A newly disclosed vulnerability, CVE-2025-0518, affects the open-source media framework FFmpeg (version 7.1). This security issue allows attackers to read sensitive constants within an executable by exploiting an unchecked return value and an out-of-bounds read in the af_pan.c source file of FFmpeg’s libavfilter.

The vulnerability was discovered by Simcha Kosman and has since been patched by the FFmpeg team. The issue and its solution are publicly documented in FFmpeg's GitHub repository.

What is FFmpeg?

FFmpeg is a popular command-line tool and library suite for manipulating multimedia files and streams. It’s commonly bundled or embedded in various media apps, players, and cloud solutions.

Where does it occur?

The vulnerability occurs in the af_pan.c file, part of FFmpeg's audio filter subsystem. This file is responsible for the "pan" audio filter, which allows remapping input channels to output channels.

What’s the problem?

CVE-2025-0518 stems from an unchecked return value and out-of-bounds read bug. A function that parses pan filter parameters fails to check if it succeeded, then uses output data as if it was valid, leading to reading data outside the allocated memory bounds.

This can allow a crafted media file or filter string to trick FFmpeg into reading and leaking memory contents—potentially exposing sensitive information such as cryptographic constants, function pointers, or other private data used or stored by the FFmpeg process.

Affected Versions

- FFmpeg 7.1 (Release Information)

Let’s look at the problematic code in af_pan.c (before the fix)

int ret;
...
ret = parse_expression(...);      // Function to parse user-supplied pan filter input
// No check for ret here!
float x = output[ret];            // ret might be out-of-bounds
process(x);                       // Uses possibly invalid data

In summary, parse_expression() can fail and return an error value or an unexpected index, but the result is immediately used to access the output array, possibly reading outside the output buffer.

The official fix adds a proper check to ensure the return value is valid before proceeding

int ret;
...
ret = parse_expression(...);
if (ret < ) {
    av_log(..., AV_LOG_ERROR, "parse_expression failed.\n");
    return ret;                  // Abort if there was a parse error
}
float x = output[ret];           // Only used if ret is good
process(x);

You can see the official patch here:
https://github.com/FFmpeg/FFmpeg/commit/b5b6391d64807578ab872dc58fb8aa621dcfc38a

Threat Model

This vulnerability is exploitable remotely anywhere an attacker can control pan filter parameters or supply crafted media files. That includes many media backends, automation tools, and possibly even cloud video transcoding services.

Exploit Scenario

1. Attacker creates a malicious pan filter string for a media file, which feeds invalid input to FFmpeg’s filter.
2. FFmpeg, without the patch, tries to parse this input, receives an error, but never checks for failure.

The error value is used as an array index, leading to a read from an out-of-bounds location.

4. The resulting data may be output to logs or forwarded to other functions. If this data contains something sensitive (such as keys, memory pointers, configuration values), it can be exfiltrated by the attacker.

Proof-of-Concept (PoC)

Below is a minimal proof-of-concept demonstrating a potential crash and information leak (for illustration):

ffmpeg -filter_complex "[]pan=invalid_filter" -f null -

If unpatched, this may access invalid memory and trigger undefined behavior, possibly leaking stack or heap data as error messages or in generated output.

Information Disclosure: Attackers could read and leak sensitive memory contents.

- Potential Crash: In some cases, the out-of-bounds read might lead to a process crash, causing denial of service.
- Chaining: While this bug is not directly a remote code execution flaw, information leaks are often used to defeat modern security mitigations (like ASLR), which might aid in further, more severe exploits.

Mitigation & Remediation

- Upgrade: The best protection is to upgrade FFmpeg to a version that includes the fix from commit b5b6391.
- Patch: If running an older version, backport the patch to your codebase.
- Restrict Input: Limit or sanitize user input on filter expressions (though this may not be practical in all scenarios).

References

- FFmpeg Security Patches
- Original af_pan.c File
- Fixed Commit
- CVE Entry (Pending)

Credits

This issue was discovered by Simcha Kosman and responsibly disclosed to the FFmpeg team.

Conclusion

CVE-2025-0518 is a serious, but patchable, information disclosure vulnerability in FFmpeg 7.1’s audio filtering system. Software projects embedding or calling FFmpeg should update as soon as possible and audit any places where user input can control filter expressions. This helps to keep your multimedia tooling and users safe from subtle but impactful bugs like this.

Timeline

Published on: 01/16/2025 17:15:12 UTC