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ASP.NET Core Negotiate LDAP authentication handles identity verification in enterprise environments.
When applications process incoming authentication requests via Negotiate, they use LDAP claim resolution.
The framework queries directory services to resolve user attributes and group memberships during login.
A critical flaw exists in how the framework processes returned LDAP search results during this flow.
The vulnerability arises because the application fails to verify the retrieved LDAP search result.
Specifically, the LDAP result is not properly verified against the expected Kerberos principal SID.
Without this crucial validation check, identity association flaws can occur within the system.
An attacker can manipulate or exploit the unverified LDAP response handling during identity mapping.
Consequently, the authentication mechanism improperly associates an authenticated identity with an unintended principal.
An unauthorized user could potentially assume privileges associated with a different LDAP principal.
The root cause stems from insufficient structural binding between Kerberos and LDAP query results.
Enterprise setups utilizing LDAP claim resolution are directly exposed to this spoofing vector.
If directory lookup results lack SID verification, identity mismatches go completely undetected.
The authentication handler blindly trusts the returned LDAP object instead of cross-referencing SIDs.
This behavior represents CWE-290, classified as Authentication Bypass by Spoofing.
Exploitation requires LDAP claim resolution to be enabled within the target application.
Network-based attackers can leverage this gap to bypass access controls or elevate privileges.
Updating affected packages and restarting or redeploying applications is mandatory to resolve it.
DailyCVE Form:
Platform: Microsoft .NET
Version: ASP.NET Core
Vulnerability : Authentication Bypass
Severity: High
date: 10/13/2026
Prediction: October Patch Tuesday
What Undercode Say:
To check installed .NET versions and manage package references, use the following bash commands:
dotnet --info dotnet list package dotnet add package Microsoft.AspNetCore.Authentication.Negotiate --version <patched_version>
Code reference for updating package configuration in project files:
<ItemGroup> <PackageReference Include="Microsoft.AspNetCore.Authentication.Negotiate" Version="[bash]" /> </ItemGroup>
Exploit: (Educational Purposes!)
An attacker positions themselves to intercept or influence LDAP query results returned to the ASP.NET Core application during Negotiate authentication. By supplying a crafted or mismatched LDAP principal entry whose SID is not validated against the authenticated Kerberos ticket, the application incorrectly binds the session to the spoofed principal. This allows the attacker to inherit elevated access rights and permissions intended only for legitimate high-privileged directory users.
Protection: from this CVE
To mitigate this vulnerability, developers must immediately update their project package references for Microsoft.AspNetCore.Authentication.Negotiate to the latest patched version provided by Microsoft. After updating the SDK or package, all running applications must be restarted. Additionally, self-contained deployments targeting impacted versions must be completely recompiled and redeployed into production environments.
Impact:
Successful exploitation of this vulnerability leads to an authentication bypass and privilege escalation vector within enterprise applications. An attacker can compromise the integrity of identity mapping, allowing unauthorized entities to gain access permissions, roles, and administrative capabilities associated with unintended LDAP principals across affected Windows, Linux, and macOS environments.
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Sources:
Reported By: github.com
Extra Source Hub:
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