CVE-2026-84127 in Firefox
Summary
by MITRE • 09/01/2026
Information disclosure in the WebExtensions component in Firefox for Android. This vulnerability was fixed in Firefox 155.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 09/01/2026
The identified security issue resides within the WebExtensions subsystem of Mozilla Firefox for Android, representing a critical class of vulnerabilities often associated with insufficient access controls or improper data handling within browser extension APIs. WebExtensions serve as the primary mechanism for third-party developers to add functionality to the browser, such as ad blocking, password management, and privacy enhancement tools. These extensions operate in a sandboxed environment designed to isolate them from the core browser code and user data; however, this isolation relies heavily on strict adherence to permission models and API restrictions. The vulnerability described indicates that under specific conditions or through particular invocation patterns of WebExtension APIs, sensitive information accessible to an extension was inadvertently exposed beyond its intended scope. This could involve leaking internal state variables, exposing private browsing history details, accessing clipboard contents without explicit user consent, or revealing network request headers that contain authentication tokens or personally identifiable information. Such disclosures typically stem from a failure in the boundary checks between the privileged browser context and the less-privileged extension context, allowing an attacker-controlled script to query data it should not have access to according to the principle of least privilege.
From a technical perspective, this flaw likely involves a misconfiguration in how permissions are validated during runtime or a logic error in the serialization of objects passed between JavaScript contexts. In many browser architectures, extensions interact with the host application through asynchronous messaging channels and API calls that return promises or callbacks. If these interfaces do not properly sanitize inputs or validate the origin and intent of the requestor before returning data, an extension can exploit this gap to retrieve information from other tabs, windows, or even system-level APIs. The impact is particularly severe in mobile environments where users often grant broad permissions to extensions for convenience, assuming that the browser engine will enforce strict isolation. An attacker who has managed to inject malicious code into a legitimate extension, or an untrusted extension installed by the user, can leverage this vulnerability to harvest sensitive data such as login credentials stored in password managers, session cookies enabling account takeover, or detailed browsing habits that compromise user privacy. This aligns with CWE-200, which classifies information exposure vulnerabilities where security-sensitive information is disclosed without explicit authorization, and maps directly to MITRE ATT&CK techniques related to Credential Access and Collection via browser extensions, specifically T1557 if the attack involves adversary-in-the-browser scenarios or data exfiltration through compromised plugins.
The operational impact of this vulnerability extends beyond immediate data theft; it undermines the trust model that allows users to safely utilize third-party tools within their web browsers. If attackers can reliably extract sensitive information from extensions, they can build sophisticated phishing campaigns by tailoring attacks based on real-time user behavior or steal session tokens to hijack active accounts without needing passwords. Furthermore, because Firefox for Android is a widely used mobile browser, the scale of potential exposure is significant. Users relying on these browsers for financial transactions, corporate communications, or personal correspondence are at heightened risk if they have installed extensions that inadvertently trigger this disclosure flaw. The existence of such vulnerabilities also highlights the complexity of maintaining secure boundaries in complex software ecosystems where multiple layers of abstraction interact dynamically. It necessitates rigorous code auditing and fuzzing of extension APIs to ensure that data flow is strictly controlled and that no unintended side effects occur during API execution.
To mitigate these risks, users should exercise caution when installing extensions, prioritizing those from reputable developers with transparent privacy policies and regular security audits. Disabling unused extensions reduces the attack surface by removing potential vectors for exploitation. Mozilla has addressed this specific issue in Firefox version 155, which includes patches that likely involve stricter validation of extension permissions, enhanced sandboxing mechanisms, or refined API return values to prevent unauthorized data leakage. It is imperative for users to update their browsers immediately upon release of such security fixes to ensure they are protected against known exploitation techniques. Additionally, developers building extensions should adhere strictly to the WebExtensions documentation regarding permission scopes and avoid requesting unnecessary privileges that could increase exposure if a vulnerability were present. Regularly reviewing extension permissions in browser settings can help identify any anomalies or overly broad access rights that might indicate potential misuse or susceptibility to such disclosure flaws.