CVE-2026-120643 in Cgi
Summary
by MITRE • 11/04/2026
CGI versions from 4.62 before 5.00 for Perl allow cross-site scripting via a raw apostrophe in URLs returned by url and self_url.
Both pass the URL through URI's canonical method, which leaves an apostrophe in the path raw and decodes the %27 that query parameter escaping produces, so an apostrophe in the request path or a parameter appears raw in the result.
An application that places the URL in a single-quoted HTML attribute or JavaScript string is open to cross-site scripting.
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Analysis
by VulDB Data Team • 11/04/2026
The vulnerability identified in Perl CGI module versions prior to 5.00, specifically affecting releases from 4.62 onwards, represents a significant security flaw rooted in improper input sanitization and output encoding practices. This issue manifests as a Cross-Site Scripting (XSS) vector that exploits the behavior of the URI canonicalization method used within the CGI module's url and self_url functions. When these methods process URLs containing raw apostrophes or percent-encoded apostrophe sequences such as %27, they fail to properly escape the resulting output for safe inclusion in HTML contexts. The core technical failure lies in the fact that while query parameter escaping may decode certain characters, the path component retains the literal apostrophe character without applying necessary HTML entity encoding. This discrepancy creates a scenario where user-controlled data is reflected back into an HTTP response with insufficient protection against script injection attacks.
From a technical perspective, the flaw arises because the CGI module passes URL components through URI's canonical method which normalizes the structure of the Uniform Resource Identifier but does not perform context-aware output encoding. If a request path or query parameter contains an apostrophe, either in its raw form or as %27, the resulting string returned by url or self_url will contain this unescaped character. When an application developer incorporates these values directly into HTML attributes delimited by single quotes or within JavaScript strings without additional sanitization, the browser interprets the closing quote of the attribute or string prematurely. This allows any subsequent content in the URL to be executed as part of the page's script context rather than being treated as data.
The operational impact of this vulnerability is severe for web applications that rely on CGI scripts generated by Perl and utilize these specific functions to embed URLs into client-side code. Attackers can craft malicious links or manipulate existing parameters to inject arbitrary JavaScript payloads. Successful exploitation enables session hijacking, credential theft, defacement of the website interface, or redirection to phishing sites. Since XSS attacks are among the most prevalent web application vulnerabilities, this flaw exposes organizations to widespread risk depending on their usage patterns and security controls surrounding user input handling. The lack of automatic escaping for single quotes in these specific CGI methods means that developers must manually ensure proper encoding, a step often overlooked during rapid development cycles.
This vulnerability aligns with CWE-79, which classifies Improper Neutralization of Input During Web Page Generation commonly known as Cross-site Scripting. It also maps to the MITRE ATT&CK technique T1059.007, specifically JavaScript execution within a browser environment via XSS. To mitigate this risk, organizations should upgrade immediately to CGI version 5.00 or later where these encoding issues have been addressed. For systems unable to upgrade promptly, developers must implement strict output encoding strategies that convert special characters including apostrophes into their corresponding HTML entities before embedding URL data into HTML attributes or JavaScript blocks. Additionally, implementing Content Security Policy headers can provide an additional layer of defense by restricting the sources from which scripts are allowed to execute, thereby reducing the impact of any successful injection attempts even if input validation is imperfect.