CVE-2026-6550: AWS Encryption SDK Auth Bypass Vulnerability

CVE-2026-6550 Overview

A cryptographic algorithm downgrade vulnerability exists in the caching layer of Amazon AWS Encryption SDK for Python before versions 3.3.1 and 4.0.5. This weakness allows 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.

The vulnerability is classified under CWE-757 (Selection of Less-Secure Algorithm During Negotiation), indicating a fundamental flaw in how the SDK handles cryptographic algorithm selection when caching is enabled. This type of vulnerability undermines the integrity guarantees that key commitment is designed to provide, potentially allowing sophisticated attackers to craft malicious ciphertexts that appear valid.

Critical Impact

Authenticated local attackers can bypass key commitment policy enforcement, enabling ciphertext to be decrypted to multiple different plaintexts, compromising data integrity in cryptographic operations.

Affected Products

• Amazon AWS Encryption SDK for Python versions prior to 3.3.1
• Amazon AWS Encryption SDK for Python versions prior to 4.0.5

Discovery Timeline

• 2026-04-20 - CVE-2026-6550 published to NVD
• 2026-04-21 - Last updated in NVD database

Technical Details for CVE-2026-6550

Vulnerability Analysis

This vulnerability stems from improper handling of key commitment policies within the SDK's caching layer. Key commitment is a cryptographic property that ensures a ciphertext can only be decrypted to a single plaintext under a single key, preventing certain classes of attacks where adversaries might craft ciphertexts that decrypt differently under different keys.

When the caching layer is enabled, the SDK fails to properly enforce key commitment policies, allowing the use of less-secure cryptographic algorithms. This creates a scenario where an authenticated local attacker with access to the shared key cache can manipulate the cryptographic operations to produce ciphertexts that violate the key commitment guarantees.

The attack requires local access and authentication, meaning the threat actor must already have some level of access to the system running the vulnerable SDK. The complexity of exploitation is elevated due to the specific conditions required—the attacker must have access to the shared cache and understand the internal workings of the caching mechanism.

Root Cause

The root cause lies in the SDK's caching layer not properly validating or enforcing key commitment policies when retrieving cached cryptographic materials. When encryption operations utilize cached keys, the algorithm selection mechanism fails to verify that the cached materials meet the current key commitment policy requirements, allowing a downgrade to algorithms that do not provide key commitment guarantees.

Attack Vector

The attack requires local access to the system where the vulnerable AWS Encryption SDK for Python is deployed. An authenticated attacker with access to the shared key cache can exploit the policy enforcement gap to:

1. Access the shared key cache used by the encryption SDK
2. Manipulate or inject entries that reference weaker algorithms
3. Trigger encryption operations that use the compromised cache entries
4. Produce ciphertexts that can decrypt to multiple plaintexts under different keys

This attack specifically targets the integrity of encrypted data rather than confidentiality, as it allows an attacker to create ambiguous ciphertexts that violate key commitment properties.

Detection Methods for CVE-2026-6550

Indicators of Compromise

• Unexpected changes to the shared key cache files or in-memory cache structures
• Encryption operations completing with algorithms that don't match the configured key commitment policy
• Anomalous cache hit patterns indicating potential cache manipulation
• Log entries showing mismatched algorithm selections during decryption operations

Detection Strategies

• Audit all applications using AWS Encryption SDK for Python and verify version numbers against affected versions (< 3.3.1 or < 4.0.5)
• Monitor for unexpected local process access to SDK cache resources
• Implement logging for cryptographic algorithm selection during encryption/decryption operations
• Review application logs for key commitment policy violations or warnings

Monitoring Recommendations

• Enable verbose logging for AWS Encryption SDK operations to capture algorithm selection events
• Implement file integrity monitoring on any persistent cache storage used by the SDK
• Monitor for unusual local authentication attempts on systems running cryptographic workloads
• Deploy runtime application self-protection (RASP) to detect unexpected SDK behavior

How to Mitigate CVE-2026-6550

Immediate Actions Required

• Upgrade AWS Encryption SDK for Python to version 3.3.1 or higher (for 3.x branch)
• Upgrade AWS Encryption SDK for Python to version 4.0.5 or higher (for 4.x branch)
• Review all applications using the SDK to identify vulnerable deployments
• Clear existing key caches after upgrading to ensure clean cache state

Patch Information

Amazon has released security patches addressing this vulnerability. Users should upgrade to the patched versions immediately:

• Version 3.x users: Upgrade to 3.3.1 or later - GitHub Release v3.3.1
• Version 4.x users: Upgrade to 4.0.5 or later - GitHub Release v4.0.5

For complete details on the vulnerability and remediation, refer to the AWS Security Bulletin 2026-017 and the GitHub Security Advisory GHSA-v638-38fc-rhfv.

Workarounds

• Disable caching in the AWS Encryption SDK if upgrading immediately is not possible
• Restrict local access to systems running the vulnerable SDK to minimize exposure
• Implement strict key commitment policies at the application level as an additional validation layer
• Isolate processes using the encryption SDK from untrusted local users

# Upgrade AWS Encryption SDK for Python
pip install --upgrade aws-encryption-sdk>=3.3.1  # For 3.x branch
# OR
pip install --upgrade aws-encryption-sdk>=4.0.5  # For 4.x branch

# Verify installed version
pip show aws-encryption-sdk | grep Version