ANT-2026-WG9N02WM · libjxl

heap-buffer-overflow medium

Severity Claude medium · Security research firm medium · Maintainer -

Discovered by Claude Mythos Preview

REPORT

Anthropic's analysis, sealed at approval. Disclosure to the maintainer was performed by Trail of Bits.

ANT-2026-WG9N02WM: Heap buffer overflow in fast lossless encoder due to undersized DC global buffer for >8-bit depth

The fast lossless encoder allocates an undersized DC global buffer when processing inputs with bit depth greater than 8, resulting in a heap buffer overflow.

Target

Project: libjxl
Discovery: static analysis — not yet dynamically reproduced

Technical Details

The DC global buffer in the fast lossless encoder is sized only for 8-bit depth; when encoding images with >8-bit depth, writes exceed the allocated buffer, causing a heap buffer overflow.

Reproduction

This finding was identified by static analysis and has not yet been dynamically reproduced. The Technical Details section above describes the code path; a trigger input is not included.

[No reproducer or sanitizer output attached — request from security-cvd@anthropic.com if needed.]

Acknowledgement

This vulnerability was discovered by Claude, Anthropic's AI assistant, and triaged by the Anthropic security team in collaboration with Anthropic Research. Please direct questions to security-cvd@anthropic.com and reference ANT-2026-WG9N02WM.

Reference: ANT-2026-WG9N02WM
Anthropic CVD Policy: https://www.anthropic.com/coordinated-vulnerability-disclosure

SECURITY RESEARCH FIRM ANALYSIS

Triage and disclosure were performed by Trail of Bits.

Verdict: true positive
Severity: medium

TIMELINE

Dates from discovery through public reveal.

2026-03-29 Reported to tracker
2026-04-09 Sent to maintainer
2026-05-09 Maintainer acknowledged
2026-06-30 Patch released
2026-08-17 Publicly revealed

PROVENANCE

SHA-3-512 hash:

b76f0794b94afea903c33261a49186bfc8d8f7b0d01ffd7473841e1cbb8318ef70da1c4510fb0c127b647c6176811ed69884969565215a6e952bc4f23ac84289

Committed 2026-04-09 11:49 PT

Revealed 2026-08-17 13:03 PT

Verify (download preimage.json)

Show preimage JSON

{
  "ant_id": "ANT-2026-WG9N02WM",
  "bug_class": "Heap Buffer Overflow",
  "claude_severity": "medium",
  "commit_sha": null,
  "created_at": "2026-03-29T20:43:07+00:00",
  "description": "The fast lossless encoder allocates an undersized DC global buffer when processing inputs with bit depth greater than 8, resulting in a heap buffer overflow.",
  "discovered_at": null,
  "location": null,
  "poc_sha256": null,
  "preimage_version": 1,
  "project": "libjxl",
  "reproduction": null,
  "technical_details": "The DC global buffer in the fast lossless encoder is sized only for 8-bit depth; when encoding images with >8-bit depth, writes exceed the allocated buffer, causing a heap buffer overflow.",
  "title": "Heap buffer overflow in fast lossless encoder due to undersized DC global buffer for >8-bit depth",
  "vendor_severity": "medium"
}