CVE-2021-32813 in Traefik
Summary
by MITRE • 08/04/2021
Traefik is an HTTP reverse proxy and load balancer. Prior to version 2.4.13, there exists a potential header vulnerability in Traefik's handling of the Connection header. Active exploitation of this issue is unlikely, as it requires that a removed header would lead to a privilege escalation, however, the Traefik team has addressed this issue to prevent any potential abuse. If one has a chain of Traefik middlewares, and one of them sets a request header, then sending a request with a certain Connection header will cause it to be removed before the request is sent. In this case, the backend does not see the request header. A patch is available in version 2.4.13. There are no known workarounds aside from upgrading.
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Analysis
by VulDB Data Team • 08/07/2021
The vulnerability described in CVE-2021-32813 affects Traefik, a widely-used HTTP reverse proxy and load balancer that serves as a critical component in modern microservices architectures and cloud-native deployments. This security issue resides in Traefik's handling of the Connection header, which is a standard HTTP header used to control connection behavior between clients and servers. The vulnerability specifically impacts versions prior to 2.4.13, making it a significant concern for organizations that have not yet updated their Traefik installations. The issue is classified under CWE-200, which deals with information exposure, as it can potentially lead to information leakage or manipulation of request headers that are critical for proper application behavior and security enforcement.
The technical flaw manifests when Traefik processes a chain of middlewares where one middleware sets a request header, but a subsequent Connection header in the request causes that header to be removed before the request reaches the backend service. This occurs because Traefik's header handling logic removes certain headers, including those specified in the Connection header, before forwarding requests to backend servers. This behavior creates a potential security risk where legitimate headers that should be preserved for backend processing are inadvertently stripped away, potentially breaking application functionality or creating unexpected security implications. The vulnerability is particularly concerning in environments where header-based authentication, authorization, or security controls depend on specific header values being passed through to backend services.
The operational impact of this vulnerability extends beyond simple header removal, as it can disrupt critical application flows and potentially create security gaps in environments where header manipulation is used for access control or application-specific security measures. When headers are stripped from requests, backend services may not receive necessary authentication tokens, session identifiers, or other security-related information that is essential for proper application behavior. This can lead to authentication failures, access control bypasses, or other security-related issues that could be exploited by attackers. The vulnerability's classification under the ATT&CK framework would fall under T1071.004 for application layer protocols and potentially T1566 for credential access, as it can impact how credentials and security information are transmitted between components.
Organizations utilizing Traefik as part of their infrastructure should prioritize immediate upgrade to version 2.4.13 or later to remediate this vulnerability. The patch addresses the core issue by modifying Traefik's header processing logic to prevent the inappropriate removal of headers that are critical for application functionality and security. No effective workarounds exist for this vulnerability, as the issue stems from fundamental header handling behavior within the Traefik proxy itself. The security implications are particularly significant in environments where Traefik is used as a gateway for microservices, where header-based security controls are prevalent, or where the integrity of request headers is essential for proper application behavior. Security teams should conduct thorough assessments of their Traefik deployments to ensure all instances are updated and monitor for any potential impacts from the header removal behavior that was previously present in vulnerable versions.