Selection of Less-Secure Algorithm During Negotiation ('Algorithm Downgrade') Affecting aws-encryption-sdk package, versions [,3.3.1)[4.0.0,4.0.5)
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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- published21 Apr 2026
- disclosed20 Apr 2026
- credit1seal
Introduced: 20 Apr 2026
CVE-2026-6550 (opens in a new tab)How to fix?
Upgrade aws-encryption-sdk to version 3.3.1, 4.0.5 or higher.
Overview
aws-encryption-sdk is an AWS Encryption SDK implementation for Python
Affected versions of this package are vulnerable to Selection of Less-Secure Algorithm During Negotiation ('Algorithm Downgrade') via the shared key cache. An attacker can bypass key commitment policy enforcement by exploiting the caching mechanism, potentially causing ciphertext to be decrypted into multiple different plaintexts.
Note:
This is only exploitable if all of the following conditions are met:
Two ESDK for Python clients with different commitment policies share a single
CachingCryptoMaterialsManagerinstance within the same process;The client with the weaker commitment policy encrypts first, warming the cache;
Both clients use matching encryption contexts;
Both clients use the pre-configured default algorithm suite.