CVE-2025-6433: 
NixOS vulnerability analysis and mitigation

CVE-2025-6433 is a WebAuthn specification compliance vulnerability in Mozilla Firefox and Thunderbird where the browser incorrectly permits WebAuthn challenges to be issued and signed on pages with invalid TLS certificates that the user has manually accepted via an exception. This violates the WebAuthn specification requirement that authentication only occur over "a secure transport established without errors." The vulnerability affects Firefox versions prior to 140.0 and Thunderbird versions prior to 140.0. It was reported by a researcher named Simon, disclosed on June 24, 2025 (Firefox) and July 2, 2025 (Thunderbird), and is rated Low severity by Mozilla. The Feedly-aggregated CVSS v3.1 score is 9.8 (Critical), though Mozilla's own advisory classifies it as low impact (Mozilla Advisory MFSA2025-51, Mozilla Advisory MFSA2025-54, Red Hat Bugzilla).

The root cause is an incomplete implementation of the WebAuthn API (CWE-295: Improper Certificate Validation) in Firefox's DOM: Web Authentication component. The WebAuthn specification (W3C WebAuthn Level 2, §5) requires that user agents only relay authentication challenges when a secure context is established via TLS without errors; Firefox previously allowed WebAuthn operations to proceed even when a certificate error override (e.g., untrusted issuer or domain mismatch) had been granted by the user. The fix, implemented by Mozilla engineer John Schanck (jschanck), disallows WebAuthn when an untrusted issuer or domain override is active, while continuing to permit it when only a certificate validity period override is used. Other browsers such as Chromium and Brave already enforced this restriction, returning NotAllowedError: WebAuthn is not supported on sites with TLS certificate errors (Mozilla Bugzilla, Mozilla Advisory MFSA2025-51).

The primary risk is that an attacker performing a DNS spoofing or man-in-the-middle attack could host a phishing site with a TLS certificate for their own domain (not the victim's), trick the user into accepting the certificate error, and then present a WebAuthn challenge using the legitimate site's relying party ID (rpid). If the user has a registered passkey for the spoofed site, Firefox would relay the challenge to the authenticator and allow the user to sign it, potentially enabling unauthorized authentication or credential misuse against the legitimate service. This undermines the phishing-resistance guarantee that is a core security property of WebAuthn/passkeys (Mozilla Bugzilla, Mozilla Advisory MFSA2025-51).

There is no public proof-of-concept exploit code and no evidence of in-the-wild exploitation as of the time of disclosure (Feedly). The vulnerability is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires a specific precondition: the victim must visit a DNS-spoofed or attacker-controlled site, encounter a TLS certificate warning, and manually grant an exception — limiting practical exploitability. The EPSS score is 0.024% (0.000240), reflecting very low probability of automated exploitation (Feedly). No threat actor attribution has been reported.

1. Setup: The attacker obtains a valid TLS certificate for their own domain (e.g., attacker.com) and sets up a phishing page that mimics a legitimate service (e.g., bank.com) that uses WebAuthn/passkeys for authentication.
2. DNS Spoofing: The attacker performs DNS spoofing or ARP poisoning to redirect the victim's DNS resolution for bank.com to the attacker's server IP.
3. Victim Visits Target: The victim navigates to bank.com in Firefox. Firefox detects that the TLS certificate presented is for attacker.com, not bank.com, and displays a certificate error warning.
4. Exception Granted: The attacker's phishing page or social engineering prompts the victim to click through the certificate warning and grant a TLS exception.
5. WebAuthn Challenge Issued: The attacker's page calls the WebAuthn API with the rpid set to bank.com and issues a credential assertion challenge.
6. Passkey Signed: Firefox (pre-140) relays the challenge to the authenticator without blocking it due to the TLS error. If the victim has a registered passkey for bank.com, they are prompted to authenticate and the challenge is signed.
7. Credential Misuse: The attacker captures the signed WebAuthn assertion and replays it against the legitimate bank.com service to authenticate as the victim (Mozilla Bugzilla).

Mozilla has released patches in Firefox 140 and Thunderbird 140, both of which enforce the WebAuthn specification requirement to block authentication challenges on connections with active TLS certificate error overrides. Users should update Firefox to version 140.0 or later and Thunderbird to version 140.0 or later immediately. As a behavioral workaround, users should avoid granting exceptions for invalid TLS certificates, particularly on sites where they use passkeys or WebAuthn-based authentication. Firefox ESR 115 and ESR 128 are listed as unaffected (Mozilla Advisory MFSA2025-51, Mozilla Advisory MFSA2025-54, Mozilla Bugzilla).

Mozilla classified the vulnerability as low severity in its official advisories, consistent with the requirement for significant user interaction (manually accepting a TLS certificate error) before exploitation is possible (Mozilla Advisory MFSA2025-51). The original bug report from researcher Simon highlighted that Chromium and Brave already blocked this scenario with an explicit NotAllowedError, framing Firefox's behavior as a spec compliance gap rather than a novel attack vector (Mozilla Bugzilla). Broader media coverage was limited, with the vulnerability bundled into roundups of the Firefox 140 and Thunderbird 140 security releases.