CVE-2021-22923 in cURL
Summary
by MITRE • 08/06/2021
When curl is instructed to get content using the metalink feature, and a user name and password are used to download the metalink XML file, those same credentials are then subsequently passed on to each of the servers from which curl will download or try to download the contents from. Often contrary to the user's expectations and intentions and without telling the user it happened.
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Analysis
by VulDB Data Team • 05/16/2025
The vulnerability described in CVE-2021-22923 represents a significant security flaw in the curl command-line tool that affects how authentication credentials are handled during metalink downloads. This issue falls under the category of improper credential handling and can be classified as a CWE-256 vulnerability, which deals with insecure credential storage and transmission. The metalink feature in curl allows users to download files from multiple sources simultaneously, but the implementation contains a critical design flaw that exposes user credentials to unintended recipients.
When a user specifies authentication credentials for downloading a metalink XML file, curl incorrectly propagates these same credentials to all subsequent download operations that are referenced within the metalink document. This behavior violates fundamental security principles of least privilege and credential isolation, as the original authentication context becomes unnecessarily extended to multiple unrelated servers. The vulnerability is particularly concerning because it operates silently without informing the user, making it difficult to detect and potentially leading to credential exposure across multiple systems.
The operational impact of this vulnerability extends beyond simple credential leakage to encompass potential authorization bypass scenarios and increased attack surface for malicious actors. Attackers who gain access to metalink documents could potentially leverage this flaw to obtain credentials for additional systems or services that are not directly connected to the initial download target. This behavior creates a chain reaction where a single compromised metalink file can result in credential compromise across multiple servers, making it a particularly dangerous vulnerability in enterprise environments where metalink functionality is commonly used for software distribution.
This vulnerability aligns with several ATT&CK techniques including T1078 Valid Accounts for maintaining access and T1566 Phishing for credential theft. The silent credential propagation mechanism makes it particularly difficult to detect through standard monitoring approaches, as network traffic analysis may not immediately reveal the unauthorized credential usage. Organizations should consider implementing network monitoring rules that flag unusual credential propagation patterns and establish strict policies for metalink usage. The vulnerability also highlights the importance of proper authentication context management and credential isolation principles that should be enforced in all network tools and applications handling user credentials.
Mitigation strategies should include immediate patching of curl versions affected by this vulnerability, implementing strict access controls for metalink file sources, and establishing monitoring procedures to detect credential propagation anomalies. Organizations should also consider disabling metalink functionality in environments where credential security is paramount, or implementing additional authentication layers that can detect and prevent unauthorized credential usage. The vulnerability serves as a reminder of the critical importance of proper credential management in network applications and the potential for seemingly minor implementation flaws to create significant security risks.