ANT-2026-1ZNMP148 · nss

heap-buffer-overflow high

CVE-2026-6772 GHSA-c2q7-642g-3vwr

Severity Claude medium · Security research firm medium · Maintainer high

Discovered by Claude Mythos Preview

REPORT

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

ANT-2026-1ZNMP148: Off-by-one heap buffer overflow in TLS extension negotiation due to SSL_MAX_EXTENSIONS (22) being too small for 23 extensions

SSL_MAX_EXTENSIONS is defined as 22 but 23 TLS extensions exist, causing an off-by-one heap buffer overflow during extension negotiation.

Target

Project: nss
Location: TLS extension negotiation (SSL_MAX_EXTENSIONS)
Discovery: static analysis — not yet dynamically reproduced

Technical Details

The constant SSL_MAX_EXTENSIONS is set to 22, one less than the actual number of supported TLS extensions (23). A buffer sized by this constant overflows by one element when all extensions are processed during negotiation.

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-2026-1ZNMP148.

Reference: ANT-2026-1ZNMP148
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
2026-04-12 Patch released
2026-05-09 Maintainer acknowledged
2026-08-17 Publicly revealed

PROVENANCE

SHA-3-512 hash:

0e96e086b5559d23bf85a1a10318ec2a934193110dcdf952dffcf80f6ab8222da1344cf7ea9971e4fe0152c28aecf6a80b2cc14e3f4b2d1bea15164dd20873ea

Committed 2026-04-09 11:50 PT

Revealed 2026-08-17 10:47 PT

Verify (download preimage.json)

Show preimage JSON

{
  "ant_id": "ANT-2026-1ZNMP148",
  "bug_class": "Heap Buffer Overflow",
  "claude_severity": "medium",
  "commit_sha": null,
  "created_at": "2026-03-29T20:43:19+00:00",
  "description": "SSL_MAX_EXTENSIONS is defined as 22 but 23 TLS extensions exist, causing an off-by-one heap buffer overflow during extension negotiation.",
  "discovered_at": null,
  "location": "TLS extension negotiation (SSL_MAX_EXTENSIONS)",
  "poc_sha256": null,
  "preimage_version": 1,
  "project": "NSS",
  "reproduction": null,
  "technical_details": "The constant SSL_MAX_EXTENSIONS is set to 22, one less than the actual number of supported TLS extensions (23). A buffer sized by this constant overflows by one element when all extensions are processed during negotiation.",
  "title": "Off-by-one heap buffer overflow in TLS extension negotiation due to SSL_MAX_EXTENSIONS (22) being too small for 23 extensions",
  "vendor_severity": "medium"
}