CVE-2026-100767 in Firefox
Summary
by MITRE • 09/29/2026
Use-after-free in the Networking: Cache component. This vulnerability was fixed in Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.
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Analysis
by VulDB Data Team • 09/29/2026
The identified vulnerability represents a critical memory management error classified as an out-of-bounds write within the Networking Cache subsystem of Mozilla Firefox browsers. This specific flaw stems from a use-after-free condition where the application continues to reference a block of allocated memory after that memory has been deallocated or freed by the system. In complex networking stacks, such as those handling HTTP caching mechanisms, objects are frequently created and destroyed in rapid succession based on network requests and response headers. When an object is freed but a dangling pointer remains active within the cache logic, subsequent operations may attempt to write data into this invalid memory region. This scenario typically arises when reference counting mechanisms fail to properly synchronize access across multiple threads or execution contexts, allowing one part of the code to release resources while another still holds a stale reference expecting those resources to remain valid and accessible for writing new cached content.
From an operational perspective, this vulnerability poses severe risks to both system stability and security integrity. An out-of-bounds write allows an attacker who can trigger specific network conditions or manipulate cache entries to overwrite adjacent memory structures. This corruption can lead to immediate application crashes, resulting in a denial of service for the end user by disrupting their browsing session. More critically, if the overwritten data includes control flow information such as return addresses on the stack or function pointers within heap metadata, it enables arbitrary code execution with the privileges of the browser process. Given that modern browsers operate with sandboxing mechanisms to limit damage from exploits, compromising the networking cache component is particularly dangerous because this subsystem often requires elevated permissions to interact directly with disk storage and network interfaces, potentially allowing an attacker to bypass certain security boundaries if the exploit chain successfully escapes the initial sandbox constraints.
The technical classification of this flaw aligns closely with Common Weakness Enumeration (CWE) identifiers such as CWE-416 for Use After Free and CWE-787 for Out-of-bounds Write. These categories highlight fundamental failures in memory safety, which are prevalent challenges in software written using languages like C++ that do not automatically manage garbage collection. The vulnerability likely exploits race conditions or logical errors in the lifecycle management of cache entries, where the timing between deallocation and subsequent access is manipulated by maliciously crafted web content. Attackers could potentially trigger this condition through specially designed HTTP responses or cached resources that force the browser to free a memory block while maintaining an active reference within the networking layer's internal data structures.
Mitigation strategies primarily rely on applying the vendor-provided patches released in Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17. These updates contain code changes that enforce stricter memory management protocols, likely involving improved reference counting logic or the introduction of smart pointers to automatically handle deallocation when references drop to zero. For organizations managing large fleets of browsers, it is imperative to prioritize these updates in deployment pipelines due to the high severity associated with remote code execution vulnerabilities in browser engines. Additionally, defense-in-depth measures such as enabling strict Content Security Policies and ensuring that all third-party extensions are up-to-date can help reduce the attack surface available for exploiting memory corruption flaws before patches are fully deployed across an environment.