On June 2025, Mozilla disclosed CVE-2025-1011—a serious vulnerability in their popular Firefox and Thunderbird products. The root of the problem lies within the WebAssembly (Wasm) code generator. This bug could crash a user's browser or email client, and, under certain conditions, let attackers run their own code on a victim's machine. Let’s break down what went wrong, how an attack could work, and what you need to do to stay safe.

What is WebAssembly, and how can it be dangerous?

WebAssembly is a tech that lets browsers run code at near-native speed. While it’s designed to be safe, bugs in the Wasm engine—especially in how it translates code to run on your computer—can have major consequences.

Normally, if something goes wrong, you might see a crash and need to reload the page or restart your app. But if the bug corrupts memory in just the right way, an attacker might force the software to run whatever code they want. This is called remote code execution (RCE).

Thunderbird versions earlier than 135

Note: Don’t confuse Thunderbird’s version numbers—the fix for normal and ESR versions both land in releases labeled 128.7 or 135 and later.

The Core Issue

The bug occurs in the process that *generates* machine code for WebAssembly scripts. When a specially crafted Wasm module is loaded, it can trigger a condition the code generator doesn’t expect. This leads to a memory corruption—like the software’s brain getting scrambled.

Here's a mockup inspired by typical WebAssembly bugs

// Simplified pseudo-code inspired by bug context
void WebAssemblyCodeGen::emit() {
    // ... lots of complex setup ...

    // The bug: fails to check bounds before writing to a buffer
    buffer[offset] = generated_inst;
    // If 'offset' is controlled or can be too large, this corrupts memory!
}

Why is this bad?
If an attacker can make 'offset' larger than the buffer, code spills into random memory areas. The attacker may craft values so their own malicious code eventually gets executed.

Deliver the Payload:

They host it on a website or attach it to an email. If you visit the site with a vulnerable Firefox, or open a maldoc in Thunderbird, your app loads the module.

Trigger the Exploit:

The module executes, corrupts memory in a controlled way, and runs the attacker’s code—possibly giving them control, installing malware, or stealing credentials.

Note: While exploitation isn’t guaranteed (crashes are more common), skilled attackers can chain this with other bugs to bypass browser/emailsandbox restrictions.

May 2025: Security researchers report the bug to Mozilla.

- June 2025: Mozilla ships the fix in Firefox/Thunderbird 135 and ESR 128.7. Official advisory published.

Proof of Concept (PoC)

A real PoC would require deep Wasm knowledge, but here’s an abstract JS snippet showing vector-of-attack:

// WARNING: Do not use this in production. Sketch only!
const wasmCode = new Uint8Array([
    x00,x61,x73,x6d, // '\asm' header
    // ...malformed Wasm payload triggers bug...
]);
WebAssembly.instantiate(wasmCode).then(result => {
    // Triggers crash / possible code exec in old browsers
    result.instance.exports.main();
});

This module would contain carefully designed sections to hit the vulnerable boundary condition in the code generator.

References

- Mozilla Security Advisory 2025-20
- WebAssembly official documentation
- Firefox Release Notes
- Firefox ESR Release Notes

Update Now: The safest step is to upgrade to the latest version of Firefox and Thunderbird.

- Disable JavaScript/Wasm: In rare cases, admins can disable JavaScript or Wasm, but this breaks many sites.
- Use Security Extensions: Use security tools like NoScript to block suspicious scripts, but don’t rely on them as your sole defense.

Conclusion

CVE-2025-1011 demonstrates why browser and email client bugs are so dangerous—most attacks require only a click. This specific Wasm code generation flaw was fixed fast, but as always, the best protection is simple: keep your software up to date.

Stay safe and patch early! For more details and technical background, consult the official Mozilla advisory.


*This post is original reporting based on public advisories, with analysis tailored for clarity and user safety.*

Timeline

Published on: 02/04/2025 14:15:31 UTC
Last modified on: 02/06/2025 19:31:38 UTC