A subtle but dangerous memory flaw quietly shipped in Firefox for six months — affecting more than 180 million users — before security researchers uncovered it.
The vulnerability allowed attackers to corrupt memory and potentially execute arbitrary code through malformed WebAssembly payloads.
“Aisle’s autonomous AI system uncovered this subtle boundary-condition vulnerability during our WebAssembly security deep dive, revealing meaningful memory-safety risks for roughly 180 million Firefox users,” Stanislav Fort, founder and chief scientist at AISLE, said in a blog post.
He added, “Mozilla moved quickly to deploy a fix. Modern browsers are some of the most secure and rigorously engineered platforms in existence, and this finding highlights the importance of continuous, AI-driven security research to keep them safe for users worldwide.”
The hidden code error that exposed Firefox users
At the core of the vulnerability (CVE-2025-13016) is a subtle pointer arithmetic mistake in Firefox’s WebAssembly garbage-collection (GC) implementation, specifically within the StableWasmArrayObjectElements class, where mismatched pointer types caused incorrect copying of inline array data.
The vulnerable code used byte-addressed pointers (uint8_t*) to determine how much data to copy, but copied the data into a buffer typed as uint16_t. When the template was instantiated for 16-bit values, std::copy() interpreted the byte-based range as a count of typed elements rather than bytes.
As a result, a buffer intended to hold N 16-bit elements received 2N elements instead, overrunning stack memory and corrupting adjacent data structures. The issue was made worse by a second flaw: the copy operation didn’t read from the correct memory location.
Instead of using the dedicated pointer for the array’s data region, the code pulled from inlineStorage(), a location that begins with internal object metadata.
That means the first bytes copied into the buffer weren’t array contents at all — they were structural information about the WebAssembly object itself. This introduces additional unpredictability and increases the chance that corrupted memory can be weaponized during exploitation.
The conditions attackers need to exploit this Firefox bug
Not every execution path in Firefox touches the flawed routine, which is why the vulnerability remained undetected for so long.
The issue occurs only when Firefox falls back to a slower, GC-enabled path for handling WebAssembly arrays — specifically during the conversion of those arrays into strings.
In a typical sequence, WebAssembly code first manipulates an array, such as a char16_t array. Firefox then attempts to convert that array into a string using a fast-path operation designed to avoid garbage collection. However, when certain conditions — most commonly memory pressure — cause that fast path to fail, the browser shifts into a GC-permitted fallback routine.
It is within this fallback that Firefox invokes the vulnerable StableWasmArrayObjectElements constructor, which executes the flawed copy operation and ultimately overflows the stack, corrupting adjacent memory.
In practical attack scenarios, an adversary could deliberately craft a malicious WebAssembly module to manipulate this sequence in their favor.
By creating arrays of specific sizes, intentionally driving the browser into memory pressure to force garbage collection, and repeatedly triggering the array-to-string conversion process, an attacker could reliably push Firefox into the vulnerable fallback path.
This creates a controlled environment in which the resulting memory corruption can be directed toward a chosen target on the stack.
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Mitigation strategies for the Firefox vulnerability
Organizations can reduce their exposure to the vulnerability by applying the latest Firefox patches and implementing additional defense-in-depth measures to limit attacker access, contain potential exploitation, and harden browser security.
- Prioritize deploying Firefox 145 or later (or ESR 140.5+) across all systems and verify version compliance organization-wide.
- Enforce enterprise browser management policies to limit high-risk features, tighten sandboxing controls, and lock down critical security configurations.
- Disable WebAssembly temporarily in environments where patching cannot occur immediately, especially on high-exposure endpoints.
- Monitor browser logs, EDR signals, and crash analytics for WebAssembly-related memory errors or unusual Firefox process behavior.
- Use network-level defenses — such as DNS filtering, secure web gateways, and domain reputation tools — to block malicious or suspicious web content.
- Deploy browser isolation or segment high-risk browsing activities to contain threats from users who regularly access untrusted sites.
- Harden endpoint and operating system defenses by enforcing exploit mitigation settings, application sandboxing, and strict least-privilege access controls.
Together, these measures help strengthen overall cyber resilience.
Editor’s note: This article first appeared on our sister publication, eSecurityPlanet.com.