Defense in depth enhancement for region parameter value in AWS SDK for .NET V4

### Summary This notification is related to the use of specific values for the region input field when calling AWS services. An actor with access to the environment in which the SDK is used could set the region input field to an invalid value. A defense-in-depth enhancement has been implemented in the AWS SDK for .NET v4. This enhancement validates that a region used to construct an endpoint URL is a valid host label. The change was released on Nov 21 2025. This advisory is informational to help customers understand their responsibilities regarding configuration security. ### Impact Customer applications could be configured to improperly route AWS API calls to non-existent or non-AWS hosts. While the SDK was functioning safely within the requirements of the shared responsibility model, additional safeguards have been added to support secure customer implementations. Impacted versions: All versions of the AWS SDK for .NET V4 prior to the 4.0.139.0 release (the AWS SDK for .NET V3 was not impacted) ### Patches On Nov 21 2025, an enhancement [1] was made to the AWS SDK for .NET v4 release, which validates the formatting of a region, providing additional safeguards. ### Workarounds No workarounds are needed, but as always you should ensure that your application is following security best practices: - Implement proper input validation in your application code - Update to the latest AWS SDK for .NET release on a regular basis - Follow AWS security best practices [2] for SDK configuration ### References Contact AWS Security via our vulnerability reporting page or email [aws-security@amazon.com](mailto:aws-security@amazon.com). ### Acknowledgement We thank Guy Arazi for bringing these customer security considerations to our attention through the coordinated disclosure process. [1] https://github.com/aws/aws-sdk-net/pull/4150 [2] https://docs.aws.amazon.com/sdk-for-net/v4/developer-guide/security.html

CVSS 3.1

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

Severity from

GitHub (reviewed advisory)

• github.com/aws/aws-sdk-net/security/advisories/GHSA-9cvc-h2w8-phrp
• github.com/aws/aws-sdk-net