ANT-2026-3CJT2BP4 · libjxl

heap-buffer-overflow high

Severity Claude high · Security research firm high · Maintainer -

Discovered by Claude Mythos Preview

REPORT

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

ANT-2026-3CJT2BP4: Heap buffer overflow in JPEG reconstruction writer due to unbounded extra zero runs

The JPEG reconstruction writer overflows a heap buffer when handling unbounded extra zero runs.

Target

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

Reproduction

This finding was identified by static analysis and has not yet been dynamically reproduced. 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-2026-3CJT2BP4.

Reference: ANT-2026-3CJT2BP4
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
high
ADVISORY

https://github.com/libjxl/libjxl/releases/tag/v0.12.0

TIMELINE

Dates from discovery through public reveal.

  1. 2026-03-29 Reported to tracker
  2. 2026-04-09 Sent to maintainer
  3. 2026-05-09 Maintainer acknowledged
  4. 2026-06-30 Patch released
  5. 2026-08-17 Publicly revealed
PROVENANCE

SHA-3-512 hash:

f1543e84d722de86582489332360c3366a0d50256a1560af8fc37f848e6b153c3e3b41d348d8988809875c3fe108bcded4954f0d61be37f0d34431ab1cec36e5

Committed 2026-04-09 11:49 PT

Revealed 2026-08-17 13:03 PT

Verify (download preimage.json)

Show preimage JSON 
{
  "ant_id": "ANT-2026-3CJT2BP4",
  "bug_class": "Heap Buffer Overflow",
  "claude_severity": "high",
  "commit_sha": null,
  "created_at": "2026-03-29T20:43:07+00:00",
  "description": "The JPEG reconstruction writer overflows a heap buffer when handling unbounded extra zero runs.",
  "discovered_at": null,
  "location": null,
  "poc_sha256": null,
  "preimage_version": 1,
  "project": "libjxl",
  "reproduction": null,
  "technical_details": null,
  "title": "Heap buffer overflow in JPEG reconstruction writer due to unbounded extra zero runs",
  "vendor_severity": "high"
}