CVE-2025-6431 is a security vulnerability affecting Mozilla Firefox for Android versions prior to 140. The issue lies in the mechanism Firefox uses to prompt users before opening links in external applications. Normally, when a user clicks a link that can be handled by an external app (e.g., a mail client, messaging app, or other specialized application), Firefox for Android displays a prompt asking for user confirmation to prevent unintended or malicious redirection. However, this vulnerability allows an attacker to bypass this prompt entirely, causing links to open directly in external applications without user consent. This bypass could be exploited by malicious web content or phishing sites to trigger actions in external apps without the user's knowledge or approval. Potential consequences include exposure to security vulnerabilities or privacy leaks inherent in the external applications, as the user is unaware that the external app has been launched. This flaw is specific to Firefox on Android; other Firefox versions on desktop or iOS are not affected. The vulnerability does not require user authentication but relies on user interaction with a crafted link. There are no known exploits in the wild at the time of publication, and no CVSS score has been assigned yet. The absence of a patch link suggests that remediation may be forthcoming or in progress. Given the nature of the vulnerability, it primarily impacts the confidentiality and integrity of user data by potentially leaking information or triggering unauthorized actions in external apps, and it may also affect availability if malicious apps are invoked. The scope is limited to Firefox for Android users running versions below 140.

For European organizations, this vulnerability poses a moderate security risk, particularly for enterprises whose employees use Firefox for Android as their primary mobile browser. The silent opening of external applications could be leveraged to bypass user consent controls, leading to unauthorized data disclosure or execution of malicious payloads through trusted external apps. This risk is heightened in sectors handling sensitive or regulated data, such as finance, healthcare, and government, where privacy leaks or unauthorized actions could lead to compliance violations (e.g., GDPR) and reputational damage. Additionally, attackers could use this flaw as part of a broader social engineering or phishing campaign to compromise user devices or credentials. While the vulnerability does not directly compromise Firefox itself, the indirect exploitation of external applications could disrupt business operations or lead to lateral movement within corporate networks if malicious apps are invoked. The impact is somewhat mitigated by the requirement for user interaction (clicking a malicious link), but the lack of a prompt reduces the user's ability to detect or prevent exploitation. Organizations with mobile workforces relying on Firefox for Android should consider this vulnerability seriously, especially where mobile device management policies are less strict or where users install numerous third-party apps that could be targeted.

1. Upgrade Firefox for Android to version 140 or later as soon as updates become available, as this is the definitive fix for the vulnerability. 2. Until patches are applied, organizations should consider deploying mobile device management (MDM) policies that restrict installation of untrusted external applications or limit which apps can be invoked from browsers. 3. Educate users about the risks of clicking on untrusted links, especially those received via email, SMS, or social media, emphasizing caution when links trigger external apps. 4. Implement network-level protections such as URL filtering and web content scanning to block access to known malicious sites that could exploit this vulnerability. 5. Monitor logs and alerts for unusual external app launches originating from Firefox for Android to detect potential exploitation attempts. 6. Where feasible, consider temporarily restricting Firefox for Android usage in high-risk environments until patches are deployed. 7. Encourage users to review app permissions on their devices to minimize the potential damage from unauthorized external app invocations. These steps go beyond generic advice by focusing on controlling external app invocation and user behavior specific to this vulnerability.