as untrusted: it doesn't send credentials to it, and it checks registry signatures and package checksums that the mirror can't forge. Several response paths consume memory or disk in proportion to what the mirror sends before any of those checks run.
Hex.HTTP.request/5 reads the whole response body into memory and decompresses it with :zlib.gunzip/1 whenever the response's Content-Encoding contains gzip, with no limit on the compressed or decompressed size. Hex doesn't ask for compressed responses, so the server decides whether this step runs. It applies to registry, package, docs and API responses.
Package tarballs have no download size limit. Hex 2.5.1 holds the whole tarball in memory and writes it to the cache, and the development branch streams it to a temporary file; the 16 MiB tarball limit is checked only when the file is unpacked. The tarball is then unpacked, into deps/<app> in 2.5.1 and into a staging directory on the development branch, before its checksums are compared with the registry.
mix hex.docs fetch, and mix hex.docs offline when docs are missing, write the docs tarball to disk and extract it with :mixhexerltar.extract/2 without the 16 MiB compressed and 128 MiB decompressed limits that mixhextarball:unpackdocs/3 applies. The signed registry doesn't cover docs tarballs, so nothing checks them.
Registry responses are also decompressed before their signature is checked, in hexcore code that Hex vendors; that part is tracked in https://github.com/hexpm/hexcore/security/advisories/GHSA-7c4r-cgf9-9jmq.
This issue affects hex: from 0.11.2 before 2.5.2.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
hexto a version that resolves this vulnerability.Fixed in 2.5.2
Event History
Frequently Asked Questions
Who is exposed to this issue?
Machines running Hex commands that fetch dependencies, package data, documentation, or API responses from a mirror configured through HEX_MIRROR_URL, HEX_MIRROR, or mirror_url are exposed. The mirror is treated as untrusted, but malicious responses can consume resources before signature and checksum validation occurs.
What does an attacker need to do to trigger the resource exhaustion?
An attacker needs control of, or the ability to influence responses from, the configured package mirror. They can send oversized responses, gzip-encoded responses that expand substantially when decompressed, or oversized package tarballs.
Are credential protections and package verification sufficient mitigation?
No. Hex does not send credentials to an untrusted mirror and verifies registry signatures and package checksums, but the affected response handling occurs before those checks. These protections do not prevent memory or disk exhaustion.
Which operations can be affected?
The issue can affect mix deps.get, mix deps.update, mix hex.package fetch, and mix hex.docs fetch. Registry, package, documentation, and API responses may be affected by unbounded response handling.
What can be done while patching is unavailable?
Avoid using untrusted or unneeded mirror endpoints, especially mirrors configured through HEX_MIRROR_URL, HEX_MIRROR, or mirror_url. Use a trusted mirror or remove the mirror configuration where operationally possible.