CVE-2026-120646 in CGI
Summary
by MITRE • 11/04/2026
CGI versions from 4.71 before 5.00 for Perl allow denial of service via a plain string dereferenced as a symbolic reference in uploadInfo.
uploadInfo dereferences its argument and calls its filename method without checking that it is an upload filehandle. A plain string naming a defined global variable, such as the parameter value from ?upload=0, is treated as a symbolic reference, and the method call on it dies.
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Analysis
by VulDB Data Team • 11/04/2026
The vulnerability identified in Perl CGI module versions prior to 5.00 represents a critical flaw in input validation and type handling within the uploadInfo function. This component of the library is responsible for processing file uploads submitted via web forms, specifically extracting metadata such as filenames from uploaded data structures. The core technical deficiency lies in the lack of rigorous type checking before performing method dispatch operations on user-supplied arguments. When a request parameter like upload=0 or any other plain string is passed to this function, the code attempts to dereference it as if it were an object capable of responding to specific methods, rather than validating that the input conforms to the expected data structure representing a valid file handle or upload object.
This behavior triggers a mechanism known in Perl programming as symbolic reference resolution. In Perl, when a string is used where an object reference is expected and no explicit type check occurs, the interpreter may attempt to resolve the string as a name of a global variable or package symbol. If this process fails because the referenced entity does not support the requested method call—in this case, invoking the filename method—the runtime environment throws an unhandled exception. This results in an abrupt termination of the script execution, manifesting as a denial of service condition for any application relying on this specific version of the CGI module to handle file uploads. The impact is particularly severe because it can be triggered remotely by sending malformed HTTP requests with specifically crafted parameters that exploit this type confusion.
From a classification perspective, this vulnerability aligns closely with CWE-20 Improper Input Validation and CWE-754: Improper Check for Unusual or Exceptional Conditions. The failure to verify the data type of the input before dereferencing it constitutes an improper check for unusual conditions, allowing attackers to induce a crash through simple parameter manipulation. In terms of offensive security frameworks such as MITRE ATT&CK, this flaw facilitates Denial of Service (T1499) by exploiting application logic errors that lead to process termination. It also touches upon aspects of Improper Neutralization of Special Elements used in an Injection Command (CWE-78) if the symbolic reference mechanism inadvertently exposes internal state or variables during the resolution phase, although the primary impact here is availability rather than confidentiality or integrity compromise.
The operational impact extends beyond simple script crashes to potential resource exhaustion and service instability. In high-throughput web environments where file uploads are common, an attacker can repeatedly send requests with these malformed parameters to exhaust server resources such as memory and CPU cycles associated with handling the exceptions and restarting processes. This makes the vulnerability a potent tool for disrupting critical business functions that depend on continuous availability of upload services. Furthermore, if error reporting is configured to display stack traces or internal variable states during crashes, it could inadvertently leak sensitive information about the underlying system architecture or application logic, compounding the security risk beyond mere denial of service.
Mitigation strategies must prioritize immediate upgrading to CGI module version 5.00 or later, where this specific type-checking flaw has been addressed by developers implementing stricter validation protocols before dereferencing inputs. For organizations unable to upgrade immediately due to legacy system constraints, defensive coding practices should be employed within the application layer. This includes wrapping upload processing logic in eval blocks to catch and handle exceptions gracefully without crashing the entire script, as well as explicitly validating that input parameters are of the expected object type or class before invoking methods on them. Additionally, implementing robust error handling policies that suppress detailed stack traces from being returned to end-users can mitigate information leakage risks associated with these crashes. Regular security audits focusing on input validation and symbolic reference usage in Perl applications will help prevent similar vulnerabilities from arising in custom code bases.