In early 2024, a critical vulnerability was discovered in Mozilla’s popular software—Firefox, Firefox Extended Support Release (ESR), and Thunderbird. The flaw, tracked as CVE-2024-3859, centers on an integer overflow and out-of-bounds (OOB) read when parsing OpenType fonts on 32-bit systems. If attackers trick users into loading a specially crafted font (for example, by visiting a malicious website), they can potentially cause a crash or, in some cases, leak memory—setting the stage for further attacks.

This write-up will simplify what went wrong, show a potential exploit, and offer links for more information. All content here is exclusive for anyone who wants both the technical and practical side of the story.

What Is CVE-2024-3859?

CVE-2024-3859 is an integer overflow vulnerability in how affected Mozilla products handle OpenType font files on 32-bit versions. This bug leads to an out-of-bounds read in memory, which attackers can exploit using a malformed font. The vulnerability exists in:

Thunderbird before 115.10

Mozilla’s advisory is here:
🔗 https://www.mozilla.org/en-US/security/advisories/mfsa2024-17/

Technical Details – What Went Wrong?

When Firefox (or Thunderbird) loads an OpenType font, it parses font tables using offsets and lengths read directly from the file. On 32-bit builds, arithmetic with those values can wrap around (overflow), causing the software to calculate a memory position that goes outside the intended buffer.

Say the code needed to allocate memory to load a font table

uint32_t length = /* read from font file */;
// This allocation may overflow on 32-bit
uint8_t *ptr = (uint8_t *) malloc(length);

memcpy(ptr, fontFileData, length);

// Reading from ptr beyond the real buffer
uint8_t value = ptr[offset]; // here, offset could be intentionally set large, leading to OOB read

If an attacker sets length to a very big value (near xFFFFFFFF), but due to overflow the OS only allocates a small buffer, when code later tries to access data in the "expected" range, it will end up reading past the end of the real memory. On 64-bit, such overflows are nearly impossible due to the much larger address space, but on 32-bit, it’s a serious risk.

A typical attack would look like this

1. Attacker makes a web page that sets a custom font using @font-face and provides a crafted malicious .otf file.
2. Victim visits attacker’s website with a vulnerable 32-bit Firefox/Thunderbird.

Browser parses the font, triggering the integer overflow and OOB read.

4. This could crash the browser (DoS), or allow the attacker to read bits of memory, possibly exposing sensitive info (like cookies, session tokens, or even steps toward remote code execution).

A simplified HTML PoC might look like this

<!DOCTYPE html>
<html>
<head>
  <style>
    @font-face {
      font-family: evilfont;
      src: url("evilfont.otf");
    }
    body { font-family: evilfont; }
  </style>
</head>
<body>
  <h1>Triggering CVE-2024-3859!</h1>
</body>
</html>

With a specially crafted evilfont.otf designed to exploit the bug. (Creating this OTF is a skill on its own—tools like ttx or otfexplorer can help in fuzzing or crafting.)

Denial of Service: Crashing the browser or mail client.

- Infoleak: If attackers can consistently read memory, they may harvest password fragments, keys, or other secrets.
- RCE Chain: On its own, this bug probably won’t let attackers run code (no write), but it can form a stepping stone combined with other bugs.

Mitigation and Fixes

- Update immediately! Firefox and Thunderbird released patches in version 125 and 115.10 ESR, respectively.
- 📝 Official Release Notes & Security Advisories

References

- Mozilla Security Advisory 2024-17
- https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2024-3859
- OpenType Font (OTF) spec
- Fuzzing OTF for vulnerabilities: fonttools/ttx

In Summary

CVE-2024-3859 shows how file parsing bugs—especially in complex formats like fonts—can translate into very real security risks on older systems. If you’re on 32-bit, patch right away or risk being blindsided by a malicious font loaded over the web or email.

Share this with your IT team, especially if you have old equipment in your environment. Stay safe!

Timeline

Published on: 04/16/2024 16:15:08 UTC
Last modified on: 07/03/2024 02:06:47 UTC