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The vulnerability identified as CVE-2026-74961 resides within the Web Audio component of Mozilla Firefox, Firefox ESR, and Thunderbird. This flaw is classified as a side-channel attack vector, specifically leveraging the high-precision timing and audio processing capabilities exposed through the Web Audio API. The core issue stems from the way the browser’s audio engine handles concurrent audio contexts, audio buffer processing, and the underlying system’s audio hardware interaction.
At its foundation, the Web Audio API provides developers with a powerful interface for manipulating audio streams, including real-time analysis, synthesis, and effects processing. This involves periodic callbacks (e.g., ScriptProcessorNode or AudioWorklet) that execute at the audio sample rate, typically 44.1 kHz or 48 kHz, which translates to a callback interval of approximately 2.3 milliseconds per sample block. Attackers can exploit this timing granularity to create a high-resolution timing side-channel that bypasses traditional cross-origin restrictions.
The attack works by observing measurable execution time differences in audio processing routines when performing specific operations on audio data arrays (e.g., FFT analysis, convolution, or gain adjustments) while simultaneously triggering memory access patterns in other processes or threads. Because the audio thread runs with elevated real-time priority, it can act as a “probe” for cache-coherency states or CPU pipeline stalls induced by memory activity in the victim context. This allows a malicious webpage to infer secret data, such as cryptographic keys or cross-origin DOM values, by correlating audio processing latency with the CPU’s cache-hit/miss patterns.
Specifically, an attacker can allocate multiple AudioContexts and AudioBuffers, then precisely measure the time taken to process a buffer using `AnalyserNode` or AudioWorkletProcessor. By varying the audio data length and observing the processing times under different system loads, the attacker can construct a timing profile that reveals microarchitectural state. This is reminiscent of previous Spectre-variant attacks but applied through the browser’s audio stack. The flaw exists because the Web Audio engine does not sufficiently isolate the timing side-effects of its internal memory allocations and JIT-compiled audio processing code, allowing a website to measure differences down to nanoseconds despite the nominal timing resolution limits imposed by performance.now().
Mozilla addressed this in Firefox 154, Firefox ESR 153.1, Thunderbird 154, and Thunderbird 153.1 by introducing random jitter into audio processing callbacks, reducing the precision of observable timing, and isolating audio worklet execution in separate, sandboxed threads with memory barriers. However, prior to these versions, the side-channel remains exploitable, making it critical for enterprise environments and high-security users to update immediately.
DailyCVE Form:
Platform: Firefox, Thunderbird
Version: <154, <153.1
Vulnerability: Web Audio Side-Channel
Severity: High
date: 2026-08-18
Prediction: Already Patched (v154)
What Undercode Say:
Analytics: The following commands and scripts can be used to verify installed versions and monitor Web Audio usage patterns.
Check Firefox version on Linux firefox --version | grep -oP 'Firefox \K[0-9.]+' Check Thunderbird version on Windows (PowerShell) (Get-Item "C:\Program Files\Mozilla Thunderbird\thunderbird.exe").VersionInfo.FileVersion Monitor audio processing events via system logs (Linux) sudo strace -e trace=ioctl -p $(pgrep -f "firefox") 2>&1 | grep -i audio Extract Web Audio API calls from browser console (JavaScript snippet) console.log(performance.now(), audioContext.currentTime);
// Node.js script to simulate WebAudio timing probes (for analytics)
const { performance } = require('perf_hooks');
const start = performance.now();
// Simulate audio buffer processing loop
for (let i = 0; i < 1000; i++) { / dummy / }
const end = performance.now();
console.log(<code>Processing window: ${end - start} ms</code>);
Exploit: (Educational Purposes!)
A proof-of-concept exploits this by creating two AudioContexts: one for generating noise and another for measuring processing latency. The attacker fills a buffer with varying data patterns, then measures the `AnalyserNode.getByteTimeDomainData()` execution time while repeatedly triggering cache flushes via SharedArrayBuffer. By correlating time deviations with known cache-line accesses in a victim process (e.g., a cross-origin iframe), the attacker can reconstruct secret bits through a sliding window analysis. The core loop is:
const ctx = new AudioContext({ sampleRate: 48000 });
const analyser = ctx.createAnalyser();
const data = new Uint8Array(analyser.fftSize);
function probe() {
const t0 = performance.now();
analyser.getByteTimeDomainData(data);
const t1 = performance.now();
return t1 - t0; // this delta exposes microarchitectural state
}
// Repeated calls with interleaved memory operations leak side-channel info.
Protection: from this CVE
Immediate mitigation requires updating to Firefox 154, Firefox ESR 153.1, Thunderbird 154, or Thunderbird 153.1 as these contain the official patches. For systems unable to update immediately, disable the Web Audio API via enterprise policy by setting `dom.webaudio.enabled` to `false` in `about:config` or through Group Policy templates. Additionally, enforce strict site isolation (fission.autostart = true) to contain cross-origin probes and deploy browser extensions that block third-party scripts from accessing AudioContext. Network-level defenses are ineffective; client-side patching is mandatory.
Impact:
Successful exploitation allows an attacker to leak sensitive information across origins, including private user data, session tokens, and memory-resident cryptographic material. The attack is executed entirely within the browser without user interaction, requiring only a malicious webpage or embedded advertisement. On multi-user systems or cloud desktops, the side-channel can also be used to infer activity from other browser tabs or even other applications sharing the same CPU core, breaking the security boundaries of the operating system’s process isolation. The risk is highest for users with high-value accounts, such as banking or enterprise dashboards, where a single leaked token can lead to account takeover.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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