CVE-2021-1723 in ASP.NET Core
Summary
by MITRE • 01/13/2021
ASP.NET Core and Visual Studio Denial of Service Vulnerability
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Analysis
by VulDB Data Team • 10/09/2024
This vulnerability affects Microsoft ASP.NET Core and Visual Studio environments, representing a critical denial of service weakness that can be exploited by remote attackers. The flaw exists in the way these frameworks handle certain HTTP requests, specifically when processing malformed or specially crafted input that triggers unexpected behavior in the request processing pipeline. The vulnerability stems from insufficient input validation mechanisms within the core HTTP request handling components, allowing malicious actors to craft requests that cause the application to consume excessive system resources or enter unstable states. This issue impacts both the server-side processing capabilities and the development environment tools, making it particularly dangerous for organizations that rely on ASP.NET Core for web application deployment and Visual Studio for development workflows. The vulnerability is classified under CWE-400 as an uncontrolled resource consumption issue, which aligns with the denial of service characteristics observed in the exploitation patterns. Attackers can leverage this weakness by sending specifically formatted HTTP requests that cause the application to enter infinite loops or consume excessive memory and CPU cycles, effectively rendering the service unavailable to legitimate users. The impact extends beyond simple service disruption as it can affect application stability, potentially leading to cascading failures in larger distributed systems where these components are integrated. The vulnerability affects multiple versions of ASP.NET Core and Visual Studio, with the most significant risk occurring in environments where applications process untrusted input from external sources. Organizations utilizing these technologies are particularly vulnerable when they fail to implement proper input sanitization measures or when they deploy applications without applying the relevant security patches. The attack surface is broad since ASP.NET Core is widely used for building web applications and services, making this vulnerability particularly attractive to threat actors seeking to disrupt online services at scale. The exploitation requires minimal technical expertise and can be automated, increasing the potential for widespread impact across different organizations and applications. This weakness also aligns with ATT&CK technique T1499.004 which describes denial of service attacks through resource exhaustion, demonstrating how this vulnerability can be weaponized in broader attack campaigns.
The technical implementation of this vulnerability involves the interaction between HTTP request parsing and the underlying ASP.NET Core request processing logic. When malformed requests are received, the framework fails to properly validate the input parameters, leading to unexpected execution paths within the application's request handling code. The flaw manifests when the system attempts to process certain combinations of headers, query parameters, or body content that cause the request pipeline to enter resource-intensive operations. This can include situations where the application attempts to parse malformed data structures or where recursive processing occurs due to improper boundary conditions in the parsing algorithms. The vulnerability is particularly concerning because it can be triggered through standard HTTP communication channels without requiring authentication or specialized access privileges, making it highly accessible to attackers. The resource exhaustion occurs at multiple levels including memory allocation, thread management, and CPU processing cycles, which can lead to complete application failure or significant performance degradation. Security researchers have identified that the vulnerability can be exploited through various attack vectors including web application firewalls, API endpoints, and direct HTTP client interactions, making comprehensive protection challenging. The weakness is exacerbated in cloud environments where applications may be subject to high traffic volumes and where the resource constraints can be quickly exhausted through targeted attacks. Organizations deploying these technologies in production environments are advised to implement immediate mitigations including input validation rules, rate limiting mechanisms, and monitoring systems to detect anomalous request patterns. The vulnerability also highlights the importance of regular security updates and patch management processes, as Microsoft has released specific patches to address this weakness in affected versions of the framework.
Mitigation strategies for this vulnerability should encompass both immediate defensive measures and long-term architectural improvements. Organizations should implement comprehensive input validation at multiple layers of their application architecture, including API gateways, web application firewalls, and application-level controls to prevent malformed requests from reaching the core ASP.NET Core processing logic. Network-level protections such as rate limiting and connection throttling should be deployed to prevent abuse of the vulnerability through excessive request volumes. The implementation of proper monitoring and alerting systems becomes critical for detecting exploitation attempts and measuring the impact of attacks on system resources. Organizations should also consider implementing circuit breaker patterns and graceful degradation mechanisms to maintain service availability even when under attack. Security teams must conduct thorough vulnerability assessments to identify all instances of affected software within their environments and prioritize patching efforts based on risk exposure. The ATT&CK framework suggests implementing defensive measures such as network segmentation and application whitelisting to limit the potential impact of exploitation. Additionally, organizations should review their incident response procedures to ensure they can effectively respond to denial of service attacks targeting these specific vulnerabilities. Regular security testing including penetration testing and vulnerability scanning should be conducted to verify that mitigations are properly implemented and functioning as intended. The vulnerability also emphasizes the need for secure coding practices and regular security training for development teams to prevent similar weaknesses from being introduced in future application code.