CVE-2026-120647 in CGI
Summary
by MITRE • 11/04/2026
CGI versions before 5.00 for Perl allow Content-Disposition parameter injection via unescaped double quotes in the header attachment filename.
header(-attachment => $name) writes $name into the quoted filename parameter of Content-Disposition without escaping double quotes.
An application that passes request data as the attachment name can be made to send a second filename parameter, changing the name the response is saved under.
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Analysis
by VulDB Data Team • 11/04/2026
The vulnerability identified in Perl CGI versions prior to 5.00 represents a significant security flaw rooted in improper input validation and output encoding within the HTTP header generation logic. Specifically, the issue affects the handling of the Content-Disposition header when generating file attachments for download responses. The core technical failure lies in the lack of escaping or sanitization applied to double quote characters contained within the filename parameter passed by the application developer via the CGI module's attachment function. When an application constructs a response using header(-attachment => $name), the system directly embeds the value of $name into the quoted string of the Content-Disposition field without performing necessary character escapes for embedded quotes. This behavior violates fundamental principles of secure web development regarding the separation of data and code within HTTP headers, leading to potential injection attacks where an attacker can manipulate the structure of the header itself rather than just its content values.
From a technical perspective, this flaw allows for Content-Disposition parameter injection, which is categorized under CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component and closely related to CWE-20 Improper Input Validation. The vulnerability exploits the way web browsers parse HTTP headers containing quoted strings. In standard MIME format, parameters within the Content-Disposition header are often enclosed in double quotes when they contain special characters or spaces. If an attacker can control part of this filename string through user input such as query parameters, form submissions, or cookie values, and that input contains a closing double quote followed by additional parameter definitions, the browser's parser will interpret these new segments as separate header attributes rather than part of the original filename. This structural manipulation enables the injection of arbitrary directives into the HTTP response headers sent to the client.
The operational impact of this vulnerability is primarily focused on user experience and data integrity during file downloads. By injecting a second filename parameter, an attacker can force the victim's web browser to save the downloaded content under a different name than intended by the application developer. For instance, if the original intent was to download report.pdf but the injected payload alters the header to specify attachment; filename="malicious.exe", the user will see and potentially execute malware disguised as a legitimate document. This technique is frequently utilized in phishing campaigns where attackers trick users into downloading malicious payloads that appear safe due to deceptive file extensions or names. Furthermore, this vulnerability can be leveraged for cross-site scripting attacks if the browser's handling of these headers interacts with other security mechanisms like Content Security Policy or if it leads to open redirect scenarios depending on how downstream components process the manipulated header values.
This type of injection is also relevant to the MITRE ATT&CK framework under techniques involving client-side behavior manipulation, specifically those that affect file system interactions and user trust indicators. The ability to alter download filenames undermines the integrity of information provided by the server, which is critical for maintaining user confidence in web applications. In enterprise environments where automated systems or scripts might rely on predictable filename structures for processing downloaded files, this vulnerability can disrupt business logic and lead to data corruption or loss if unexpected file names are processed incorrectly by downstream tools.
Mitigation strategies must focus on strict input validation and proper output encoding at the point of header generation. Developers should ensure that any user-supplied data used in HTTP headers is strictly validated against a whitelist of allowed characters, typically alphanumeric characters plus safe punctuation like hyphens and underscores. Alternatively, when constructing Content-Disposition headers, developers must escape double quotes within filename strings by replacing them with escaped sequences such as backslash-quote or using HTML entity encoding if appropriate for the context, although standard HTTP header escaping usually requires doubling the quote or removing it entirely depending on browser compatibility requirements. Upgrading to CGI version 5.00 or later resolves this issue as newer versions implement proper sanitization routines that neutralize special characters before they are embedded into headers. Additionally, implementing Content Security Policy directives can help mitigate some downstream effects by restricting how browsers handle downloaded content and preventing the execution of files from untrusted sources, although upgrading the library remains the primary remediation step to address the root cause at the application layer.