CVE-2020-1476: .NET Framework Privilege Escalation Flaw

CVE-2020-1476 Overview

CVE-2020-1476 is an elevation of privilege vulnerability that exists when ASP.NET or .NET web applications running on Internet Information Services (IIS) improperly allow access to cached files. An attacker who successfully exploits this vulnerability could gain unauthorized access to restricted files that should otherwise be protected from access.

The vulnerability requires local access to the system and allows an attacker to send specially crafted requests to an affected server to access cached content that the attacker should not have permissions to view. This represents a significant confidentiality risk for organizations running vulnerable .NET Framework versions on Windows systems with IIS.

Critical Impact

Successful exploitation allows attackers with local access to read restricted cached files from ASP.NET and .NET web applications, potentially exposing sensitive configuration data, credentials, or other confidential information stored in cache.

Affected Products

• Microsoft .NET Framework 2.0 SP2, 3.5, 3.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8
• Microsoft Windows Server 2008 SP2/R2, 2012/R2, 2016, 2019
• Microsoft Windows 7 SP1, 8.1, RT 8.1, 10 (multiple versions including 1607, 1709, 1803, 1809, 1903, 1909, 2004)

Discovery Timeline

• August 17, 2020 - CVE-2020-1476 published to NVD
• February 23, 2026 - Last updated in NVD database

Technical Details for CVE-2020-1476

Vulnerability Analysis

This elevation of privilege vulnerability stems from improper access control mechanisms within the caching subsystem of ASP.NET and .NET Framework when running on IIS. The vulnerability allows a local attacker to bypass intended access restrictions to cached files, enabling the reading of sensitive data that should be protected.

The attack requires local access to the vulnerable system, meaning the attacker must already have some level of access to the machine hosting the IIS web server. Once local access is obtained, the attacker can craft malicious requests that exploit the improper handling of cached file access controls. The vulnerability does not require user interaction and does not require elevated privileges to exploit, making it accessible to any authenticated local user.

The impact is limited to confidentiality—the vulnerability allows reading of restricted files but does not permit modification (integrity) or disruption of service (availability). However, the confidentiality impact is high, as cached files may contain sensitive application data, session information, or configuration details.

Root Cause

The root cause of CVE-2020-1476 lies in the improper implementation of access control checks within the ASP.NET and .NET Framework caching mechanism when operating under IIS. The framework fails to properly validate access permissions when serving cached content, allowing requests from unauthorized local users to retrieve files that should be restricted based on their access level.

When web applications cache content for performance optimization, the caching layer should enforce the same access controls as the original resource. In this case, the validation logic contains a flaw that allows bypass of these access restrictions under specific conditions.

Attack Vector

The attack vector for CVE-2020-1476 is local, requiring the attacker to have existing access to the affected system. The exploitation process involves:

1. The attacker gains local access to a system running IIS with vulnerable ASP.NET or .NET Framework applications
2. The attacker identifies web applications that utilize caching functionality
3. The attacker crafts and sends specially formatted requests designed to access cached files
4. Due to improper access validation, the request bypasses normal access controls
5. The attacker receives the contents of restricted cached files

The vulnerability does not require any special privileges beyond local system access, and no user interaction is needed for successful exploitation.

Detection Methods for CVE-2020-1476

Indicators of Compromise

• Unusual access patterns to IIS cached content directories from local processes
• Unexpected read operations on temporary ASP.NET files or cached application data
• Local users accessing web application cache stores that they should not normally interact with
• Anomalous request patterns in IIS logs originating from local addresses targeting cached resources

Detection Strategies

• Monitor Windows Security Event logs for suspicious file access events (Event IDs 4663, 4656) targeting IIS and ASP.NET cache directories
• Implement file integrity monitoring on ASP.NET temporary files and cache directories
• Configure audit policies to track access attempts to %SystemRoot%\Microsoft.NET\Framework\ directories
• Deploy endpoint detection solutions to identify unusual local process behavior interacting with IIS caching components

Monitoring Recommendations

• Enable detailed IIS logging with client IP tracking to identify unusual local access patterns
• Configure SentinelOne agents to monitor for suspicious access to .NET Framework cache directories
• Implement Windows Advanced Audit Policy for object access on sensitive IIS configuration and cache folders
• Create alerts for local processes making repeated requests to cached web application resources

How to Mitigate CVE-2020-1476

Immediate Actions Required

• Apply the Microsoft security update for CVE-2020-1476 immediately on all affected systems
• Inventory all systems running .NET Framework on IIS and prioritize patching based on exposure
• Restrict local access to servers running IIS web applications to only essential personnel
• Review and limit user accounts with local access to production web servers

Patch Information

Microsoft has released security updates to address this vulnerability. The patches modify how ASP.NET and .NET Framework handle requests to cached files, ensuring proper access control validation is performed before serving cached content.

Detailed patch information and download links are available in the Microsoft Security Advisory for CVE-2020-1476.

Organizations should apply the appropriate .NET Framework update based on their installed version and Windows operating system. Multiple cumulative updates are available depending on the specific combination of .NET Framework version and Windows version in use.

Workarounds

• Implement strict access controls on IIS server systems to limit local user access
• Use application-level caching alternatives that enforce access controls at the application layer
• Consider disabling or reducing the use of output caching in ASP.NET applications until patches can be applied
• Deploy network segmentation to isolate web servers from general user access

# Verify installed .NET Framework versions to identify systems requiring patching
reg query "HKLM\SOFTWARE\Microsoft\NET Framework Setup\NDP" /s

# Check IIS status and application pools
%windir%\system32\inetsrv\appcmd list apppool

# Review current .NET Framework security updates
wmic qfe list | findstr /i "KB"