.NET: Security feature bypass in Negotiate authentication
Summary
A flaw in the Unix Negotiate authentication of the .NET runtime causes the server to fail open: a request is accepted when required channel-binding information is missing. An unauthenticated attacker with a network position to relay credentials and user interaction (UI:R) can bypass the security feature. CVSS 3.1 is 6.8 (Medium). The flaw only affects .NET 11 prerelease builds on Linux and macOS according to the sources reviewed, and no exploitation is known.
Affected
- Microsoft .NET 11.0 on Linux (official CVE record: 11.0.0-preview.1 to below 11.0.0)
- Microsoft .NET 11.0 on macOS (official CVE record: 11.0.0-preview.1 to below 11.0.0)
- Microsoft's announcement mirror lists Linux previews 1-6 and release candidates 1-3, and macOS previews 1-6, so the exact range is conflicting and must be checked against the MSRC advisory
- Not listed as affected: .NET 8, .NET 9, .NET 10, Windows
Fixed in
- Upgrade to .NET 11.0.0 or later
- Self-contained applications must be rebuilt and redeployed, not only updated at machine level
Workaround
Unknown. No vendor workaround is documented. Until patched, avoid server-side Negotiate with extended protection set to Always on Linux or macOS prerelease builds.
Exploitation
- No evidence of in-the-wild exploitation; CISA SSVC lists exploitation as none
- Not listed in CISA KEV according to the research sources
- No public proof-of-concept found; the fix pull request is public
- Rapid7 rates exploitation as less likely
Our recommendation
- Inventory .NET 11 prerelease SDKs and runtimes on Linux and macOS servers, including development, test and CI hosts
- Move any .NET 11 prerelease workloads to .NET 11.0.0 or later and rebuild self-contained applications
- Until patched, avoid server-side Negotiate with extended protection set to Always on prerelease builds
- If no such systems exist, the priority is low
Information from Microsoft (Patch Tuesday November 2026)
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft Security Update Guide: CVE-2026-118746 msrc.microsoft.com
- .NET announcement issue 498 github.com
- .NET runtime issue 137282 github.com
- .NET runtime pull request 137268 (fix) github.com
- .NET and .NET Framework November 2026 servicing updates devblogs.microsoft.com
- BleepingComputer: Microsoft November 2026 Patch Tuesday www.bleepingcomputer.com
Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 11 Nov 2026, 10:41.
Original description (NVD)
Not failing securely ('failing open') in .NET allows an unauthorized attacker to bypass a security feature over a network.
NVD references (1)
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.
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