CVE-2022-23516 in Loofah
Summary
by MITRE • 12/14/2022
Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. Loofah >= 2.2.0, < 2.19.1 uses recursion for sanitizing CDATA sections, making it susceptible to stack exhaustion and raising a SystemStackError exception. This may lead to a denial of service through CPU resource consumption. This issue is patched in version 2.19.1. Users who are unable to upgrade may be able to mitigate this vulnerability by limiting the length of the strings that are sanitized.
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Analysis
by VulDB Data Team • 11/05/2025
The vulnerability identified as CVE-2022-23516 affects the Loofah library, a widely used Ruby gem designed for HTML and XML document manipulation that relies on Nokogiri as its foundation. This library serves as a critical component in many web applications for processing and sanitizing user-generated content, making its security implications particularly significant. The flaw manifests in versions greater than or equal to 2.2.0 but less than 2.19.1, where the sanitization process for CDATA sections employs a recursive approach that can be exploited by malicious actors to exhaust system resources.
The technical root cause of this vulnerability lies in the recursive implementation used for CDATA section sanitization within the Loofah library. When processing malformed or specially crafted input containing deeply nested CDATA sections, the recursive algorithm consumes stack space with each recursive call. This recursive approach creates a potential stack overflow condition that ultimately results in a SystemStackError exception being raised. The vulnerability operates under CWE-674, which specifically addresses "Uncontrolled Recursion," and represents a classic denial of service scenario where the system's call stack becomes exhausted through excessive recursive calls.
The operational impact of this vulnerability extends beyond simple system instability, as it creates a reliable denial of service condition that can be triggered by any application utilizing the affected Loofah versions. Attackers can craft malicious input containing deeply nested CDATA sections to cause the application to consume excessive CPU resources and eventually crash or become unresponsive. This type of vulnerability aligns with ATT&CK technique T1499.004, which covers "Resource Exhaustion" through malicious input processing. The vulnerability is particularly dangerous in web applications where user input is processed through Loofah's sanitization functions, as it can be exploited through common attack vectors such as form submissions, API endpoints, or any input handling mechanism that processes HTML/XML content.
The mitigation strategies for this vulnerability encompass both immediate and long-term solutions. The primary and recommended approach involves upgrading to Loofah version 2.19.1 or later, which contains the patched implementation that eliminates the recursive approach for CDATA section processing. For organizations unable to perform immediate upgrades, a temporary workaround involves implementing input length limitations for strings processed through Loofah's sanitization functions. This approach reduces the potential depth of recursive calls by constraining the size of input data that can be processed. Additionally, implementing proper input validation and sanitization at multiple layers of the application can provide defense in depth, while monitoring for unusual CPU usage patterns can help detect exploitation attempts. Organizations should also consider implementing rate limiting and input sanitization controls to prevent abuse of vulnerable endpoints, ensuring that even if exploitation occurs, the impact remains contained and manageable within the application's operational boundaries.