CVE-2026-120648 in CGI
Summary
by MITRE • 11/04/2026
CGI versions before 5.00 for Perl allow HTML injection via text containing --> in comment.
Arguments are placed between <!-- and --> without escaping, so text containing --> closes the comment early.
An application that passes request data to comment can be made to inject HTML into the page.
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Analysis
by VulDB Data Team • 11/04/2026
The Common Gateway Interface (CGI) module for Perl, specifically versions prior to 5.00, contains a critical input validation flaw related to how it handles comments within CGI scripts. This vulnerability stems from an insufficient sanitization of user-supplied data when that data is embedded directly into HTML comment blocks without proper escaping or encoding. The core technical issue lies in the parser's inability to distinguish between legitimate comment delimiters and maliciously crafted sequences intended to break out of the comment context. When a CGI script accepts input parameters and places them within an HTML comment structure, defined by opening tags like <!-- and closing tags like -->, it assumes that any content provided will not contain characters that alter this structural integrity. However, because the module does not escape or validate these inputs against special HTML entities, an attacker can inject sequences such as double hyphens followed by a greater-than sign (-->). This specific sequence acts as a valid closing delimiter for an HTML comment in standard web browsers and parsers.
From a technical perspective, this flaw represents a classic case of improper neutralization of special elements used in an HTML context. When the application processes a request containing --> within its parameters, the parser interprets this input not merely as text but as the end of the current HTML comment block. Consequently, any subsequent content included by the CGI script after that point is no longer treated as part of the hidden comment section. Instead, it becomes active, executable markup or plain text visible to the client browser. This breaks the intended isolation of the data and allows an attacker to inject arbitrary HTML tags into the page structure. For instance, if a developer uses this input within a larger script that appends additional content after the user's input, those subsequent elements may be exposed prematurely or manipulated in unexpected ways depending on how the rest of the document is structured.
The operational impact of this vulnerability extends beyond simple data exposure and can facilitate Cross-Site Scripting (XSS) attacks if the injected HTML includes script tags or event handlers that execute client-side code. Even without executable scripts, an attacker could inject malicious hyperlinks, images with tracking pixels, or other elements designed to phish users, steal session cookies via DOM manipulation, or deface the website's appearance. This undermines the integrity and confidentiality of the web application by allowing untrusted data to influence the rendering engine in unintended ways. The severity is particularly high because it affects a foundational component used for dynamic content generation on many legacy Perl-based systems, potentially impacting numerous applications that rely on this module for handling form submissions or URL parameters within comment sections.
This vulnerability aligns with CWE-79, which classifies improper neutralization of input during web page generation as Cross-site Scripting (XSS). It also maps to the MITRE ATT&CK framework under techniques related to Client-side Injection and potentially Command Line Interface Exploitation if the injection leads to broader system compromise through social engineering vectors enabled by the injected content. The lack of output encoding is a fundamental security anti-pattern that violates best practices for secure coding, particularly when dealing with dynamic data insertion into HTML contexts.
To mitigate this vulnerability, organizations must upgrade the CGI module to version 5.00 or later, where these input validation and escaping mechanisms have been corrected. For systems unable to immediately patch due to legacy constraints, developers should implement rigorous output encoding for all user-supplied data before it is inserted into any HTML context, including comments. This involves converting special characters such as <, >, &, ", and ' into their corresponding HTML entities (e.g., <, >). Additionally, implementing Content Security Policy headers can help mitigate the impact of successful injections by restricting the sources from which scripts and other resources can be loaded. Regular security audits focusing on input validation practices in CGI scripts are essential to prevent similar flaws in related applications.