EUVD-2026-23943
GHSA-v638-38fc-rhfv
Severity by source
CVSS:4.0/AV:L/AC:H/AT:P/PR:L/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Primary rating from NVD · only source for this CVE.
CVSS VectorNVD
CVSS:4.0/AV:L/AC:H/AT:P/PR:L/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Lifecycle Timeline
6Blast Radius
ecosystem impact- 1 pypi packages depend on aws-encryption-sdk (1 direct, 0 indirect)
Ecosystem-wide dependent count for version 2.0.0.
DescriptionCVE.org
Cryptographic algorithm downgrade in the caching layer of Amazon AWS Encryption SDK for Python before version 3.3.1 and before version 4.0.5 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.
To remediate this issue, users should upgrade to version 3.3.1, 4.0.5 or above.
AnalysisAI
Cryptographic algorithm downgrade in AWS Encryption SDK for Python's caching layer allows authenticated local attackers to bypass key commitment policy enforcement through a shared key cache, enabling decryption of single ciphertext to multiple different plaintexts. Affected versions include Python 2 up to 2.5.1, Python 3 up to 3.3.0, and Python 4 up to 4.0.4. AWS has released vendor patches (versions 3.3.1, 4.0.5, and later) to remediate the vulnerability, which requires local access and authenticated credentials but has no known public exploit.
Technical ContextAI
The AWS Encryption SDK for Python implements cryptographic operations with key commitment policies to ensure that encrypted data binds to a specific decryption key, preventing key confusion attacks. The vulnerability resides in the caching layer (CWE-757: Selection of Less-Secure Cryptographic Algorithm) where cached key material may be leveraged to downgrade the effective cryptographic algorithm protections. When applications share a key cache across authenticated users or contexts, an authenticated local threat actor can manipulate cache behavior to bypass policy enforcement mechanisms, effectively allowing the same ciphertext to decrypt to different plaintexts depending on which cached key is selected during decryption. This breaks the cryptographic guarantee that each ciphertext is bound to exactly one plaintext under a given key.
RemediationAI
Upgrade AWS Encryption SDK for Python to version 3.3.1, 4.0.5, or above as released by AWS. For Python 2 users still in maintenance windows, contact AWS Support for guidance on the appropriate patch version for your environment. No workarounds are available for the underlying caching mechanism flaw. If immediate patching is not feasible, organizations should implement compensating controls by disabling key caching entirely (with awareness of performance trade-offs - each encryption/decryption operation will require full key derivation or KMS calls, increasing latency), isolating key caches to single-user or single-request scopes to prevent cross-user cache poisoning, or segregating applications using shared caching into separate processes or containers to eliminate cache sharing vectors. Verify patch deployment by confirming version numbers in your Python environment (pip show aws-encryption-sdk or equivalent) and reviewing AWS security bulletin 2026-017 and GitHub release notes at https://github.com/aws/aws-encryption-sdk-python/releases/tag/v4.0.5 and https://github.com/aws/aws-encryption-sdk-python/releases/tag/v3.3.1.
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External POC / Exploit Code
Leaving vuln.today