CVE-2026-100820 in Firefox
Summary
by MITRE • 09/29/2026
Privilege escalation in the Address Bar component. This vulnerability was fixed in Firefox ESR 153.4, Firefox 157, and Firefox ESR 140.17.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 09/29/2026
The identified security issue represents a critical privilege escalation flaw located within the address bar component of Mozilla Firefox browsers. Address bars serve as primary interfaces for user interaction with web content, handling URL parsing, navigation requests, and display of site identity indicators such as lock icons or domain names. A vulnerability in this specific subsystem can allow an attacker to manipulate how the browser interprets or displays network addresses, potentially leading to unauthorized access controls being bypassed or security boundaries being eroded. The resolution of this flaw was implemented across multiple release channels, specifically Firefox ESR 153.4, Firefox 157, and Firefox Extended Support Release (ESR) version 140.17, indicating that the defect existed in prior versions of these branches and required targeted patches to restore proper security posture.
From a technical perspective, privilege escalation vulnerabilities within browser components often stem from improper validation of input data or incorrect handling of state transitions during navigation events. In the context of an address bar flaw, this may involve scenarios where maliciously crafted URLs or scripts exploit parsing inconsistencies to execute code with higher privileges than intended, such as gaining access to internal browser APIs that are normally restricted to trusted contexts. Such flaws can enable attackers to bypass same-origin policies, inject content into privileged frames, or manipulate the user interface to deceive users into interacting with untrusted elements under false pretenses of legitimacy. The specific mechanics likely involved a failure in sanitizing or validating URL schemes, hostnames, or path components before they were processed by higher-level security modules within the browser engine.
The operational impact of this vulnerability is significant due to its potential for facilitating further attacks on end-users. If exploited successfully, an attacker could escalate privileges from untrusted web content to trusted chrome code, effectively gaining control over sensitive user data such as cookies, local storage, or browsing history. This level of access can lead to account takeover, session hijacking, or the installation of persistent malware through drive-by download techniques that rely on elevated permissions. Furthermore, because address bars are central to user trust indicators, compromising this component may also facilitate phishing attacks by allowing attackers to spoof legitimate domain names or security badges, thereby deceiving users into disclosing credentials or sensitive information.
To mitigate risks associated with such vulnerabilities, organizations and individual users must ensure that all instances of Firefox are updated to the specified patched versions immediately. Regular patch management cycles should be enforced to minimize exposure windows for newly disclosed flaws. Additionally, implementing strict content security policies (CSP) on web applications can reduce the attack surface by limiting how scripts interact with browser components. Security awareness training for users remains essential, as technical mitigations alone cannot prevent social engineering attacks that exploit human trust in interface elements like address bars. Monitoring for anomalous behavior related to URL navigation and privilege usage within enterprise environments can also aid in detecting potential exploitation attempts before significant damage occurs.
This vulnerability aligns with common classification standards such as CWE-269, which covers improper privilege assignment where an actor is granted more privileges than necessary or intended. It may also relate to CWE-74 if the flaw involves injection of malicious code through unsanitized input in the address bar context. In terms of adversary tactics, this type of vulnerability supports techniques described in the MITRE ATT&CK framework under Initial Access and Execution phases, particularly those involving browser exploitation for privilege escalation or defense evasion by masking malicious activity behind legitimate-looking interface elements. Understanding these mappings helps security teams prioritize remediation efforts based on broader threat intelligence landscapes and known attack patterns targeting web browsers.