ANT-2026-H4RM2RMD · htslib

oob-read 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-H4RM2RMD: Out-of-bounds access in CRAM xpack/xrle transform cache via undersized block_by_id array

The CRAM xpack/xrle transform cache performs out-of-bounds array access because the block_by_id array is undersized.

Target

Project: htslib
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-H4RM2RMD.

Reference: ANT-2026-H4RM2RMD
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.

  1. 2026-03-29 Reported to tracker
  2. 2026-04-09 Sent to maintainer
  3. 2026-05-09 Maintainer acknowledged
  4. 2026-07-08 Patch released
  5. 2026-08-17 Publicly revealed
PROVENANCE

SHA-3-512 hash:

7c8e6d1f5da78207f427e23d88809c38ddd70da25e949a58884c71302b7ffb4debaf03f4de1354251319268cbaf73c086ee6edfc0c42885ddc09e9f536399aa2

Committed 2026-04-09 11:49 PT

Revealed 2026-08-17 14:12 PT

Verify (download preimage.json)

Show preimage JSON 
{
  "ant_id": "ANT-2026-H4RM2RMD",
  "bug_class": "Out-of-Bounds Access",
  "claude_severity": "medium",
  "commit_sha": null,
  "created_at": "2026-03-29T20:42:57+00:00",
  "description": "The CRAM xpack/xrle transform cache performs out-of-bounds array access because the block_by_id array is undersized.",
  "discovered_at": null,
  "location": null,
  "poc_sha256": null,
  "preimage_version": 1,
  "project": "htslib",
  "reproduction": null,
  "technical_details": null,
  "title": "Out-of-bounds access in CRAM xpack/xrle transform cache via undersized block_by_id array",
  "vendor_severity": "medium"
}