Formbricks JWT Signature Verification Bypass Account Takeover
CVE-2025-59934 affects Formbricks, an open source survey platform, prior to version 4.0.1. The vulnerability is caused by improper JWT validation: the application decodes tokens using jwt.decode but does not verify the JWT signature. The same flawed validator is used in both the email-verification token login flow and the password-reset server action. As described, the validator also fails to validate other critical claims and properties, including expiration, issuer, and audience. Because of this, an attacker who knows a victim’s valid user.id can forge an arbitrary JWT, including one using an alg:"none" header, and have it accepted as authentic by the application. This can be used to authenticate as the victim and reset the victim’s password, resulting in account takeover. The issue is patched in Formbricks 4.0.1.
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