CVE-2026-109723 in Firefox
Summary
by MITRE • 10/13/2026
Integer overflow in the Layout: Text and Fonts component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5.
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Analysis
by VulDB Data Team • 10/13/2026
The identified security issue resides within the Layout, Text, and Fonts subsystem of Mozilla Firefox, specifically involving an integer overflow condition during text rendering operations. Integer overflows occur when a calculation results in a value that exceeds the maximum capacity of the data type used to store it, causing the number to wrap around to a negative or unexpectedly small positive value. In the context of browser layout engines, such as Gecko which powers Firefox, this flaw typically manifests when processing complex font metrics, glyph positioning, or text shaping algorithms where arithmetic operations are performed on dimensions like width, height, or offset values without adequate bounds checking. When an attacker can influence these input parameters through maliciously crafted web content, the resulting miscalculation may lead to incorrect memory allocation sizes or buffer boundary violations during subsequent rendering processes.
From a technical perspective, this vulnerability aligns with CWE-190, which defines integer overflow or wraparound errors. The exploitation vector generally involves crafting HTML and CSS structures that trigger specific code paths within the text layout engine where arithmetic operations on font-related metrics are executed. If the browser fails to validate that intermediate calculation results remain within expected positive bounds before proceeding to memory allocation or array indexing, it can result in a heap-based buffer overflow or use-after-free scenario depending on how the flawed value is utilized downstream by the rendering pipeline. This type of flaw allows for potential out-of-bounds reads or writes, which are critical security risks as they can lead to arbitrary code execution if an attacker successfully leverages memory corruption primitives available within the browser's sandbox environment.
The operational impact of this vulnerability is significant because it affects the core stability and security boundary of the web rendering engine. Successful exploitation could allow a remote attacker to execute arbitrary code on the victim’s system by bypassing modern browser mitigations such as ASLR, DEP, or stack canaries if the memory corruption leads to control flow hijacking. Even without full code execution, integer overflows in layout engines often facilitate information disclosure vulnerabilities where sensitive data from adjacent memory regions is leaked through side-channel effects or direct out-of-bounds reads. This compromises user privacy and system integrity, particularly when users visit maliciously designed websites that exploit these rendering inconsistencies to trigger the faulty logic paths within the text processing subsystem.
Mozilla addressed this issue in Firefox version 158 and Firefox Extended Support Release (ESR) version 153.5 by implementing stricter validation checks for arithmetic operations involving font metrics and layout dimensions. The fix likely involves adding explicit bounds checking before performing calculations that determine buffer sizes or array indices, ensuring that any potential overflow is detected and handled safely rather than allowing the wrapped value to propagate through the rendering pipeline. Users are strongly advised to update their browsers immediately to these patched versions to mitigate the risk of exploitation. Additionally, organizations should ensure that automated patch management systems are configured to deploy these updates promptly across all endpoints running Firefox or its derivatives.
In terms of industry standards and threat modeling, this vulnerability is relevant to ATT&CK technique T1203, which covers Exploitation for Client Execution, as it represents a client-side attack vector targeting the browser application itself. Security researchers analyzing similar vulnerabilities often reference CWE-787 regarding out-of-bounds write access resulting from integer overflows. The remediation strategy emphasizes defensive programming practices such as using safe arithmetic libraries that detect overflow conditions before they occur and enforcing strict type safety in low-level rendering code. Continuous monitoring for new CVEs related to browser layout engines remains essential, given the complexity of modern web standards and the frequent updates required to maintain a secure browsing environment against evolving exploitation techniques.