CVE-2026-96958

Overview

.NET is a free, open-source, cross-platform framework for building modern apps and powerful cloud services. It consists of a runtime and a developer platform made up of tools, programming languages, and libraries for building many different types of applications.

A denial of service vulnerability (CVE-2026-96958) has been identified in the XML canonicalization used by .NET's Signed XML implementation, System.Security.Cryptography.Xml, which allows an unauthenticated attacker to exhaust CPU on an application that verifies attacker-supplied signed XML by sending elements that carry very large numbers of namespace declarations.

Per CWE-407: Inefficient Algorithmic Complexity, an algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker.

This issue affects .NET 6, including NES for .NET releases through 6.0.46. System.Security.Cryptography.Xml ships in-box in the ASP.NET Core and Windows Desktop shared frameworks, so every ASP.NET Core, WPF, and Windows Forms application carries it without a package reference, and it is also published as a standalone NuGet package. Applications are exposed when they verify or compute signatures over untrusted XML through SignedXml, as SAML, WS-Federation, and WS-Trust token handling does. Microsoft's advisory lists the System.Security.Cryptography.Xml package for 8.0, 9.0, 10.0 and the 11.0 previews as affected; .NET 6 is past end of life and is not assessed there.

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.

Details

Module Info

• Product: .NET
• Affected packages: Microsoft.AspNetCore.App and Microsoft.WindowsDesktop.App (in-box copies of System.Security.Cryptography.Xml); System.Security.Cryptography.Xml (standalone NuGet package)
• Affected versions:
  
  • Microsoft.AspNetCore.App, Microsoft.WindowsDesktop.App >= 6.0.0 <= 6.0.46
  • System.Security.Cryptography.Xml >= 6.0.0 <= 6.0.2.2
• Package manager: NuGet, Windows Installer, Docker
• GitHub repository: https://github.com/dotnet/runtime
• Published packages: Download .NET (Linux, macOS, and Windows); System.Security.Cryptography.Xml on NuGet
• Fixed in: NES for .NET 6.0.47 (System.Security.Cryptography.Xml 6.0.2.3)

Vulnerability Info

This High-severity vulnerability is found in the canonicalization transforms of System.Security.Cryptography.Xml as shipped for .NET 6. Before computing or checking a digest, SignedXml canonicalizes the signed content with C14N or exclusive C14N, which tracks the attributes and namespace declarations in scope for every element it renders. Nothing bounded how many an element could carry, and the cost of tracking them grew quadratically, so a single element with a very large number of namespace declarations could tie up a CPU core during one signature check.

NES for .NET 6.0.47 renders at most 512 attributes and 512 namespace declarations per element in both C14N and exclusive C14N, configurable through the AppContext setting System.Security.Cryptography.Xml.DangerousMaxCanonicalizationAttributesPerElement. Documents that exceed the limit are rejected with a CryptographicException.

CVE-2026-58280, CVE-2026-69651, CVE-2026-96958, CVE-2026-96959 and CVE-2026-96960 were reported separately and are addressed together by the same Signed XML hardening in NES for .NET 6.0.47, matching Microsoft's fix.

This vulnerability has been present since at least .NET 6.0 and likely earlier.

Mitigation

.NET 6 is End-of-Life and will not receive any updates to address this issue. For more information see .NET and .NET Core official support policy.

Users of the affected components should apply one of the following mitigations:

• Upgrade affected applications to one of:
  
  • NES for .NET 6.0.47 or later
  • System.Security.Cryptography.Xml 8.0.5 or later (.NET 8)
  • System.Security.Cryptography.Xml 9.0.21 or later (.NET 9)
  • System.Security.Cryptography.Xml 10.0.13 or later (.NET 10)
• Microsoft's advisory also directs .NET 8, 9 and 10 users to install the October runtime (8.0.32, 9.0.21 or 10.0.13), which updates the in-box copy.
• Leverage a commercial support partner like HeroDevs for post-EOL security support, including NES for .NET 6.0.47 or later. Applications with an explicit System.Security.Cryptography.Xml package reference should also move it to 6.0.2.3 from the NES feed, because NuGet otherwise keeps resolving Microsoft's unpatched 6.0.2.

Credits

Mateusz Gierblinski with Curios, Leandro Batista, Kevin Schmidt, and Dr.-Ing Ashar Javed (@soaj1664ashar), as credited in Microsoft's advisory.