ANT-2026-NGTJ413W · ffmpeg/ffmpeg

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 Ada Logics.

ANT-2026-NGTJ413W: This is a genuine heap-buffer-overflow vulnerability in ffmpeg's ON2AVC audio decoder.

A heap buffer overflow with a WRITE operation was identified in ffmpeg's ON2AVC (On2 Audio for Video Codec) decoder. The overflow is triggered by a fuzzer-generated malformed ON2AVC audio stream supplied to the decoder entrypoint. An attacker able to deliver crafted ON2AVC audio to an application using ffmpeg for decoding could trigger this out-of-bounds heap write.

Target

Project: ffmpeg

Technical Details

Heap buffer overflow with out-of-bounds write in ffmpeg's ON2AVC audio decoder when processing malformed input.

Crash signature: heap-buffer-overflow WRITE (ON2AVC decoder)

Reproduction

Reproduce against the target as described under Technical Details.

[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-NGTJ413W.

Reference: ANT-2026-NGTJ413W
Anthropic CVD Policy: https://www.anthropic.com/coordinated-vulnerability-disclosure

SECURITY RESEARCH FIRM ANALYSIS

Triage and disclosure were performed by Ada Logics.

Verdict: true positive
Severity: high

UPSTREAM FIX

The change that resolved this finding.

diff --git a/libavcodec/on2avc.c b/libavcodec/on2avc.c
index 0a912cc30b1fd..41ada55e1a4b0 100644
--- a/libavcodec/on2avc.c
+++ b/libavcodec/on2avc.c
@@ -866,6 +866,12 @@ static int on2avc_decode_frame(AVCodecContext * avctx, AVFrame *frame,
             av_log(avctx, AV_LOG_ERROR, "No subframes present\n");
             return AVERROR_INVALIDDATA;
         }
+        if (num_frames > INT_MAX / ON2AVC_SUBFRAME_SIZE) {
+            av_log(avctx, AV_LOG_ERROR,
+                   "Too many subframes (%d); per-frame sample count overflows\n",
+                   num_frames);
+            return AVERROR_INVALIDDATA;
+        }
 
         /* get output buffer */
         frame->nb_samples = ON2AVC_SUBFRAME_SIZE * num_frames;

https://github.com/FFmpeg/FFmpeg/commit/331b3e9dea484f4974bbff2596e6e31c3d5e4685

TIMELINE

Dates from discovery through public reveal.

2026-03-26 Reported to tracker
2026-05-28 Sent to maintainer
2026-06-09 Patch released
2026-08-17 Publicly revealed

PROVENANCE

SHA-3-512 hash:

d06d3e204d9da2ea0b35d2a28085d39ab5c1a5311322511d8e09757c756d08227d8136fc82f621634e5fbf6f397529473de34f6664937620858241dca80dd6af

Committed 2026-05-28 08:01 PT

Revealed 2026-08-17 16:43 PT

Verify (download preimage.json)

Show preimage JSON

{
  "ant_id": "ANT-2026-NGTJ413W",
  "bug_class": "heap-buffer-overflow",
  "claude_severity": "high",
  "commit_sha": null,
  "created_at": "2026-03-27T01:44:14+00:00",
  "description": "A heap buffer overflow with a WRITE operation was identified in ffmpeg's ON2AVC (On2 Audio for Video Codec) decoder. The overflow is triggered by a fuzzer-generated malformed ON2AVC audio stream supplied to the decoder entrypoint. An attacker able to deliver crafted ON2AVC audio to an application using ffmpeg for decoding could trigger this out-of-bounds heap write.",
  "discovered_at": null,
  "location": null,
  "poc_sha256": null,
  "preimage_version": 1,
  "project": "ffmpeg",
  "reproduction": null,
  "technical_details": null,
  "title": "This is a genuine heap-buffer-overflow vulnerability in ffmpeg's ON2AVC audio decoder.",
  "vendor_severity": "high"
}