A weak robustness vulnerability exists in the AWS Encryption SDKs for Java, Python, C and Javalcript prior to versions 2.0.0. 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 2.0.0 or later.