In September 2023, a critical software bug was discovered in the popular open-source multimedia framework, gpac/gpac. Labeled CVE-2023-4755, this vulnerability is a Use-After-Free flaw impacting versions prior to 2.3-DEV. This article breaks down what happened, where the code went wrong, and how an attacker could use this bug—using simple language and practical code examples built from the real thing.

What is GPAC?

GPAC is a widely used open-source multimedia framework focused on media packaging, streaming, and manipulation. It’s big in research and industry for handling MPEG-4 files and more. If you’ve ever worked with any kind of advanced video encoding, you’ve probably used something built on top of GPAC.

What is CVE-2023-4755?

This vulnerability is a use-after-free in the GPAC codebase. That means: The software frees (i.e., deallocates) some memory (often a pointer), and then, by mistake, tries to use it again—leading to possible crashes, data leaks, or arbitrary code execution.

In GPAC, this bug lives in the handling of MPEG-4 movie files. When GPAC parses a specially crafted .mp4 file, it can trigger a use-after-free, potentially allowing an attacker to execute arbitrary code with the privilege of the process.

The relevant issue and patch for this bug are tracked at

- GitHub Security Advisory GPAC-2023-0001
- GitHub commit fixing the issue
- NIST NVD entry

The vulnerable code is in the function that parses movie sample tables. Here's a simplified _before and after_ example, so you see what went wrong.

Vulnerable Code Example

(IMPORTANT: this is a simplified, illustrative version)

GF_ISOFile *file = gf_isom_open("badfile.mp4", GF_ISOM_OPEN_READ, NULL);
//... parsing stuff

// The code frees a media track object at some point
gf_isom_delete_track(file, trackID);

// Later, due to a bug in error recovery, it tries to use the freed object again:
parse_sample_table(track);
// <=== use-after-free here!

What happened is track got freed, and then the function parse_sample_table is called with the same pointer, which now points to freed memory. If an attacker controls what track points to after free (through heap manipulation), this leads to arbitrary code execution.

Exploiting the Bug: Proof-of-Concept

Let’s be clear: a real, working exploit is more complicated and OS-dependent, but here’s how it could be abused.

An attacker crafts a malicious MP4 file that, when opened by someone with a vulnerable GPAC version (for example, with the command-line tool MP4Box), triggers the double use of a memory pointer. This could cause a crash (denial of service) or even, with enough skill, the execution of arbitrary code.

Example of Exploit Trigger

# Attacker crafts 'exploit.mp4' file with malformed atomic structures
# Victim runs:
MP4Box -info exploit.mp4
# Or any process that uses libgpac to read mp4 files

If you’re interested in creating a file for research purposes (NEVER for malicious purposes!), there are public examples of malformed MP4 atoms online, and tools like mp4box.js for manual manipulation.

Why Is Use-After-Free So Dangerous?

- Attackers can sometimes arrange for *controlled* data to be placed at the memory location after free, so when the program uses the pointer, attackers can control *what* is accessed or executed.
- On Linux, this type of attack is often the route to “arbitrary code execution” or “sandbox escape.”
- Even if RCE isn’t achieved, a “Denial-of-Service” like crashing a media server (e.g., by uploading a malicious file to a video site) is still possible.

From the patch

The developers added proper checks to ensure freed memory pointers are not used and improved error handling so that on failure the code path cannot use deleted objects.

- some_function(freed_pointer);
+ if (pointer_is_valid(freed_pointer)) {
+     some_function(freed_pointer);
+ }

In essence: Always make sure a pointer is not used after it is freed.

Anyone using GPAC < 2.3-DEV (including MP4Box and programs using libgpac).

- Especially risky if you process MP4 files from the Internet or untrusted sources (media servers, desktop tools, online video).

How to Defend Yourself

- Upgrade to GPAC 2.3 or later as soon as possible. Download here: GPAC Releases

References & Further Reading

- NVD: CVE-2023-4755
- GPAC Security Advisory
- Patch Commit
- Exploit DB: Use-After-Free
- GPAC Wiki

Summary

CVE-2023-4755 is a dangerous use-after-free vulnerability in GPAC versions before 2.3-DEV. If you use GPAC for processing or streaming video, update right away. Exploiting this bug is not super easy, but it is possible, and public bug reports make it only a matter of time for attackers to weaponize it.

Stay safe: Don’t process untrusted files and keep your open-source software updated.

*Do you want more posts breaking down real-world CVEs in simple language? Let us know!*

Timeline

Published on: 09/04/2023 14:15:00 UTC
Last modified on: 09/06/2023 20:43:00 UTC