AWS Encryption SDK (ESDK) for Python is a client-side encryption library. An issue exists where, under certain circumstances, a specific cryptographic algorithm downgrade in the caching layer might allow an authenticated local threat actor to bypass key commitment policy enforcement via a shared key cache, resulting in ciphertext that can be decrypted to multiple different plaintexts.
A weak robustness vulnerability exists in the AWS Encryption SDKs for Java, Python, C and Javalcript Due to the non-committing property of AES-GCM (and other AEAD ciphers such as AES-GCM-SIV or (X)ChaCha20Poly1305) used by the SDKs to encrypt messages, an attacker can craft a unique cyphertext which will decrypt to multiple different results, and becomes especially relevant in a multi-recipient setting. We recommend users update their SDK to or later.
This advisory addresses several LOW severity issues with streaming signed messages and restricting processing of certain types of invalid messages. This ESDK supports a streaming mode where callers may stream the plaintext of signed messages before the ECDSA signature is validated.