CVE-2026-125103: FFmpeg through 9.0.2 Uninitialized Heap Disclosure via MLZ Decompressor in ALS Decoder

Q: Who is exposed to this issue?

Applications that use affected FFmpeg versions to decode MPEG-4 ALS audio are exposed when they process attacker-controlled ALS files. Exploitation requires local access and user interaction, such as a user opening or importing a crafted file.

Q: What does an attacker need to provide?

The attacker needs a crafted MPEG-4 ALS file containing malformed initial or out-of-range MLZ string codes. Those codes can cause the decoder to copy uninitialized heap memory into the decoded audio output.

Q: What is the impact of successful exploitation?

Successful exploitation can disclose uninitialized heap bytes through decoded audio output. The provided severity vector indicates confidentiality impact only; no integrity or availability impact is identified.

Q: How can I determine whether my deployment is affected?

Check whether your application uses FFmpeg through version 9.0.2 and decodes MPEG-4 ALS content. The vulnerable function is ff_mlz_decompression() in libavcodec/mlz.c.

Published Nov 9, 2026

Updated

FFmpeg through 9.0.2 contains an information disclosure vulnerability in ffmlzdecompression() in libavcodec/mlz.c, which accepts string codes beyond the dictionary's next free code. Attackers can supply a crafted MPEG-4 ALS file with malformed initial or out-of-range codes to copy uninitialized heap bytes into decoded audio output.

Affected Software

1 affected component

FFmpeg FFmpeg<=9.0.2

Event History

Nov 9, 2026

CVE Published

via MITRE·07:10 PM

Data Sourced

via MITRE·07:10 PM

DescriptionSeverityWeakness

Data Sourced

via NVD·07:16 PM

DescriptionSeverityWeakness

Frequently Asked Questions

Who is exposed to this issue?

Applications that use affected FFmpeg versions to decode MPEG-4 ALS audio are exposed when they process attacker-controlled ALS files. Exploitation requires local access and user interaction, such as a user opening or importing a crafted file.

What does an attacker need to provide?

The attacker needs a crafted MPEG-4 ALS file containing malformed initial or out-of-range MLZ string codes. Those codes can cause the decoder to copy uninitialized heap memory into the decoded audio output.

What is the impact of successful exploitation?

Successful exploitation can disclose uninitialized heap bytes through decoded audio output. The provided severity vector indicates confidentiality impact only; no integrity or availability impact is identified.

How can I determine whether my deployment is affected?

Check whether your application uses FFmpeg through version 9.0.2 and decodes MPEG-4 ALS content. The vulnerable function is ff_mlz_decompression() in libavcodec/mlz.c.

CVE-2026-125103: FFmpeg through 9.0.2 Uninitialized Heap Disclosure via MLZ Decompressor in ALS Decoder

Affected Software

Event History

Frequently Asked Questions

Who is exposed to this issue?

What does an attacker need to provide?

What is the impact of successful exploitation?

How can I determine whether my deployment is affected?

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Resources

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Frequently Asked Questions

Who is exposed to this issue?

What does an attacker need to provide?

What is the impact of successful exploitation?

How can I determine whether my deployment is affected?