CVE-2023-28835 in Server
Summary
by MITRE • 03/30/2023
Nextcloud server is an open source home cloud implementation. In affected versions the generated fallback password when creating a share was using a weak complexity random number generator, so when the sharer did not change it the password could be guessable to an attacker willing to brute force it. It is recommended that the Nextcloud Server is upgraded to 24.0.10 or 25.0.4. This issue only affects users who do not have a password policy enabled, so enabling a password policy is an effective mitigation for users unable to upgrade.
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Analysis
by VulDB Data Team • 04/20/2023
The vulnerability identified as CVE-2023-28835 affects Nextcloud Server versions prior to 24.0.10 and 25.0.4, representing a significant security weakness in the file sharing functionality of this popular open source cloud storage platform. This issue stems from the implementation of fallback password generation when creating shares, where the system employs a weak random number generator that produces predictable and guessable passwords. The vulnerability specifically impacts scenarios where users do not actively set custom passwords for shared content, relying instead on the system-generated defaults. The flaw creates an exploitable condition where attackers with sufficient computational resources can perform brute force attacks against these weakly generated passwords, potentially gaining unauthorized access to shared files and directories within the Nextcloud environment.
The technical implementation of this vulnerability can be categorized under CWE-330 Use of Insufficiently Random Values, which specifically addresses the use of weak random number generators in security-critical contexts. The weakness manifests in the password generation algorithm that fails to produce cryptographically secure random values, making the fallback passwords susceptible to prediction and enumeration attacks. This vulnerability directly relates to the principle of least privilege and secure authentication practices as outlined in the NIST SP 800-63B standard for digital identity management. The insufficient randomness in password generation violates fundamental security requirements for cryptographic operations, particularly when dealing with authentication mechanisms that are meant to provide access control boundaries.
The operational impact of CVE-2023-28835 extends beyond simple unauthorized file access, potentially enabling attackers to compromise entire shared directories and access sensitive data that users intended to keep private. This vulnerability affects organizations using Nextcloud for business file sharing, collaboration platforms, and personal cloud storage solutions where share links are commonly used. The risk is particularly elevated in environments where users do not have password policies enforced, as these systems lack the additional security layer that would prevent weak password generation. Attackers can exploit this weakness through automated brute force tools that systematically test common password patterns against the predictable fallback values, making the attack surface significantly larger than typical authentication bypass scenarios. The vulnerability also demonstrates poor security hygiene in the software development lifecycle, as cryptographic functions should undergo rigorous testing and validation before deployment in production environments.
Organizations can mitigate this vulnerability through several approaches, with the primary recommendation being the immediate upgrade to Nextcloud Server versions 24.0.10 or 25.0.4 where the issue has been patched. The vulnerability is particularly relevant to the ATT&CK framework's T1110 credential access techniques, specifically targeting the use of weak or predictable passwords as a means of unauthorized access. For organizations unable to perform immediate upgrades, enabling a password policy serves as an effective compensating control that forces users to create stronger passwords, thereby eliminating the reliance on the vulnerable fallback mechanism. Additional mitigation strategies include implementing rate limiting on share creation requests, monitoring for unusual sharing patterns, and conducting regular security assessments of shared content. The vulnerability also highlights the importance of following security best practices such as those outlined in the OWASP Top Ten, particularly focusing on secure authentication and session management. Organizations should also consider implementing multi-factor authentication for critical shared resources and establishing incident response procedures to detect and respond to potential exploitation attempts.