CVE-2026-34720: Zammad SSO Authentication Bypass Flaw

CVE-2026-34720 Overview

CVE-2026-34720 is an authentication bypass vulnerability in Zammad, a web-based open source helpdesk and customer support system. The vulnerability exists in Zammad's Single Sign-On (SSO) mechanism, which fails to verify whether authentication headers originate from a trusted SSO proxy or gateway before processing them. This weakness allows attackers to potentially forge SSO headers and bypass authentication controls, gaining unauthorized access to the helpdesk system.

Critical Impact

Attackers could bypass SSO authentication by injecting forged headers, potentially gaining unauthorized access to Zammad helpdesk systems and sensitive customer support data.

Affected Products

• Zammad versions prior to 7.0.1
• Zammad versions prior to 6.5.4

Discovery Timeline

• April 8, 2026 - CVE-2026-34720 published to NVD
• April 8, 2026 - Last updated in NVD database

Technical Details for CVE-2026-34720

Vulnerability Analysis

This vulnerability is classified under CWE-346 (Origin Validation Error), which occurs when a product does not properly verify that the source of data or communication is valid. In the context of Zammad's SSO implementation, the application processes SSO headers without first validating that they originate from a legitimate and trusted SSO proxy or identity gateway.

SSO implementations typically rely on reverse proxies or identity providers to inject authentication headers (such as X-Remote-User or similar custom headers) after successful authentication. The receiving application must verify these headers come from a trusted source, often by checking the request originates from an expected IP address or through cryptographic signatures. Zammad's SSO mechanism was not performing this verification, creating an authentication bypass condition.

Root Cause

The root cause is the absence of origin validation for SSO authentication headers. The Zammad application accepted and processed SSO headers without confirming they came from a configured, trusted SSO proxy or gateway. This allows direct requests to the application to include forged SSO headers that would be processed as if they came from a legitimate identity provider.

Attack Vector

An attacker with network access to the Zammad instance could craft HTTP requests containing forged SSO authentication headers. Without proper origin validation, Zammad would accept these headers as legitimate, allowing the attacker to authenticate as any user by simply injecting the appropriate header values. This requires the attacker to know or guess valid username identifiers used by the target organization.

The attack requires network access to the Zammad application and knowledge of the SSO header format expected by the application. In environments where Zammad is exposed directly to the internet without proper network segmentation, the attack surface is significantly increased.

Detection Methods for CVE-2026-34720

Indicators of Compromise

• Authentication logs showing SSO-based logins from unexpected IP addresses or network segments
• Successful SSO authentications bypassing the configured identity provider
• Login events occurring without corresponding entries in the SSO proxy or identity provider logs
• Unusual administrative actions performed by accounts that authenticated via SSO from non-standard sources

Detection Strategies

• Implement log correlation between Zammad authentication logs and identity provider logs to detect mismatched SSO events
• Monitor for SSO authentication requests originating from IP addresses outside the trusted proxy range
• Configure network monitoring to detect direct connections to Zammad that bypass the SSO proxy layer
• Review Zammad access logs for authentication patterns inconsistent with normal SSO traffic flows

Monitoring Recommendations

• Enable verbose authentication logging in Zammad to capture source IP addresses and authentication methods
• Set up alerts for SSO authentication events from IP addresses not matching your identity provider infrastructure
• Monitor for unusual user session creation patterns that may indicate forged authentication attempts
• Implement network-level monitoring to ensure all production traffic flows through the designated SSO proxy

How to Mitigate CVE-2026-34720

Immediate Actions Required

• Upgrade Zammad to version 7.0.1 or 6.5.4 immediately to address this vulnerability
• Implement network-level restrictions ensuring only the trusted SSO proxy can reach the Zammad application
• Review authentication logs for any signs of unauthorized SSO-based access
• Audit current user sessions and terminate any suspicious sessions that may have originated from forged SSO authentication

Patch Information

Zammad has addressed this vulnerability in versions 7.0.1 and 6.5.4. The fix implements proper origin validation for SSO headers, ensuring they are only accepted from configured trusted sources. Organizations should upgrade to these versions or later to remediate the vulnerability. For detailed information, refer to the GitHub Security Advisory.

Workarounds

• Configure firewall rules to ensure the Zammad application only accepts connections from the trusted SSO proxy IP addresses
• Place Zammad behind a reverse proxy that strips incoming SSO headers from external requests before they reach the application
• Implement network segmentation to prevent direct access to Zammad from untrusted network segments
• Consider temporarily disabling SSO authentication and using alternative authentication methods until the patch can be applied

# Example: Configure firewall to restrict access to Zammad from SSO proxy only
# Replace SSO_PROXY_IP with your actual SSO proxy IP address
iptables -A INPUT -p tcp --dport 443 -s SSO_PROXY_IP -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -j DROP