CVE-2026-47303 Overview
CVE-2026-47303 is an authentication bypass vulnerability in ASP.NET Core caused by reliance on assumed-immutable data. An authorized attacker can manipulate data the application treats as tamper-proof to elevate privileges over a network. Microsoft published the advisory on July 14, 2026.
The weakness is classified under [CWE-90] (Improper Neutralization of Special Elements used in an LDAP Query). Exploitation requires low privileges and no user interaction, and results in high impact to confidentiality, integrity, and availability of affected ASP.NET Core applications.
Critical Impact
An authenticated attacker with low privileges can bypass authentication controls in ASP.NET Core and elevate to higher-privileged roles across networked applications, compromising confidentiality, integrity, and availability.
Affected Products
- ASP.NET Core (specific versions listed in the Microsoft advisory)
- Web applications and APIs built on affected ASP.NET Core runtime versions
- Services relying on ASP.NET Core authentication and authorization middleware
Discovery Timeline
- 2026-07-14 - CVE-2026-47303 published to NVD
- 2026-07-15 - Last updated in NVD database
Technical Details for CVE-2026-47303
Vulnerability Analysis
The vulnerability originates in how ASP.NET Core handles data that the framework or application assumes cannot be modified by a client. When authentication or authorization decisions rely on such assumed-immutable values, an attacker with a valid low-privileged account can supply or influence that data to obtain elevated privileges.
Because the flaw sits in the authentication pathway, it can be reached over the network without user interaction. Exploitation does not require chaining with additional bugs. The scope remains unchanged, but confidentiality, integrity, and availability of the target application are all affected.
Root Cause
The root cause is an authentication bypass by assumed-immutable data, mapped to [CWE-90]. ASP.NET Core code paths trust an input, header, claim, or serialized token value as immutable when a client can in fact modify it. Once modified, the application grants access decisions based on attacker-controlled state.
Attack Vector
The attack vector is network-based. An authenticated user with limited privileges sends crafted requests to an ASP.NET Core application, altering data that the framework treats as trustworthy. The server processes the modified value during authentication or authorization, granting privileges the caller should not have.
No public proof-of-concept, exploit tooling, or exploitation in the wild has been reported at publication. The vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog. Refer to the Microsoft CVE-2026-47303 Advisory for authoritative technical details.
Detection Methods for CVE-2026-47303
Indicators of Compromise
- Unexpected role or claim changes for user accounts without corresponding administrative activity in application audit logs.
- Authentication events where a low-privileged account subsequently performs actions restricted to higher-privileged roles.
- Malformed or tampered authentication tokens, cookies, or headers reaching ASP.NET Core endpoints.
Detection Strategies
- Correlate authentication logs with authorization decisions to flag sessions that gain privileges mid-session without a legitimate elevation event.
- Inspect ASP.NET Core middleware logs for anomalies in claims parsing, token validation failures, or repeated attempts to submit modified identity data.
- Deploy application-layer monitoring on identity-carrying request fields such as JWT claims, cookies, and custom headers.
Monitoring Recommendations
- Enable verbose logging on ASP.NET Core authentication and authorization middleware and forward logs to a centralized SIEM for correlation.
- Track privilege changes and administrative actions per user account and alert on deviations from established baselines.
- Monitor outbound activity from web tiers for signs of post-exploitation lateral movement following successful privilege elevation.
How to Mitigate CVE-2026-47303
Immediate Actions Required
- Apply the security update referenced in the Microsoft CVE-2026-47303 Advisory to all affected ASP.NET Core runtimes and SDKs.
- Inventory internal and external applications built on ASP.NET Core and prioritize patching internet-facing workloads.
- Rotate authentication secrets, signing keys, and session tokens after patching to invalidate any tampered credentials.
Patch Information
Microsoft has issued guidance and updates for this vulnerability. Administrators should consult the Microsoft CVE-2026-47303 Advisory for the current list of fixed ASP.NET Core versions, download links, and deployment guidance. Update both the runtime and any bundled hosting components.
Workarounds
- Restrict network access to ASP.NET Core applications using firewalls, reverse proxies, or web application firewalls until patches are applied.
- Enforce server-side revalidation of identity claims and role assignments on every privileged action rather than trusting client-supplied state.
- Audit custom authentication handlers and claims transformers for reliance on client-controlled values treated as immutable.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
