CVE-2022-31118 is a medium-severity vulnerability affecting Nextcloud Server, an open-source personal cloud solution widely used for file sharing and collaboration. The vulnerability arises from improper allocation of resources without adequate limits or throttling (CWE-770) in the federated sharing feature of Nextcloud. Specifically, an attacker can perform brute force attacks to determine whether federated sharing is enabled on a target Nextcloud instance. More critically, the attacker can attempt to brute force access tokens used for federated shares. These tokens consist of alphanumeric characters (a-z, A-Z, 0-9) with a length of 15 characters, which, while complex, may still be susceptible to brute forcing if no rate limiting or throttling mechanisms are in place. The vulnerability affects Nextcloud versions prior to 22.2.9, versions from 23.0.0 up to but not including 23.0.6, and versions from 24.0.0 up to but not including 24.0.2. The recommended mitigation is to upgrade to fixed versions 22.2.9, 23.0.6, or 24.0.2. For users unable to upgrade immediately, disabling federated sharing via the Admin Sharing settings is advised to prevent exploitation. No known exploits have been reported in the wild as of the publication date (August 4, 2022). The vulnerability does not require user interaction or authentication to attempt brute forcing, increasing its risk profile. However, the complexity of the token and the need for repeated attempts without throttling are key factors in exploitation feasibility.

For European organizations using Nextcloud Server, especially those leveraging federated sharing for inter-organizational collaboration, this vulnerability poses a risk to confidentiality and potentially integrity. Successful brute forcing of access tokens could allow unauthorized access to shared files and data, leading to data leakage or unauthorized data manipulation. This is particularly concerning for sectors handling sensitive information such as government agencies, healthcare providers, financial institutions, and critical infrastructure operators. The absence of throttling means attackers can attempt numerous guesses rapidly, increasing the likelihood of token compromise over time. Additionally, compromised federated shares could be used as a pivot point for further lateral movement within connected networks. Although availability impact is limited, the breach of confidentiality and integrity can have significant regulatory and reputational consequences under GDPR and other European data protection laws.

1. Immediate upgrade of Nextcloud Server to versions 22.2.9, 23.0.6, or 24.0.2 to apply official patches addressing this vulnerability. 2. For environments where immediate upgrade is not feasible, disable federated sharing via the Admin Sharing settings (`index.php/settings/admin/sharing`) to prevent brute force attempts on access tokens. 3. Implement network-level rate limiting and intrusion detection systems to monitor and block excessive authentication or token validation attempts targeting Nextcloud instances. 4. Enforce strong access controls and logging around federated sharing features to detect anomalous access patterns. 5. Educate administrators on monitoring Nextcloud logs for repeated failed token validation attempts. 6. Consider deploying Web Application Firewalls (WAFs) with custom rules to detect and block brute force patterns against federated sharing endpoints. 7. Regularly audit federated shares and access tokens to revoke any suspicious or unused tokens. 8. Integrate Nextcloud with centralized authentication and multi-factor authentication (MFA) solutions where possible to reduce reliance on token-based access alone.