CVE-2020-8911: 
cAdvisor vulnerability analysis and mitigation

A padding oracle vulnerability (CVE-2020-8911) was discovered in the AWS S3 Crypto SDK for GoLang versions prior to V2. The vulnerability exists in the SDK's implementation that allows users to encrypt files with AES-CBC without computing a Message Authentication Code (MAC) on the data (AWS Blog, GitHub Advisory).

The vulnerability stems from the SDK's support for CBC mode encryption without a message authentication code (MAC) that checks the ciphertext prior to decryption. This implementation flaw leads to a padding oracle issue similar to those found in SSL/TLS. The SDK's V1 allowed users to encrypt files with AES-CBC mode without proper integrity checks, although AES-GCM was available as an alternative encryption option (AWS Blog).

An attacker with write access to an S3 bucket and the ability to observe decryption failures (without seeing the plaintext) could potentially reconstruct the plaintext data. The attack requires approximately 128 attempts per byte of the encrypted content. This vulnerability poses insider risks and privilege escalation risks, potentially circumventing KMS controls for stored data (GitHub Advisory).

The exploitation requires specific conditions: write access to the affected S3 bucket, access to an endpoint that reveals decryption failures without exposing the plaintext, and the use of CBC option as the content cipher during encryption. A proof of concept exploit has been developed and documented (GitHub Advisory).

The vulnerability has been addressed in Version 2 of the S3 Crypto SDK by disabling encryption with CBC mode for new files. However, existing files that were encrypted with CBC mode remain vulnerable until they are re-encrypted using AES-GCM. Users are advised to upgrade to Version 2 of the SDK to prevent creating new vulnerable files (AWS Blog).