CVE-2026-107145 in twofactor_gauthenticator
Summary
by MITRE • 10/27/2026
The twofactor_gauthenticator plugin for Roundcube is vulnerable to brute-forcing of the second-factor code. Due to the lack of rate limiting, an attacker who obtains user's password can submit arbitrary 6-digit codes, resulting in second-factor bypass and unauthorized access to the system.
This issue was fixed in commit 4640889.
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Analysis
by VulDB Data Team • 10/27/2026
The twofactor_gauthenticator plugin for Roundcube presents a critical authentication weakness that undermines the security benefits of two-factor authentication. While multi-factor authentication is designed to provide an additional layer of protection beyond simple password verification, this specific implementation fails to enforce rate limiting on the second factor validation process. This architectural oversight allows an attacker who has already compromised or obtained a user's primary credentials through phishing, credential stuffing, or other means to bypass the secondary security control entirely. The vulnerability stems from the absence of mechanisms such as account lockout policies, progressive delays between attempts, or CAPTCHA challenges during the verification of the time-based one-time password generated by Google Authenticator compatible applications.
From a technical perspective, the flaw resides in the input validation and session management logic within the plugin's authentication handler. When a user submits their credentials along with a six-digit code for verification, the server processes these inputs without tracking the number of failed attempts associated with that specific session or account identifier. Consequently, an automated script can iterate through all possible combinations of the six-digit numeric code at high speed. Since there are only one million potential permutations (from 000000 to 999999), and considering typical user behavior where codes may be predictable or weakly randomized in some implementations, a brute-force attack is computationally feasible within seconds using standard hardware resources. This effectively reduces the two-factor authentication mechanism back to single-factor security based solely on knowledge of the password.
The operational impact of this vulnerability is severe, as it directly leads to unauthorized access to the Roundcube webmail interface and any systems integrated with it via Single Sign-On or shared credential stores. An attacker can gain full control over a user's email account without needing physical possession of their authenticator device or knowledge of the secret seed used to generate codes. This compromise allows for further lateral movement within an organization, access to sensitive communications, potential data exfiltration, and use of the compromised identity for phishing campaigns against colleagues. The ease of exploitation means that even sophisticated security postures relying on this plugin are effectively neutralized if rate limiting is not configured at either the application level or a preceding network perimeter device such as a Web Application Firewall.
This vulnerability aligns with CWE-307, which describes Improper Restriction of Excessive Authentication Attempts, and maps to MITRE ATT&CK technique T1110.001, representing Brute Force: Password Guessing. Although the primary attack vector targets the second factor rather than the password itself, the underlying principle remains the exploitation of insufficient controls against repeated authentication attempts. To mitigate this risk, administrators must ensure that the plugin is updated to version containing commit 4640889 or later, which implements appropriate rate limiting logic. Additionally, organizations should enforce account lockout policies at the application level and consider implementing CAPTCHA challenges after a small number of failed login attempts. Network-level protections such as IP-based throttling can also serve as an effective compensating control to slow down automated brute-force attacks while waiting for upstream patches or configuration changes to take effect.