CVE-2025-25200 in koa
Summary
by MITRE • 02/12/2025
Koa is expressive middleware for Node.js using ES2017 async functions. Prior to versions 0.21.2, 1.7.1, 2.15.4, and 3.0.0-alpha.3, Koa uses an evil regex to parse the `X-Forwarded-Proto` and `X-Forwarded-Host` HTTP headers. This can be exploited to carry out a Denial-of-Service attack. Versions 0.21.2, 1.7.1, 2.15.4, and 3.0.0-alpha.3 fix the issue.
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Analysis
by VulDB Data Team • 01/20/2026
The vulnerability identified as CVE-2025-25200 affects the Koa web framework for Node.js, a popular middleware framework that leverages ES2017 async functions for building expressive web applications. This security flaw specifically targets the parsing logic for HTTP headers, namely X-Forwarded-Proto and X-Forwarded-Host, which are commonly used in proxy configurations to determine the original protocol and host information when requests pass through load balancers or reverse proxies. The vulnerability stems from the use of a malicious regular expression pattern that exhibits exponential backtracking behavior, creating a path for malicious actors to exploit through crafted header values.
The technical implementation of this vulnerability involves a poorly constructed regular expression that processes the X-Forwarded-Proto and X-Forwarded-Host headers without proper input validation or sanitization. When an attacker crafts specially designed header values that trigger the regex engine's backtracking mechanism, the system can become overwhelmed with computational work, leading to significant performance degradation or complete service unavailability. This type of vulnerability falls under CWE-1321, which specifically addresses Regular Expression Denial of Service (ReDoS) attacks, where malicious input causes the regular expression engine to consume excessive CPU resources. The regex pattern likely contains nested quantifiers or alternations that create exponential time complexity when processing certain input strings, making it particularly susceptible to exploitation in high-traffic environments.
The operational impact of this vulnerability extends beyond simple service disruption, as it can affect the entire application availability and potentially impact downstream systems that depend on the Koa framework. Organizations using affected versions of Koa are at risk of experiencing denial-of-service conditions that can last from minutes to hours depending on the scale of the attack and the system's computational resources. The vulnerability affects multiple version lines including 0.21.x, 1.7.x, 2.15.x, and the alpha release of 3.0.0, indicating that it has been present across several major releases and potentially affects a wide range of applications. This vulnerability also aligns with ATT&CK technique T1499.004, which covers Network Denial of Service attacks, where adversaries leverage application-level weaknesses to exhaust system resources and make services unavailable to legitimate users.
The fix implemented in versions 0.21.2, 1.7.1, 2.15.4, and 3.0.0-alpha.3 addresses the core issue by replacing the problematic regular expression with a more robust and secure parsing mechanism. This remediation approach ensures that header parsing operations maintain predictable performance characteristics regardless of input values. Organizations should prioritize updating their Koa framework installations to the patched versions to eliminate this attack vector. Additionally, implementing proper input validation at the application level and monitoring for unusual header patterns can serve as additional defensive measures. The vulnerability also highlights the importance of regular security audits and dependency updates, as similar issues can occur in other frameworks and libraries that rely on regular expression parsing for header or URL validation. Security teams should consider implementing automated scanning tools to identify vulnerable dependencies and establish processes for rapid patch deployment when similar vulnerabilities are discovered in their technology stack.