CVE-2026-42210 in Webmin
Summary
by MITRE • 07/20/2026
Webmin is a web-based system administration tool for Unix-like servers. Prior to version 2.640, for Webmin accounts that require a second authentication factor (typically TOTP), an attacker with knowledge of the username and password can bypass the 2FA requirement by using Basic authentication. Webmin is a web-based system administration tool for Unix-like servers. As a workaround, apply the patch from commit da18a16c84ae5c0b78cad79609cb0efb174000ec manually.
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Analysis
by VulDB Data Team • 07/20/2026
This vulnerability represents a critical authentication bypass flaw in Webmin versions prior to 2640, where the security mechanism designed to protect administrative accounts is fundamentally undermined by improper authentication flow handling. The issue stems from how Webmin processes authentication requests for accounts configured with two-factor authentication, specifically Time-based One-Time Passwords, creating a pathway for attackers to circumvent the additional security layer through a simple manipulation of authentication protocols. The vulnerability occurs because the system fails to properly validate that all authentication factors have been successfully verified before granting access, allowing malicious actors to exploit the Basic authentication mechanism to gain unauthorized access to administrative functions while bypassing the second factor requirements.
The technical implementation flaw manifests in the authentication processing logic where Webmin's authentication subsystem does not adequately enforce the requirement for all configured authentication factors to be satisfied. When a user attempts to authenticate with valid credentials but without completing the second factor verification, the system should reject the connection entirely or require proper completion of both factors. However, the vulnerability allows an attacker who has already obtained valid username and password credentials to leverage Basic authentication headers to bypass the TOTP validation process entirely. This represents a classic case of inadequate session management and authentication flow control that violates fundamental security principles for multi-factor authentication systems.
The operational impact of this vulnerability is severe and far-reaching within Unix-like server environments where Webmin serves as the primary administrative interface. Attackers who successfully exploit this flaw gain full administrative privileges without the protection that two-factor authentication was designed to provide, potentially leading to complete system compromise, data exfiltration, privilege escalation, and persistent access within the target environment. The vulnerability affects any organization relying on Webmin for server administration where accounts have been configured with TOTP as a second factor, creating a significant attack surface that could be exploited by threat actors with minimal technical sophistication. This weakness undermines the entire security posture of systems that depend on Webmin for administrative access control.
The mitigation strategy involves applying the patch referenced in commit da18a16c84ae5c0b78cad79609cb0efb174000ec, which addresses the core authentication flow issue by ensuring that all configured authentication factors are properly validated before access is granted. Organizations should immediately update their Webmin installations to version 2.640 or later, as this patch specifically resolves the vulnerability by implementing proper authentication factor validation. Additionally, system administrators should review and verify their current Webmin configurations to ensure that TOTP requirements are properly enforced and that no accounts are configured with incomplete authentication settings. The fix aligns with security best practices outlined in CWE-305 Authentication Bypass Through Multiple Authentication Factors and addresses techniques described in MITRE ATT&CK matrix under Initial Access and Privilege Escalation phases, where adversaries often exploit weak authentication controls to gain unauthorized access to systems.
This vulnerability demonstrates the critical importance of proper authentication flow implementation and the dangers of assuming that basic authentication mechanisms can be safely combined with additional security layers without proper validation. The flaw represents a failure in the principle of least privilege and proper access control enforcement, as it allows attackers to bypass security controls that were explicitly implemented to protect sensitive administrative functions. Organizations should conduct comprehensive security assessments of their Webmin installations and review all authentication configurations to ensure that multi-factor authentication is properly enforced across all administrative interfaces. The patch addresses the vulnerability through improved session management and authentication validation processes, ensuring that all configured authentication factors must be successfully completed before any administrative access is granted, thereby restoring the intended security posture for protected accounts.