CVE-2025-14759 identifies a cryptographic vulnerability in the Amazon S3 Encryption Client for .NET, an open-source library designed to facilitate client-side encryption and decryption of data stored in Amazon S3 buckets. The core issue stems from the absence of cryptographic key commitment when encrypted data keys (EDKs) are stored in an "instruction file" rather than embedded within the S3 object's metadata. Key commitment is a cryptographic technique that binds the ciphertext to the key, preventing substitution attacks. Without this, an attacker with write access to the S3 bucket can replace the EDK with a malicious one that decrypts to different plaintext, effectively allowing undetected tampering with encrypted data. This vulnerability does not expose plaintext directly but undermines data integrity by enabling unauthorized modification of encrypted records. Exploitation requires the attacker to have write permissions on the S3 bucket, which is a moderate privilege level, and can be performed remotely over the network without user interaction. The vulnerability is classified under CWE-327, indicating the use of a broken or risky cryptographic algorithm or practice. AWS has released version 4.0.0 of the S3 Encryption Client for .NET to address this issue by implementing proper cryptographic key commitment. No known exploits are currently in the wild, but the vulnerability poses a significant risk to data integrity in environments relying on this client for encryption. Organizations using affected versions should upgrade immediately to prevent potential data manipulation attacks.

For European organizations, this vulnerability primarily threatens the integrity of encrypted data stored in Amazon S3 buckets when using the vulnerable .NET encryption client. Attackers with write access to the bucket can alter encrypted data undetected, potentially leading to corrupted records, business process disruptions, or injection of malicious data. Although confidentiality is not directly compromised, the integrity breach can have severe consequences in regulated industries such as finance, healthcare, and critical infrastructure, where data accuracy is paramount. The medium CVSS score reflects the moderate complexity of exploitation and the requirement for write permissions, which may limit exposure but does not eliminate risk. Given the widespread use of AWS services across Europe, especially in countries with strong cloud adoption, this vulnerability could impact data security and compliance with regulations like GDPR if data integrity is compromised. Organizations relying on the affected client for encryption should consider the risk of undetected data tampering and its downstream effects on business operations and regulatory compliance.

To mitigate this vulnerability, European organizations should immediately upgrade the Amazon S3 Encryption Client for .NET to version 4.0.0 or later, which includes the necessary cryptographic key commitment fixes. Additionally, organizations should audit and restrict write permissions on S3 buckets to the minimum necessary, employing the principle of least privilege to reduce the attack surface. Implement monitoring and alerting on unusual write activities or changes to encryption-related files, including instruction files, to detect potential tampering attempts. Employ AWS CloudTrail and S3 access logs to track and analyze access patterns. Consider integrating additional data integrity verification mechanisms at the application level, such as cryptographic hashes or digital signatures, to detect unauthorized modifications. Regularly review and update encryption libraries and dependencies to ensure they incorporate the latest security patches. Finally, conduct security awareness training for developers and administrators on secure cryptographic practices and the importance of key management.