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The CVE-2026-74936 vulnerability resides in the interaction between the JavaScript engine (SpiderMonkey) and the WebAssembly (Wasm) runtime within Mozilla Firefox and Thunderbird. WebAssembly is a binary instruction format designed for high-performance execution in the browser, and its integration with JavaScript allows seamless function calls and memory sharing. The core issue is a use-after-free (UAF) condition that occurs when the garbage collector improperly handles the lifecycle of a WebAssembly instance object while a JavaScript reference to its exported function or memory still exists. Specifically, when a WebAssembly module is instantiated, the engine allocates a `WasmInstanceObject` which holds pointers to the compiled code, linear memory, and tables. If the developer creates a closure or a promise that captures this instance and then forces a garbage collection via `gc()` or by allocating pressure, the engine may free the instance’s underlying resources because the reference count becomes temporarily zero. However, due to a race condition in the concurrent marking phase, a stale pointer to the freed memory can remain on the JavaScript stack or in a register. When the JavaScript code subsequently invokes the exported function or accesses the memory buffer, the engine dereferences this stale pointer, leading to a classic use-after-free. An attacker can craft a malicious WebAssembly module that exports a function which, upon invocation, triggers the freeing of its own instance through nested callbacks and explicit memory pressure. The freed memory region can then be reallocated with attacker-controlled data, such as a fake object vtable or a crafted array buffer, enabling type confusion. Once the stale pointer is reused, the engine treats the reallocated memory as a legitimate WebAssembly instance, allowing the attacker to hijack the control flow by overwriting function pointers or manipulating the linear memory bounds. This vulnerability is particularly dangerous because WebAssembly runs in a privileged context relative to the JavaScript sandbox, and successful exploitation can lead to arbitrary code execution at the user’s privilege level. The vulnerability affects all versions prior to Firefox 154, Firefox ESR 140.14, and Firefox ESR 153.1, as well as the corresponding Thunderbird releases. The fix involved adding robust lifetime checks and reference counting barriers during garbage collection phases to ensure that any active JavaScript reference prevents the underlying instance from being prematurely finalized.
DailyCVE Form:
Platform: Mozilla Firefox Thunderbird
Version: Before 154 ESR
Vulnerability : Use-after-free WebAssembly
Severity: Critical
date: August 18 2026
Prediction: Fixed in 154
What Undercode Say:
Check your Firefox version
firefox --version
For ESR users
firefox-esr --version
Detect vulnerable versions (returns true if vulnerable)
if [[ $(firefox --version | grep -E "1[0-9][0-9]." | cut -d' ' -f3 | cut -d'.' -f1) -lt 154 ]]; then
echo "VULNERABLE to CVE-2026-74936"
fi
Simulate the GC pressure trigger (Node.js style)
node -e "global.gc(); console.log('GC triggered');"
Log analysis to check for crashes
dmesg | grep -i "firefox.segfault" | tail -20
// Minimal code to reproduce the reference lifecycle bug (conceptual)
const wasmCode = new Uint8Array([0x00, 0x61, 0x73, 0x6d, ...]); // valid module
const module = new WebAssembly.Module(wasmCode);
const instance = new WebAssembly.Instance(module);
const exportFunc = instance.exports.func;
// Force GC while a closure holds the instance indirectly
setTimeout(() => { globalThis.gc(); }, 0);
// After GC, calling exportFunc may crash due to UAF
exportFunc();
Exploit: (Educational Purposes!)
A practical exploit leverages the UAF to overwrite the freed WasmInstanceObject‘s `memory` pointer. By allocating a large `ArrayBuffer` immediately after triggering the free, the attacker can control the content of the reused memory. The overwritten vtable pointer points to a shellcode stub, and invoking an exported function triggers the redirect. Below is a high-level JavaScript/Wasm snippet demonstrating the trigger sequence, not a full weaponized exploit.
// Step 1: Create a Wasm module with an exported function that allocates and deallocates
const code = new WebAssembly.Module(wasmBytes);
const inst = new WebAssembly.Instance(code, {});
const leak = inst.exports.leak; // function that creates a closure
// Step 2: Invoke the leak and immediately request GC
leak();
if (typeof gc !== 'undefined') gc();
// Step 3: Reallocate the freed memory with a crafted buffer
const fake = new Uint8Array(new ArrayBuffer(0x1000));
// Step 4: Call the exported function again to use the stale pointer
inst.exports.trigger(); // leads to control-flow hijack
Protection: from this CVE
- Immediately update to Firefox 154, Firefox ESR 140.14, or Firefox ESR 153.1, and the equivalent Thunderbird versions.
- Enable strict site isolation and disable WebAssembly via `about:config` (set `javascript.options.wasm` to
false) if not required, though this may break many web apps. - Apply operating system-level sandboxing (e.g., Firejail, AppArmor) to limit the impact of a potential exploit.
- Regularly clear browser caches and avoid untrusted WebAssembly modules from unknown origins.
Impact:
Successful exploitation allows an unauthenticated remote attacker to execute arbitrary code with the privileges of the current user. This can lead to complete browser takeover, data exfiltration, installation of malware, and potentially sandbox escape on affected systems. Since WebAssembly is widely used in gaming, media processing, and cloud applications, the attack surface is broad, making this a critical remote-code-execution vulnerability with high confidentiality, integrity, and availability impact.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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