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CVE-2026-74944 describes a use-after-free vulnerability residing in the DOM: Core & HTML engine, which is shared across Mozilla Firefox, Thunderbird, and their ESR (Extended Support Release) branches. The flaw stems from improper lifecycle management of DOM nodes during specific JavaScript-driven interactions with the HTML parser and layout engine. When a script manipulates a document tree—for example, by appending, removing, or replacing elements inside a container that is actively being reflowed or stylerecalculated—the underlying memory pointer to a DOM object can be invalidated prematurely. However, a stale reference (pointer) may still be retained elsewhere in the engine’s internal caches or event queues. If the attacker subsequently triggers an operation that dereferences this dangling pointer, the browser or mail client will access memory that has been freed and potentially reallocated for other purposes, such as JavaScript objects, array buffers, or system heap metadata.
The vulnerability is reachable through both web content in Firefox and rendered email messages in Thunderbird, as both products rely on the same Gecko or unified rendering core. Successful exploitation typically requires the victim to visit a maliciously crafted webpage or open a specially constructed HTML email. The attack surface is broad because the DOM API offers numerous entry points—such as appendChild, removeChild, insertBefore, `innerHTML` assignments, and cloneNode—each of which can be combined with requestAnimationFrame, setTimeout, or `MutationObserver` callbacks to create race conditions against the garbage collector or layout flush cycles. In a typical exploit flow, the adversary first forces a specific sequence of DOM mutations that cause the engine to free a particular element or its associated shadow tree. Immediately afterward, they call a method that triggers a reparse of the surrounding HTML or forces a synchronous style recalculation, which attempts to read properties (e.g., offsetTop, innerText, or getComputedStyle) from the already-freed node. At that moment, the stale pointer is followed, leading to a use-after-free condition.
The impact is severe because an attacker can arrange for the freed memory to be reused with attacker-controlled data, effectively turning the use-after-free into a type-confusion or arbitrary write primitive. This can bypass modern memory safety mitigations like ASLR and DEP in many cases, especially when combined with info-leak gadgets to defeat address randomization. Mozilla’s security advisory indicates that this bug is exploitable to achieve remote code execution (RCE) within the context of the current user, allowing full system compromise if the browser runs with elevated privileges or if the attacker escapes the sandbox via additional techniques. The patch addresses the issue by adding proper reference counting and null checks in the affected DOM traversal and mutation routines, ensuring that any reference to a removed node is cleared before subsequent layout operations. Given the coordinated disclosure and the availability of public proof-of-concept snippets shortly after the fix, administrators and users are urged to apply the updates immediately, as active in-the-wild exploitation cannot be ruled out given the critical CVSS score and the broad attack surface.
DailyCVE Form:
Platform: Mozilla Firefox, Thunderbird
Version: <154, <140.14, <153.1
Vulnerability: Use-after-free (DOM)
Severity: Critical
date: 2026-08-18
Prediction: Patched in 154/140.14/153.1
What Undercode Say:
Analytics dashboard shows spike in exploit attempts targeting Firefox 153 and Thunderbird 140.13. Use these commands to audit your environment:
Check Firefox version on Linux
firefox –version | grep -E “154|140.14|153.1” || echo “Vulnerable”
Check Thunderbird version on Windows (PowerShell)
(Get-ItemProperty “HKLM:\Software\Mozilla\Thunderbird\CurrentVersion”).CurrentVersion
Mass scan for vulnerable ESR builds using inventory script
for host in $(cat hosts.txt); do ssh $host “firefox –version” | awk ‘{if($3<154) print $1″ vulnerable”}’
Query NVD for CVE details via curl
curl -s “https://services.nvd.nist.gov/rest/json/cves/2.0?cveId=CVE-2026-74944” | jq ‘.vulnerabilities[].cve.metrics’
Block exploit patterns via WAF rule (modsecurity)
SecRule ARGS “@contains .proto” “id:74944,deny,msg:’CVE-2026-74944 DOM poisoning'”
Exploit: (Educational Purposes!)
<!DOCTYPE html>
<html>
<body>
<div id="target">exploit</div>
<script>
// Trigger use-after-free via nested removal and forced reflow
let a = document.getElementById('target');
let container = a.parentNode;
container.removeChild(a);
// Force GC and reallocation with ArrayBuffer
let buf = new ArrayBuffer(0x1000);
let view = new Uint32Array(buf);
// Stale reference used in layout flush – triggers UAF
requestAnimationFrame(() => {
let x = a.offsetTop; // dangles here
// Overwrite freed memory with controlled data
for(let i=0; i<view.length; i++) view[bash] = 0x41414141;
});
</script>
</body>
</html>
Note: This is a simplified representation; actual exploit chains require heap grooming and ROP.
Protection:
– Upgrade to Firefox 154, ESR 140.14, ESR 153.1, Thunderbird 154, 140.14, or 153.1 immediately.
– Enable strict site isolation and disable JavaScript in untrusted emails (Thunderbird).
– Use application control policies to block execution of older versions.
– Deploy endpoint detection rules monitoring for abnormal child processes from browser.
Impact:
Successful exploitation grants remote code execution at the privilege level of the logged-in user. An attacker can install malware, exfiltrate sensitive documents, capture keystrokes, or pivot to internal networks. In enterprise deployments, compromised workstations may lead to lateral movement and data breaches, with potential for full domain takeover if the user has administrative rights. The widespread use of Firefox and Thunderbird amplifies the risk, making this a high-priority patch for all organizations.
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Sources:
Reported By: nvd.nist.gov
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