CVE-2025-6433 in Firefox
Summary
by MITRE • 06/24/2025
If a user visited a webpage with an invalid TLS certificate, and granted an exception, the webpage was able to provide a WebAuthn challenge that the user would be prompted to complete. This is in violation of the WebAuthN spec which requires "a secure transport established without errors". This vulnerability affects Firefox < 140.
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Analysis
by VulDB Data Team • 08/17/2025
This vulnerability represents a critical breakdown in the WebAuthn authentication protocol implementation within Firefox browsers prior to version 140. The flaw occurs when users encounter websites with invalid TLS certificates and choose to proceed despite the security warnings. Under normal circumstances, the WebAuthn specification mandates that authentication challenges must only be accepted over secure transport channels that have been established without errors, as defined by the secure channel requirements. However, this vulnerability allows malicious webpages to bypass these critical security checks by exploiting the user's explicit decision to accept an invalid certificate. The technical implementation fails to properly validate that the underlying transport layer meets all security requirements before permitting WebAuthn operations to proceed, creating a dangerous pathway for credential theft and authentication bypass attacks.
The operational impact of this vulnerability extends beyond simple certificate validation failures and represents a fundamental flaw in Firefox's security architecture that undermines the entire WebAuthn framework. When users grant certificate exceptions, the browser should not automatically enable additional authentication mechanisms that rely on the integrity of the transport layer. This vulnerability creates a scenario where attackers can exploit user trust in the browser's certificate exception handling to gain unauthorized access to WebAuthn-enabled services. The flaw directly violates the security principle of least privilege and creates a persistent attack vector that can be leveraged across multiple authentication systems. Security researchers have identified this as a significant risk to enterprise environments where users may encounter legitimate certificate issues and subsequently grant exceptions, unknowingly enabling credential harvesting attacks.
From a cybersecurity perspective, this vulnerability aligns with several attack patterns documented in the MITRE ATT&CK framework, particularly those related to credential access and privilege escalation techniques. The flaw enables adversaries to perform pass-the-hash attacks or credential theft operations by exploiting the gap between certificate validation and authentication mechanism activation. Organizations implementing WebAuthn for multi-factor authentication are particularly vulnerable as this flaw essentially nullifies the security benefits of the protocol when users encounter certificate warnings. The vulnerability also relates to CWE-295, which addresses improper certificate validation, and CWE-312, which covers exposure of sensitive information through improper handling of authentication mechanisms. This represents a critical gap in browser security that can be exploited by attackers with minimal technical skill to compromise authentication systems that should be protected by strong transport layer security.
The recommended mitigations for this vulnerability include immediate deployment of Firefox version 140 or later, which contains the necessary security patches to properly enforce WebAuthn transport requirements. Organizations should also implement comprehensive monitoring of certificate exceptions within their networks and establish clear policies for handling certificate warnings. Security teams should consider disabling WebAuthn functionality for domains known to have certificate issues until proper certificate management is established. Additionally, user education programs should emphasize the dangers of accepting certificate exceptions and the potential security implications of proceeding with authentication operations on insecure connections. Network administrators should also implement certificate pinning strategies and ensure that all internal and external web services maintain valid SSL/TLS certificates to prevent users from encountering the problematic scenarios that trigger this vulnerability.