CVE-2023-45148 in Nextcloud
Summary
by MITRE • 10/25/2023
Nextcloud is an open source home cloud server. When Memcached is used as `memcache.distributed` the rate limiting in Nextcloud Server could be reset unexpectedly resetting the rate count earlier than intended. Users are advised to upgrade to versions 25.0.11, 26.0.6 or 27.1.0. Users unable to upgrade should change their config setting `memcache.distributed` to `\OC\Memcache\Redis` and install Redis instead of Memcached.
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Analysis
by VulDB Data Team • 10/25/2023
The vulnerability CVE-2023-45148 affects Nextcloud server installations that utilize Memcached as their distributed memory cache system. This security flaw specifically impacts the rate limiting mechanism within Nextcloud's authentication and access control systems. The issue arises when Nextcloud employs Memcached for distributed caching operations, creating a scenario where the rate limiting counters can be unexpectedly reset during normal operational conditions. This unexpected reset occurs at intervals that are significantly earlier than the intended rate limiting duration, effectively undermining the security controls designed to prevent brute force attacks and unauthorized access attempts.
The technical implementation of this vulnerability stems from how Nextcloud manages distributed cache invalidation when Memcached serves as the memcache.distributed backend. When cache entries expire or are removed from Memcached, the rate limiting counters that track failed authentication attempts or access violations are inadvertently reset to their initial values. This behavior creates a window of opportunity for attackers to bypass rate limiting protections that should normally prevent rapid successive authentication attempts or access control violations. The flaw manifests as a race condition or cache invalidation timing issue where legitimate security measures are circumvented due to improper handling of distributed cache state management.
The operational impact of this vulnerability extends beyond simple authentication bypasses and represents a significant weakening of Nextcloud's security posture. Attackers can exploit this flaw to perform repeated authentication attempts against user accounts without encountering the normal rate limiting protections that would typically slow or block such activities. This vulnerability particularly affects systems that rely heavily on Nextcloud's built-in security controls for protecting user access and preventing automated attacks. The implications are especially concerning for organizations using Nextcloud as their primary file sharing and collaboration platform, where unauthorized access could result in data breaches, privilege escalation, or service disruption. The vulnerability affects Nextcloud versions prior to 25.0.11, 26.0.6, and 27.1.0, making it a widespread concern across multiple release branches.
Organizations affected by this vulnerability should prioritize upgrading to the patched versions mentioned in the advisory, as this represents the most straightforward and effective mitigation approach. However, for deployments unable to perform immediate upgrades, the recommended workaround involves changing the distributed cache configuration from Memcached to Redis. This solution addresses the core issue by leveraging a different caching mechanism that does not exhibit the same cache invalidation behavior. The transition to Redis also aligns with industry best practices for distributed caching in security-critical applications, as Redis provides more predictable cache behavior and better handling of expiration events. This configuration change effectively isolates the rate limiting functionality from the problematic Memcached implementation while maintaining the overall caching capabilities required for Nextcloud's performance optimization.
This vulnerability maps to CWE-362, which describes concurrent execution using shared resource vulnerabilities, and relates to the broader category of race condition issues in distributed systems. The ATT&CK framework categorizes this as a privilege escalation or credential access technique, specifically under T1110.003 for credential stuffing or T1110.004 for password spraying, where the reduced effectiveness of rate limiting enables attackers to conduct more aggressive authentication attempts. The vulnerability demonstrates the importance of proper cache invalidation handling in security-critical applications and highlights how seemingly minor configuration choices can have significant security implications. Organizations should also consider implementing additional monitoring and alerting around authentication attempts to detect potential exploitation of this vulnerability during the remediation period.