CVE-2021-32741 in Server
Summary
by MITRE • 07/13/2021
Nextcloud Server is a Nextcloud package that handles data storage. In versions prior to 19.0.13, 20.011, and 21.0.3, there was a lack of ratelimiting on the public share link mount endpoint. This may have allowed an attacker to enumerate potentially valid share tokens. The issue was fixed in versions 19.0.13, 20.0.11, and 21.0.3. There are no known workarounds.
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Analysis
by VulDB Data Team • 07/15/2021
The vulnerability CVE-2021-32741 affects Nextcloud Server versions prior to 19.0.13, 20.0.11, and 21.0.3, representing a critical security flaw in the public share link functionality that exposes the system to unauthorized enumeration attacks. This issue resides in the public share link mount endpoint where the application fails to implement proper rate limiting mechanisms, creating a significant attack surface for malicious actors seeking to discover valid share tokens through systematic probing. The vulnerability is categorized under CWE-307, which addresses improper restriction of repeated accesses to protected resources, making it particularly dangerous as it allows for automated exploitation through brute force techniques.
The technical flaw manifests when attackers attempt to access public share links through the mount endpoint without sufficient rate limiting controls. This absence of rate limiting enables attackers to rapidly iterate through potential share tokens, systematically testing valid links while avoiding detection mechanisms that might otherwise flag suspicious activity. The vulnerability specifically targets the endpoint responsible for mounting public shares, which serves as a critical entry point for unauthorized access to shared data. Attackers can exploit this weakness to enumerate valid share tokens through automated scripts, potentially gaining access to sensitive data that has been publicly shared through Nextcloud's platform. The lack of rate limiting creates a scenario where attackers can perform high-volume requests without triggering protective measures, making it easier to discover valid tokens through systematic enumeration rather than relying on random guessing.
The operational impact of this vulnerability extends beyond simple token enumeration, as it fundamentally compromises the security model of Nextcloud's public sharing feature. When attackers successfully enumerate valid share tokens, they gain unauthorized access to files and data that were intended to be protected by the share link mechanism, potentially leading to data breaches, information disclosure, and unauthorized data access. This vulnerability particularly affects organizations that rely heavily on public sharing for collaboration, as it undermines the trust model that users place in share links. The exposure of valid tokens through automated enumeration can result in widespread unauthorized access to sensitive information, especially in environments where users create numerous public shares for various purposes including document collaboration, file distribution, and temporary access to specific datasets. Organizations may experience significant reputational damage and regulatory compliance issues if sensitive data is accessed through this vulnerability, as it represents a failure in access control mechanisms that should have prevented such unauthorized enumeration attempts.
Mitigation strategies for CVE-2021-32741 require immediate deployment of the patched versions 19.0.13, 20.0.11, and 21.0.3, as there are no known workarounds available for this specific vulnerability. System administrators should prioritize updating their Nextcloud installations to ensure proper rate limiting is implemented at the public share link mount endpoint. The fix addresses the root cause by implementing appropriate rate limiting controls that prevent excessive requests to the endpoint, thereby making automated enumeration attacks significantly more difficult. Organizations should also implement monitoring and logging of share link access patterns to detect potential exploitation attempts, as the implementation of rate limiting helps prevent the rapid iteration that enables successful enumeration. Additionally, security teams should review existing share link configurations and consider implementing additional access controls for sensitive data shared through Nextcloud's public sharing functionality. This vulnerability aligns with ATT&CK technique T1566, which covers credential harvesting through social engineering and automated attacks, as the enumeration process represents a form of automated credential discovery that can be used to gain unauthorized access to shared resources. The mitigation approach should include comprehensive testing of the updated systems to ensure that rate limiting is properly functioning and that legitimate users are not experiencing performance degradation due to overly restrictive controls.