CVE-2026-100801 in Firefox
Summary
by MITRE • 09/29/2026
Privilege escalation in the DLL Services component. This vulnerability was fixed in Firefox ESR 153.4, Firefox 157, and Firefox ESR 140.17.
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Analysis
by VulDB Data Team • 09/29/2026
The identified vulnerability represents a critical privilege escalation flaw within the Dynamic Link Library (DLL) services architecture of Mozilla Firefox browsers. This security issue stems from improper handling of DLL loading mechanisms, which are fundamental to how Windows-based applications resolve and execute external code libraries at runtime. In typical software operations, when an application requires functionality provided by a shared library, it must locate that file on the disk or in memory. The flaw arises because the browser does not sufficiently enforce strict path validation or secure loading practices during this resolution process. This oversight allows an attacker to manipulate the environment in which Firefox operates, specifically targeting the sequence and location of DLL searches. By exploiting weaknesses such as unquoted service paths or insecure directory permissions, a malicious actor can place a rogue DLL file in a directory that is searched before the legitimate system library. When Firefox attempts to load this component, it inadvertently executes the attacker-controlled code instead of the intended safe module.
From a technical perspective, this vulnerability aligns with CWE-427, which describes Uncontrolled Search Path for an Element. This category of weakness occurs when software searches for resources using paths that are not fully qualified or are susceptible to modification by unauthorized users. In the context of Firefox on Windows, if the application relies on relative paths or directories with weak access control lists, a local attacker can pre-load malicious code into memory before the legitimate DLL is loaded. This technique leverages the inherent trust relationship between the browser process and its dependencies. Once the rogue DLL executes within the context of the Firefox process, it inherits all privileges associated with that user session. If the victim is running as an administrator or has elevated permissions for specific system tasks, this escalation can provide the attacker with significant control over the operating system, potentially leading to full system compromise rather than just a browser-level breach.
The operational impact of this vulnerability is severe due to its potential for local privilege escalation. An attacker who successfully exploits this flaw does not merely gain access to browsing data or session cookies; they achieve code execution within the context of the user running Firefox. This means that any actions performed by the malicious DLL are treated as if they were initiated by the legitimate user. Consequently, an adversary could install persistent backdoors, create new administrative accounts, exfiltrate sensitive files stored on the local machine, or pivot to other systems within a network environment. The severity is further amplified because this type of vulnerability often requires only physical access or low-level remote code execution capabilities to trigger, making it a valuable tool for post-exploitation activities where an attacker seeks to move from a foothold in the browser to full system control.
Mitigation strategies primarily involve applying the vendor-provided patches that address these DLL loading inconsistencies. Users and administrators must ensure they are running Firefox ESR 153.4, Firefox 157, or Firefox ESR 140.17 or later versions where this specific flaw has been remediated. These updates likely implement stricter path validation checks, enforce absolute paths for critical DLLs, or adjust security descriptors to prevent unauthorized users from placing files in susceptible directories. Beyond software updates, system hardening measures should be employed to reduce the attack surface. This includes ensuring that standard user accounts do not have write access to program installation directories and configuring Windows Defender Application Control or similar technologies to restrict which binaries can execute on the endpoint. Regular monitoring of DLL load events using tools like Sysmon can also help detect anomalous loading patterns indicative of this exploitation attempt before it leads to privilege escalation.
In terms of industry classification, this vulnerability is mapped to MITRE ATT&CK technique T1055, specifically sub-technique T1055.012 which covers DLL side-loading or injection. This framework categorization highlights the method by which attackers bypass security controls by leveraging legitimate system processes and libraries. Understanding this mapping helps security teams prioritize detection rules focused on unusual DLL loads originating from browser executables into non-standard directories. By aligning defensive measures with these established standards, organizations can better anticipate similar attack vectors in other applications that rely on Windows DLL services. The prompt release of patches for Firefox ESR and standard releases underscores the importance of maintaining an agile patch management cycle to close such gaps before they are widely exploited in the wild.