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.
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)
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.
No workarounds are needed, but as always developers should ensure that their application is following security best practices:
Contact AWS Security via the vulnerability reporting page or email aws-security@amazon.com.
AWS SDK for .NET v4 thanks 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
4.0.3.3Exploitability
AV:NAC:HPR:NUI:NScope
S:UImpact
C:LI:NA:N3.7/CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N