Microsoft Security Advisory CVE-2026-26127 – .NET Denial of Service Vulnerability

Executive summary

Microsoft is releasing this security advisory to provide information about a vulnerability in .NET 9.0 and .NET 10.0. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.

A denial of service vulnerability exists in .NET and Microsoft.Bcl.Memory due to an out-of-bounds read when decoding malformed Base64Url input.

Announcement

Announcement for this issue can be found at https://github.com/dotnet/announcements/issues/384

CVSS Details

• Version: 3.1
• Score: 7.5
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H/E:U/RL:O/RC:C
• Severity: High
• Weakness: CWE-129 (Improper Validation of Array Index); CWE-125 (Out-of-bounds Read)

Affected Platforms

• Platforms: All
• Architectures: All

Affected Products

.NET 9

.NET 10

Microsoft.Bcl.Memory 9.0.14

Microsoft.Bcl.Memory 10.0.4

How to fix the issue

To update the Microsoft.Bcl.Memory NuGet package, apply one of the following methods:

NuGet Package Manager UI in Visual Studio:

• Open the project in Visual Studio.
• Right-click on the project in Solution Explorer and select "Manage NuGet Packages..." or navigate to "Project > Manage NuGet Packages".
• In the NuGet Package Manager window, select the "Updates" tab. This tab lists packages with available updates from the configured package sources.
• Select the package(s) to update. A specific version can be chosen from the dropdown, or the latest available version can be selected.
• Click the "Update" button.

Using the NuGet Package Manager Console in Visual Studio:

• Open the project in Visual Studio.
• Navigate to "Tools > NuGet Package Manager > Package Manager Console".
• To update a specific package to its latest version, run the following Update-Package command:

Update-Package -Id Microsoft.Bcl.Memory

Using the .NET CLI (Command Line Interface):

• Open a terminal or command prompt in the project's directory.
• To update a specific package to its latest version, run the following command:

dotnet package update Microsoft.Bcl.Memory

Once the NuGet package reference has been updated, the application must be recompiled and redeployed. Additionally, it is recommended to update the runtime and/or SDKs, but unless the application targets .NET 9 or higher, updating the runtime and/or SDK is not a required step to patch this vulnerability.

Other Information

Reporting Security Issues

If a potential security issue has been found in a supported version of .NET, it should be reported to the Microsoft Security Response Center (MSRC) via the MSRC Researcher Portal. Further information can be found in the MSRC Report an Issue FAQ.

Security reports made through MSRC may qualify for the Microsoft .NET Bounty. Details of the Microsoft .NET Bounty Program including terms and conditions are at https://aka.ms/corebounty.

Support

Questions about this issue can be asked on GitHub in the .NET GitHub organization. The main repos are located at https://github.com/dotnet/runtime. The Announcements repo (https://github.com/dotnet/Announcements) will contain this bulletin as an issue and will include a link to a discussion issue. Questions can be asked in the linked discussion issue.

Disclaimer

The information provided in this advisory is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.

External Links

CVE-2026-26127

Revisions

V1.0 (March 10, 2026): Advisory published.