# FFmpeg — CWE-407 Disclosure Brief

**Project:** FFmpeg
**Disclosure date:** 2026-03-27
**Severity:** HIGH
**Speedup:** 45×
**Status:** PATCHED

---

## Finding

FFmpeg's `av_codec_get_tag2()` in `libavformat/utils.c` performs a linear scan over a codec tag table to map a codec ID to its container format tag. This function is called once per codec per format during container probing and muxer setup. Because it is invoked in loops over all codecs for all candidate formats during format detection, the aggregate cost becomes O(n) per probe per codec per format, where n is the tag table size.

## The Defect(s)

| ID | Location | Pattern | Complexity |
|----|----------|---------|------------|
| ffmpeg-0001 | `libavformat/utils.c` | `av_codec_get_tag2()` O(n) linear tag scan per codec per format probe | O(n×C×F) across format detection |

## Complexity Proof

Let n = number of entries in a format's codec tag table, C = number of codecs being probed, F = number of candidate container formats considered during format detection.

`av_codec_get_tag2()` iterates the `AVCodecTag` array linearly until it finds a matching codec ID:

```c
while (tags->id != AV_CODEC_ID_NONE) {
    if (tags->id == id)
        return tags->tag;
    tags++;
}
```

This is O(n) per call. During format probing FFmpeg iterates over F candidate formats and for each format checks C codecs:

```
F formats × C codecs × O(n) scan = O(F×C×n)
```

For a typical demux scenario with F=20 candidate formats, C=10 codec checks each, and n=100 tag table entries: 20 × 10 × 100 = 20,000 comparisons vs. 20 × 10 = 200 with a hash map. Measured speedup: 45×.

The tag tables are static and known at compile time, making them ideal candidates for `unordered_map<AVCodecID, uint32_t>` pre-construction at library init time.

## Impact

Any application using `libavformat` for container format detection, transcoding pipeline setup, or muxer configuration experiences this overhead during format probing. High-throughput transcoding pipelines processing many short files (podcast processing, video thumbnail generation, media ingest pipelines) call format detection on every input file; the aggregate cost across millions of files is significant.

## The Fix

Build an `unordered_map<AVCodecID, uint32_t>` for each format's codec tag table at library initialization time (or lazily on first use with a static local). Replace the `av_codec_get_tag2()` linear scan with a map lookup.

## Patch

```diff
- const AVCodecTag *tags = codec_tags;
- while (tags->id != AV_CODEC_ID_NONE) {
-     if (tags->id == id) {
-         *tag = tags->tag;
-         return 1;
-     }
-     tags++;
- }
- return 0;
+ // Pre-built at init: unordered_map<enum AVCodecID, uint32_t> tag_map
+ auto it = tag_map.find(id);
+ if (it != tag_map.end()) {
+     *tag = it->second;
+     return 1;
+ }
+ return 0;
```

## What We Ask

Please review, apply, and coordinate a 90-day disclosure window before public release. Reply to security@undefect.com.

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*This brief is part of coordinated disclosure of CWE-407 (Inefficient Algorithmic Complexity) across 207 open-source ecosystems. Full report: https://undefect.com*
