Microsoft Security Advisory CVE-2026-118747 - .NET Denial of Service Vulnerability

Executive summary

Microsoft is releasing this security advisory to provide information about a vulnerability in .NET. This advisory provides guidance on updating affected applications and components.

Colliding CBOR map keys can hash-flood uniqueness checks; the fix uses seeded Marvin hashing.

Announcement

Announcement for this issue can be found at https://github.com/dotnet/announcements/issues/501

CVSS Details

• Version: 3.1
• Severity: High
• Score: 7.5 (base score)
• 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
• Weakness: CWE-407

Affected Platforms

• Platforms: Any supported OS
• Architectures: All

Affected Packages

The tables list affected package versions and fixed versions. These are independently versioned NuGet packages, not runtime patch versions.

8.0 - standalone packages

Package	Affected version	Patched version
System.Formats.Cbor	>= 8.0.0-0, < 8.0.2	8.0.2

9.0 - standalone packages

Package	Affected version	Patched version
System.Formats.Cbor	>= 9.0.0-0, < 9.0.24	9.0.24

Update 9.0 applications to System.Formats.Cbor 9.0.24; 9.0.23 is not a fix. The fixed 10.0.15 package, which includes a net9.0 target, remains an alternative.

10.0 - standalone packages

Package	Affected version	Patched version
System.Formats.Cbor	>= 10.0.0-0, < 10.0.15	10.0.15

11.0 - standalone packages

Package	Affected version	Patched version
System.Formats.Cbor	>= 11.0.0-0, < 11.0.0	11.0.0

Advisory FAQ

How do I know if I am affected?

Check the application's System.Formats.Cbor package reference against the affected ranges above. Updating only the installed runtime does not update this independently referenced package.

How do I fix the issue?

Update each applicable package reference to its patched version listed above, then rebuild and redeploy the application. Updating only the installed runtime does not update an application's standalone package references.

Other Information

Reporting Security Issues

If you have found a potential security issue in a supported version of .NET, please report it 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

You can ask questions about this issue 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. You can ask questions 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-2026-118747

Acknowledgements

• Anonymous

Revisions

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