CVE-2020-5409 in Concourse
Summary
by MITRE
Pivotal Concourse, most versions prior to 6.0.0, allows redirects to untrusted websites in its login flow. A remote unauthenticated attacker could convince a user to click on a link using the OAuth redirect link with an untrusted website and gain access to that user's access token in Concourse. (This issue is similar to, but distinct from, CVE-2018-15798.)
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Analysis
by VulDB Data Team • 10/18/2020
The vulnerability identified as CVE-2020-5409 affects Pivotal Concourse versions prior to 6.0.0 and represents a critical security flaw in the authentication system's OAuth redirect mechanism. This issue specifically targets the login flow where Concourse fails to properly validate redirect URLs, creating an avenue for attackers to manipulate the authentication process. The vulnerability stems from insufficient input validation and lacks proper sanitization of redirect parameters, allowing malicious actors to craft deceptive authentication flows that can lead to unauthorized access. The flaw operates within the OAuth 2.0 authentication framework, which is widely adopted across enterprise platforms and cloud services, making it particularly concerning given the prevalence of this authentication method.
The technical implementation of this vulnerability enables attackers to construct malicious OAuth redirect URLs that point to untrusted domains controlled by the attacker. When a legitimate user attempts to authenticate through Concourse, they are redirected to the attacker-controlled website instead of the intended authentication endpoint. This misdirection allows the attacker to capture the user's access token and potentially gain elevated privileges within the Concourse environment. The vulnerability is classified as a redirect vulnerability under CWE-601 and aligns with the ATT&CK technique T1566 for phishing and credential access through manipulated authentication flows. The flaw essentially creates a trust relationship between the Concourse platform and malicious domains, undermining the entire authentication security model.
The operational impact of CVE-2020-5409 extends beyond simple credential theft, as successful exploitation can lead to complete compromise of user accounts and potentially the entire Concourse deployment. Attackers can leverage stolen access tokens to perform unauthorized operations such as creating or modifying pipelines, accessing sensitive build artifacts, and manipulating continuous integration workflows. This vulnerability particularly affects organizations that rely on Concourse for their CI/CD processes, as compromised access tokens could enable attackers to inject malicious code into the build pipeline or exfiltrate sensitive data. The attack vector requires user interaction through phishing emails or malicious links, making it difficult to detect and prevent without proper security awareness training and robust authentication controls.
Organizations should implement immediate mitigations including upgrading to Concourse version 6.0.0 or later where this vulnerability has been addressed through proper redirect URL validation and sanitization. Security configurations should enforce strict validation of redirect URLs to ensure they originate from trusted domains within the organization's infrastructure. Network-level controls such as web application firewalls and DNS filtering can provide additional layers of protection by blocking access to known malicious domains. The remediation process should include comprehensive security testing of authentication flows and implementation of proper logging to detect suspicious redirect activities. Organizations should also review their OAuth configurations and ensure that redirect URIs are explicitly defined and validated rather than accepting arbitrary URLs. This vulnerability highlights the importance of proper input validation in authentication systems and demonstrates how seemingly minor flaws in redirect handling can create significant security risks in enterprise environments.