CVE-2026-120640 in CGI
Summary
by MITRE • 11/04/2026
CGI versions before 5.00 for Perl allow the POSTDATA, PUTDATA, PATCHDATA and XForms:Model parameters to be set from the query string or form fields.
CGI stores a non-form request body under POSTDATA, PUTDATA or PATCHDATA, and an XForms body under XForms:Model, but also accepts these names from the query string, a form field or a multipart part, so an application cannot tell a client-sent value from the real body.
A cross-site HTML form can post POSTDATA={"admin":true} to an endpoint that reads JSON from param('POSTDATA'), a request forgery that needs no CORS preflight.
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Analysis
by VulDB Data Team • 11/04/2026
The vulnerability identified in CGI versions prior to 5.00 for Perl represents a critical flaw in how the module handles HTTP request parameters, specifically concerning body data injection through query strings and form fields. This issue stems from an architectural ambiguity where the software fails to distinguish between legitimate client-supplied values passed via standard web mechanisms and actual HTTP method-specific payload data. By allowing POSTDATA, PUTDATA, PATCHDATA, and XForms:Model parameters to be populated not only by their respective request bodies but also through query string arguments or form fields, the CGI module creates a dangerous conflation of context that undermines the integrity of server-side logic relying on these variables for secure operation.
From a technical perspective, this flaw enables an attacker to inject arbitrary data into parameters that are typically reserved for structured body content such as JSON payloads or XML forms. When an application retrieves values using methods like param('POSTDATA'), it expects to receive the raw HTTP POST body. However, due to this vulnerability, if a request includes POSTDATA in its query string or form submission, the module will prioritize or merge that input with the actual body data. This behavior effectively bypasses any server-side validation that assumes the parameter originates solely from the message body, allowing an attacker to manipulate critical application state by simply appending parameters to URLs or submitting standard HTML forms without needing complex headers or CORS preflight requests.
The operational impact of this vulnerability is severe, particularly in applications that process JSON data for administrative functions or configuration changes. An attacker can craft a cross-site request forgery attack where they trick an authenticated user into submitting a form that sets POSTDATA to a malicious payload, such as {"admin":true}. Because the server interprets this injected parameter as valid body content, it may execute privileged actions on behalf of the victim without their consent or knowledge. This bypasses traditional CSRF protections that rely on checking for specific headers like Content-Type application/json, which are not required when data is passed via query strings or standard form submissions.
This vulnerability maps directly to CWE-94 Improper Control of Generation of Code ('Code Injection') and CWE-200 Information Exposure if sensitive body content is leaked through parameter manipulation. In terms of the MITRE ATT&CK framework, this technique aligns with T1566.002 Phishing: Spearphishing Link or T1078 Valid Accounts when combined with social engineering to induce a victim to trigger the malicious request. The lack of CORS preflight requirement makes it particularly dangerous as it can be exploited from simple HTML pages hosted on any domain, facilitating widespread exploitation without triggering browser security mechanisms designed to prevent cross-origin data theft.
Mitigation strategies must focus on strict input validation and separation of concerns within the application logic. Developers should avoid relying solely on CGI module parameters for critical body content; instead, they should read the raw HTTP request body directly using lower-level I/O operations that are not susceptible to query string injection. Additionally, implementing robust CSRF tokens in all state-changing forms is essential to prevent unauthorized actions initiated by third-party sites. Upgrading to CGI version 5.00 or later resolves this issue as it corrects the parameter handling logic to strictly differentiate between body data and form/query parameters, thereby restoring the intended security boundaries of the HTTP protocol implementation.