CVE-2025-14759 identifies a cryptographic vulnerability in the Amazon S3 Encryption Client for .NET, specifically related to the absence of cryptographic key commitment when storing the encrypted data key (EDK) in an "instruction file" instead of the standard S3 metadata record. Key commitment is a cryptographic technique that binds the ciphertext to the key, preventing substitution attacks. Without this, an attacker with write permissions to the S3 bucket can replace the EDK with a maliciously crafted one that decrypts to different plaintext, effectively enabling data tampering or integrity violations. This vulnerability falls under CWE-327, indicating the use of a broken or risky cryptographic algorithm or protocol. The attack vector is network-based (AV:N), requires low privileges (PR:L), and no user interaction (UI:N), but has a higher attack complexity (AC:H) due to the need for write access and knowledge of the encryption client behavior. The impact is primarily on data integrity (I:H), with no direct confidentiality or availability impact. AWS has released a fix in version 3.2.0 of the S3 Encryption Client for .NET, which implements proper key commitment to prevent such substitution attacks. No known exploits are reported in the wild as of now.

For European organizations, this vulnerability poses a significant risk to the integrity of encrypted data stored in S3 buckets when using the vulnerable .NET encryption client. Attackers with write access to the bucket could manipulate encrypted data keys, causing data to decrypt incorrectly or maliciously altered, potentially leading to corrupted data, application errors, or unauthorized data manipulation. This can undermine trust in data integrity, disrupt business processes relying on accurate data, and complicate forensic investigations. While confidentiality is not directly compromised, the integrity breach can have cascading effects on compliance with data protection regulations such as GDPR, especially if data tampering affects personal or sensitive information. Organizations relying heavily on AWS S3 for critical data storage and using the .NET encryption client are at higher risk. The medium CVSS score reflects moderate exploitability and significant integrity impact, emphasizing the need for timely remediation.

European organizations should immediately upgrade the Amazon S3 Encryption Client for .NET to version 3.2.0 or later to ensure proper cryptographic key commitment is enforced. Additionally, organizations should audit their S3 bucket permissions to restrict write access strictly to trusted users and services, minimizing the risk of unauthorized key substitution. Implement monitoring and alerting on changes to S3 bucket contents, especially instruction files related to encryption metadata. Conduct regular integrity checks on encrypted data and validate encryption client versions in use across development and production environments. Incorporate cryptographic best practices by reviewing encryption key management policies and ensure that all cryptographic libraries are up to date. Finally, consider implementing additional layers of data integrity verification outside of the encryption client, such as application-level checksums or digital signatures, to detect tampering attempts.