Summary
Microsoft Security Advisory CVE-2026-45491 – .NET Tampering Vulnerability
Full technical description
Executive Summary
Microsoft is releasing this security advisory to provide information about a vulnerability in System.Formats.Tar. This advisory also provides guidance on what developers can do to update their applications to remove this vulnerability.
A tampering vulnerability exists in the TarFile.ExtractToDirectory method where a symlink path traversal enables an attacker to perform arbitrary file writes outside the intended extraction directory.
Announcement
Announcement for this issue can be found at https://github.com/dotnet/announcements/issues/404
Affected Platforms
- Platforms: All
- Architectures: All
Affected Packages
The vulnerability affects any Microsoft .NET project if it uses any of affected package versions listed below
.NET 10
| Package name | Affected version | Patched version |
|---|---|---|
| Microsoft.NETCore.App.Runtime | >= 10.0.0, <= 10.0.8 | 10.0.9 |
.NET 9
| Package name | Affected version | Patched version |
|---|---|---|
| Microsoft.NETCore.App.Runtime | >= 9.0.0, <= 9.0.16 | 9.0.17 |
.NET 8
| Package name | Affected version | Patched version |
|---|---|---|
| Microsoft.NETCore.App.Runtime | >= 8.0.0, <= 8.0.27 | 8.0.28 |
Advisory FAQ
How do I know if I am affected?
If using a package listed in affected packages, you're exposed to the vulnerability.
How do I fix the issue?
- To fix the issue please install the latest version of .NET 8.0, .NET 9.0, or .NET 10.0, as appropriate. If you have installed one or more .NET SDKs through Visual Studio, Visual Studio will prompt you to update Visual Studio, which will also update your .NET SDKs.
- If your application references the vulnerable package, update the package reference to the patched version.
Once you have installed the updated runtime or SDK, restart your apps for the update to take effect.
Additionally, if you've deployed self-contained applications targeting any of the impacted versions, these applications are also vulnerable and must be recompiled and redeployed.
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.
Acknowledgements
Anonymous, Ashmit Sharma, Ky0toFu ,phrolo7a
Revisions
V1.0 (June 9, 2026): Advisory published.
Impact
CVE-2026-45491 has a CVSS score of 6.8 (Medium). The vector is requires local access, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (8.0.28, 9.0.17, 10.0.9); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
Microsoft.NETCore.App.Runtime.linux-x64 to 8.0.28 or later; Microsoft.NETCore.App.Runtime.linux-x64 to 9.0.17 or later; Microsoft.NETCore.App.Runtime.linux-x64 to 10.0.9 or later; Microsoft.NETCore.App.Runtime.win-x64 to 8.0.28 or later; Microsoft.NETCore.App.Runtime.win-x64 to 9.0.17 or later; Microsoft.NETCore.App.Runtime.win-x64 to 10.0.9 or later; Microsoft.NETCore.App.Runtime.osx-arm64 to 8.0.28 or later; Microsoft.NETCore.App.Runtime.osx-arm64 to 9.0.17 or later; Microsoft.NETCore.App.Runtime.osx-arm64 to 10.0.9 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-45491? CVE-2026-45491 is a medium-severity security vulnerability in Microsoft.NETCore.App.Runtime.linux-x64 (nuget), affecting versions >= 8.0.0, <= 8.0.27. It is fixed in 8.0.28, 9.0.17, 10.0.9.
- How severe is CVE-2026-45491? CVE-2026-45491 has a CVSS score of 6.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which packages are affected by CVE-2026-45491?
Microsoft.NETCore.App.Runtime.linux-x64(nuget) (versions >= 8.0.0, <= 8.0.27)Microsoft.NETCore.App.Runtime.win-x64(nuget) (versions >= 8.0.0, <= 8.0.27)Microsoft.NETCore.App.Runtime.osx-arm64(nuget) (versions >= 8.0.0, <= 8.0.27)
- Is there a fix for CVE-2026-45491? Yes. CVE-2026-45491 is fixed in 8.0.28, 9.0.17, 10.0.9. Upgrade to this version or later.
- Is CVE-2026-45491 exploitable, and should I be worried? Whether CVE-2026-45491 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-45491 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-45491?
- Upgrade
Microsoft.NETCore.App.Runtime.linux-x64to 8.0.28 or later - Upgrade
Microsoft.NETCore.App.Runtime.linux-x64to 9.0.17 or later - Upgrade
Microsoft.NETCore.App.Runtime.linux-x64to 10.0.9 or later - Upgrade
Microsoft.NETCore.App.Runtime.win-x64to 8.0.28 or later - Upgrade
Microsoft.NETCore.App.Runtime.win-x64to 9.0.17 or later - Upgrade
Microsoft.NETCore.App.Runtime.win-x64to 10.0.9 or later - Upgrade
Microsoft.NETCore.App.Runtime.osx-arm64to 8.0.28 or later - Upgrade
Microsoft.NETCore.App.Runtime.osx-arm64to 9.0.17 or later - Upgrade
Microsoft.NETCore.App.Runtime.osx-arm64to 10.0.9 or later
- Upgrade