CVE-2026-118747 — System.Formats.Cbor
HIGHCVE-2026-118747 is a high-severity (CVSS 7.5) CWE-407 vulnerability in System.Formats.Cbor. A fix is available for System.Formats.Cbor — see the affected versions and patch details below.
Microsoft Security Advisory CVE-2026-118747 - .NET Denial of Service Vulnerability
Real-World Exposure
System.Formats.Cbor.NETSystem.Formats.Cbor.NETSystem.Formats.Cbor.NETSystem.Formats.CborReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.
Description
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
<a name="affected-packages"></a>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
<a name="how-affected"></a>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.
<a name="how-fix"></a>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
Acknowledgements
- Anonymous
Revisions
V1.0 (November 10, 2026): Advisory published.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | System.Formats.Cbor | ≥ 8.0.0-0&&< 8.0.2 | 8.0.2dotnet add package System.Formats.Cbor --version 8.0.2 |
| .NETNuGet | System.Formats.Cbor | ≥ 9.0.0-0&&< 9.0.24 | 9.0.24dotnet add package System.Formats.Cbor --version 9.0.24 |
| .NETNuGet | System.Formats.Cbor | ≥ 10.0.0-0&&< 10.0.15 | 10.0.15dotnet add package System.Formats.Cbor --version 10.0.15 |
| .NETNuGet | System.Formats.Cbor | ≥ 11.0.0-0&&< 11.0.0 | 11.0.0dotnet add package System.Formats.Cbor --version 11.0.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for System.Formats.Cbor, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update System.Formats.Cbor to 8.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-118747 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2026-118747 in your dependencies?
Find it across NuGet, including transitive dependencies.