CVE-2021-3777 in nodejs-tmpl
Summary
by MITRE • 09/15/2021
nodejs-tmpl is vulnerable to Inefficient Regular Expression Complexity
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Analysis
by VulDB Data Team • 09/17/2021
The vulnerability identified as CVE-2021-3777 affects the nodejs-tmpl package, which is a template engine for node.js applications. This package is widely used for generating dynamic content by processing templates with embedded JavaScript expressions. The vulnerability stems from an inefficient regular expression implementation that can be exploited to cause significant performance degradation and potential denial of service conditions. The issue manifests when the template engine processes certain input patterns that trigger catastrophic backtracking in the regular expression engine, leading to exponential execution time growth.
The technical flaw resides in the regular expression patterns used within the nodejs-tmpl package for parsing template syntax. When malicious input is processed through the template engine, specific character sequences can cause the regular expression engine to perform an excessive number of backtracking operations. This behavior is classified as inefficient regular expression complexity and represents a variant of the broader class of vulnerabilities documented under CWE-1321. The vulnerability allows an attacker to craft input strings that cause the regular expression engine to enter a state where it repeatedly backtracks through the same input positions, exponentially increasing processing time with minimal input size increases.
From an operational impact perspective, this vulnerability creates a significant risk for applications that rely on nodejs-tmpl for dynamic content generation, particularly those handling untrusted user input. The inefficient regular expression complexity can be exploited to perform denial of service attacks against web applications, where attackers submit carefully crafted template inputs that cause the server to consume excessive CPU resources and potentially become unresponsive. This vulnerability affects the availability aspect of the application security triad and can be particularly dangerous in high-traffic environments where a single malicious request could impact overall system performance and responsiveness.
The exploitation of this vulnerability aligns with techniques described in the MITRE ATT&CK framework under the T1499.004 subtechnique for Network Denial of Service, where attackers target inefficient regular expression implementations to consume system resources. The vulnerability also relates to the broader category of resource exhaustion attacks that can be classified under the ATT&CK tactic of Resource Exhaustion. Organizations using nodejs-tmpl should implement input validation and sanitization measures to prevent malicious regular expression patterns from reaching the vulnerable parsing logic. The recommended mitigation involves updating to patched versions of the nodejs-tmpl package, implementing regular expression complexity limits, and applying proper input validation to prevent exploitation of the inefficient regex patterns. Additionally, security teams should consider implementing monitoring for unusual CPU usage patterns that might indicate exploitation attempts and establish rate limiting mechanisms to prevent abuse of template processing functionality.