AWS Encryption SDK for Python: Key commitment policy bypass via shared key cache

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.7 · 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 · score source

EPSS

• CVE-2026-6550: 0.10% estimated exploitation activity in the next 30 days; percentile 0.68 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary AWS Encryption SDK (ESDK) for Python is a client-side encryption library. An issue exists where, under certain circumstances, a specific cryptographic algorithm downgrade in the caching layer 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.

Impact This issue requires all of the following conditions to be true: (1) Two ESDK for Python clients with different commitment policies share a single CachingCryptoMaterialsManager instance within the same process. (2) The client with the weaker commitment policy encrypts first, warming the cache. (3) Both clients use matching encryption contexts. (4) Both clients use the pre-configured default algorithm suite. These conditions may occur during a migration from ESDK for Python v1 to newer versions, as v1 did not support key commitment. When the weaker-policy client encrypts first, the…

Published by the source

24 Apr 2026, 15:59 UTC

Last modified by the source

13 Jul 2026, 16:43 UTC

Source last checked

25 Oct 2026, 04:38 UTC

1. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-v638-38fc-rhfv

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# Patch Next advisory brief: CVE-2026-6550

Advisory: AWS Encryption SDK for Python: Key commitment policy bypass via shared key cache
IDs: CVE-2026-6550, GHSA-v638-38fc-rhfv
Record: https://patchnext.com/advisories/cve-2026-6550 (revision 2, source checked 25 Oct 2026, 04:38 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: aws-encryption-sdk 4.0.5, 3.3.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI aws-encryption-sdk: affected >= 2.0.0, < 3.3.1; >= 4.0.0, < 4.0.5; fixed in 3.3.1, 4.0.5

## Suggested inspection (read-only)
- `python -m pip show aws-encryption-sdk`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on aws-encryption-sdk.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-v638-38fc-rhfv (OSV record GHSA-v638-38fc-rhfv lists affected ranges: PyPI aws-encryption-sdk >=2.0.0 =4.0.0 <4.0.5 (fixed))

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