CVE-2021-32727 in Nextcloud
Summary
by MITRE • 07/13/2021
Nextcloud Android Client is the Android client for Nextcloud. Clients using the Nextcloud end-to-end encryption feature download the public and private key via an API endpoint. In versions prior to 3.16.1, the Nextcloud Android client skipped a step that involved the client checking if a private key belonged to a previously downloaded public certificate. If the Nextcloud instance served a malicious public key, the data would be encrypted for this key and thus could be accessible to a malicious actor. The vulnerability is patched in version 3.16.1. As a workaround, do not add additional end-to-end encrypted devices to a user account.
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Analysis
by VulDB Data Team • 07/16/2021
The vulnerability identified as CVE-2021-32727 affects the Nextcloud Android client application that serves as the mobile interface for the Nextcloud cloud storage platform. This particular flaw resides within the implementation of the end-to-end encryption feature, which is a critical security mechanism designed to protect user data from unauthorized access. The vulnerability specifically impacts versions prior to 3.16.1, where the client application fails to perform a crucial validation step in the key management process. The Nextcloud platform employs a public key infrastructure approach where users maintain pairs of public and private keys to encrypt and decrypt their data, with the public keys being distributed through API endpoints and private keys being stored locally on the client devices. The compromised security model stems from the client's failure to verify that the private key it downloads corresponds to the public certificate that was previously obtained from the server, creating a potential attack vector that undermines the fundamental security assumptions of the encryption system.
The technical flaw manifests in the client's handling of cryptographic key pairs during the end-to-end encryption process, which aligns with CWE-327, a weakness related to the use of insecure cryptographic algorithms or improper implementation of cryptographic functions. When a Nextcloud instance serves a malicious public key, the Android client continues to proceed with encryption operations without validating the key association, effectively allowing data to be encrypted using an attacker-controlled public key. This creates a scenario where sensitive information is accessible to malicious actors who have compromised the Nextcloud instance, as they can intercept the encrypted data and potentially decrypt it using their own private key that corresponds to the malicious public key they provided. The vulnerability represents a breakdown in the certificate validation process that should ensure cryptographic key integrity, specifically violating the principle of key binding that ensures private keys are properly associated with their corresponding public certificates.
The operational impact of this vulnerability extends beyond simple data confidentiality breaches, as it compromises the entire trust model of the end-to-end encryption system. Attackers who gain control of a Nextcloud instance can manipulate the encryption process to their advantage, potentially accessing user data that should remain protected. This weakness creates a persistent security risk for all users of affected versions, particularly those who store sensitive information on Nextcloud servers. The vulnerability is particularly concerning because it operates at the client-side level, meaning that even if the server-side infrastructure remains secure, the client application's failure to validate cryptographic integrity can still result in data exposure. The implications affect not only individual users but also organizations that rely on Nextcloud for secure file storage and collaboration, as the vulnerability undermines the cryptographic protection that users expect from end-to-end encryption implementations.
The security implications of CVE-2021-32727 align with several ATT&CK framework techniques including T1566, which covers social engineering attacks, and T1552, which addresses credentials theft and access. The vulnerability creates an attack surface where malicious actors can exploit the trust relationship between the client and server to gain unauthorized access to encrypted data. The patch released in version 3.16.1 addresses this issue by implementing proper key validation mechanisms that ensure private keys are correctly associated with their corresponding public certificates before encryption operations are performed. Organizations should prioritize updating to version 3.16.1 or later to remediate this vulnerability, as the workaround of not adding additional encrypted devices to user accounts is insufficient to provide comprehensive protection. The fix implements proper cryptographic validation procedures that align with industry standards for secure key management and certificate validation, ensuring that the integrity of the end-to-end encryption system is maintained throughout the client-server communication process. This vulnerability demonstrates the critical importance of proper cryptographic implementation and validation in client-side applications, as even minor oversights in key management can lead to significant security compromises.