CVE-2021-3859 in Undertow
Summary
by MITRE • 08/26/2022
A flaw was found in Undertow that tripped the client-side invocation timeout with certain calls made over HTTP2. This flaw allows an attacker to carry out denial of service attacks.
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Analysis
by VulDB Data Team • 11/28/2024
The vulnerability identified as CVE-2021-3859 resides within the Undertow web server implementation, specifically targeting its handling of HTTP/2 protocol communications. This flaw manifests as a client-side invocation timeout issue that occurs when specific types of calls are made through HTTP/2 connections, creating a condition where legitimate requests can be disrupted or terminated prematurely. The vulnerability affects systems that utilize Undertow as their web server framework, particularly those implementing HTTP/2 protocol support for enhanced performance and connection management. Security researchers discovered that under certain conditions, the server's HTTP/2 implementation fails to properly manage timeout mechanisms, leading to unexpected behavior that can be exploited by malicious actors.
The technical root cause of this vulnerability stems from how Undertow processes HTTP/2 requests and manages their associated timeouts. When specific sequences of HTTP/2 frames are received or processed, the server's internal timeout handling logic becomes inconsistent, causing legitimate client connections to be prematurely terminated or suspended. This behavior creates a window where an attacker can manipulate the timing and sequence of HTTP/2 communications to trigger the timeout mechanism repeatedly, effectively exhausting resources or disrupting service availability. The flaw operates at the protocol level, specifically within Undertow's HTTP/2 stack implementation, making it particularly dangerous as it can be exploited without requiring authentication or elevated privileges. This type of vulnerability is classified under CWE-400 as an "Uncontrolled Resource Consumption" and aligns with ATT&CK technique T1499.004 for "Endpoint Denial of Service" through resource exhaustion.
The operational impact of CVE-2021-3859 extends beyond simple service disruption, as it can lead to cascading failures within applications that depend on stable HTTP/2 connections. Attackers can leverage this vulnerability to perform sustained denial of service attacks against web applications, potentially causing system instability, performance degradation, or complete service unavailability. The vulnerability is particularly concerning in high-traffic environments where HTTP/2 connections are frequently established and maintained, as the attack can quickly consume available resources and connections. Organizations running Undertow-based applications may experience increased error rates, degraded user experience, and potential data loss if the service becomes unavailable. The flaw can be exploited to target specific endpoints or to create general service disruption across entire application platforms, making it a significant risk for web applications that rely heavily on HTTP/2 for performance optimization.
Mitigation strategies for CVE-2021-3859 should focus on both immediate patching and operational hardening measures. The primary solution involves updating Undertow to versions that contain fixes for the HTTP/2 timeout handling issue, which typically includes improvements to the connection management logic and proper timeout implementation. Organizations should also implement connection rate limiting and monitoring to detect anomalous patterns that might indicate exploitation attempts. Network-level protections such as firewall rules that limit HTTP/2 traffic or implement connection pooling restrictions can provide additional defense in depth. Security teams should also consider implementing automated alerting for unusual timeout patterns or connection termination events that could indicate exploitation of this vulnerability. The mitigation approach aligns with ATT&CK tactics T1566 for malicious exploitation and T1499 for endpoint denial of service, requiring both defensive measures and active monitoring to prevent successful exploitation. Regular security assessments and vulnerability scanning should be conducted to ensure that systems remain protected against similar issues in the HTTP/2 implementation.