CVE-2026-117877 in Firefox
Summary
by MITRE • 10/27/2026
Use-after-free in the Layout: Text and Fonts component. This vulnerability was fixed in Firefox 159 and Firefox ESR 153.6.
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Analysis
by VulDB Data Team • 10/27/2026
The identified vulnerability represents a critical memory management error classified as an out-of-bounds write, specifically manifesting through a use-after-free condition within the Layout: Text and Fonts component of the Mozilla Gecko rendering engine. This type of flaw occurs when the application attempts to access or modify memory that has already been deallocated by the system's garbage collector or explicit free operations. In the context of web browsers, such vulnerabilities are particularly dangerous because they allow an attacker who controls a malicious webpage to execute arbitrary code with the privileges of the victim user. The specific component involved handles the complex calculations and rendering logic required for displaying text, fonts, and layout structures on a web page. When these elements undergo dynamic changes during page load or interaction, such as through JavaScript manipulation or CSS animations, the browser must frequently allocate and deallocate memory buffers to manage font metrics and glyph positioning data.
The technical root cause lies in the lifecycle management of internal objects representing text runs or font features. If a reference to an object is retained after its underlying memory has been freed due to a race condition or improper pointer invalidation, subsequent operations targeting that stale reference will write to arbitrary locations in the heap. This out-of-bounds write can corrupt adjacent data structures, overwrite function pointers, or alter control flow mechanisms within the browser process. Such corruption enables an attacker to achieve remote code execution by carefully crafting exploit payloads that leverage the memory layout of the Firefox binary. The severity is heightened because modern browsers employ various mitigation techniques like ASLR and DEP, but sophisticated use-after-free exploits can often bypass these protections through information disclosure or heap spraying techniques to gain precise control over program execution flow.
From an operational perspective, this vulnerability poses a severe risk to users browsing untrusted content. An attacker could host a specially crafted web page that triggers the vulnerable code path when loaded in Firefox versions prior to 159 and ESR 153.6. Upon successful exploitation, the attacker gains full control over the browser process, which can lead to further system compromise depending on user privileges. This includes stealing sensitive data such as cookies, saved passwords, and session tokens stored by the browser. It also allows for keylogging, screen capture, or pivoting into internal network resources if the victim is connected to a corporate environment. The impact extends beyond individual users to organizations relying on Firefox ESR versions for standardized deployment, where unpatched systems remain vulnerable until updates are applied through enterprise distribution channels.
The vulnerability aligns with CWE-416, Use After Free, which describes situations where pointers continue to be used after they have been freed, leading to unpredictable behavior and potential security breaches. In terms of the MITRE ATT&CK framework, this flaw facilitates initial access via Tactic TA0001, specifically through techniques associated with Exploit Public-Facing Application (T1190) or potentially Command and Scripting Interpreter if combined with other vectors for post-exploitation activities like Data from Local System (T1005). The exploitation chain typically involves crafting a malicious HTML document that triggers the specific layout engine bug, followed by heap grooming to position sensitive data in predictable memory locations, allowing the out-of-bounds write to overwrite critical structures.
Mitigation strategies primarily rely on timely software updates. Users and administrators must ensure that Firefox is updated to version 159 or later for standard releases, or Firefox ESR 153.6 or later for extended support release channels. For organizations managing large fleets of endpoints, automated patch management systems should be configured to prioritize these security updates due to the high severity and active exploitation potential associated with memory corruption vulnerabilities in rendering engines. Additionally, deploying browser hardening policies such as disabling JavaScript execution on untrusted sites can reduce the attack surface, although this may impact usability for legitimate web applications. Security teams should also monitor threat intelligence feeds for indicators of compromise related to exploit kits targeting Gecko-based browsers and implement network-level controls like sandboxing or application whitelisting to limit the potential damage if a vulnerability is successfully exploited before patches are applied.