CVE-2026-120650 in CGI
Summary
by MITRE • 11/04/2026
CGI versions before 5.00 for Perl allow memory, CPU and disk exhaustion via missing or unenforced request size, parameter and upload limits.
$CGI::POST_MAX defaults to no limit, and when set is not enforced for chunked multipart bodies or for the boundary line of a multipart body without a boundary= parameter. Bodies saved as PUTDATA, POSTDATA or PATCHDATA uploads are read into memory in one go, and nothing limits the number of parameters or uploads.
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Analysis
by VulDB Data Team • 11/04/2026
The Common Gateway Interface module for Perl, specifically versions prior to 5.00, contains critical configuration defaults that expose systems to severe resource exhaustion attacks. The core issue stems from the default behavior of the $CGI::POST_MAX variable, which is initialized with no limit on the size of POST requests by design. While this setting allows developers flexibility in handling large data transfers, it creates a significant security vulnerability when deployed without explicit overrides or additional protective layers such as web server configuration limits. An attacker can exploit this lack of enforcement to send extremely large HTTP payloads that consume excessive memory and CPU resources on the target system, potentially leading to denial-of-service conditions for both the application and underlying infrastructure.
The technical flaw is multifaceted, involving failures in parsing multipart form data correctly under specific edge cases. Even when $CGI::POST_MAX is explicitly set to a non-zero value intended to cap request sizes, the module fails to enforce this limit for chunked transfer-encoded multipart bodies. Furthermore, it neglects to validate the boundary line of a multipart body if that line lacks an explicit boundary parameter. These parsing inconsistencies allow malicious actors to bypass size restrictions entirely by structuring their HTTP requests in ways that evade standard length checks. This means that security controls relying on this module for input validation are effectively neutralized against crafted payloads, leaving the application vulnerable to abuse.
In addition to request body size issues, there is no mechanism within CGI.pm versions before 5.00 to limit the number of parameters or file uploads contained within a single HTTP request. When data is processed via PUTDATA, POSTDATA, or PATCHDATA methods, the module reads the entire input stream into memory at once rather than processing it in chunks. This all-at-once ingestion strategy means that a large volume of small requests with numerous fields can rapidly deplete available RAM on the server. The combination of unlimited parameter counts and unrestricted upload sizes creates an ideal vector for resource exhaustion attacks, where attackers send many concurrent requests designed to maximize memory consumption without necessarily transmitting massive individual files.
The operational impact of these vulnerabilities is substantial, primarily manifesting as denial-of-service scenarios that degrade or completely halt service availability. By exploiting the missing limits on request size and parameter counts, adversaries can exhaust server resources such as RAM and CPU cycles, causing legitimate users to experience timeouts or connection failures. In environments where CGI scripts are used for high-traffic web applications, this vulnerability can be leveraged in distributed denial-of-service attacks with minimal effort from the attacker side. The lack of enforcement also complicates incident response efforts because standard monitoring tools may not immediately flag these requests as anomalous if they do not exceed typical size thresholds but instead rely on volume and parameter density to cause harm.
Mitigation strategies must address both the application-level configuration and broader architectural defenses. Developers should explicitly set $CGI::POST_MAX to a reasonable value that reflects expected business requirements, ensuring it is strictly enforced for all types of multipart bodies including chunked transfers. It is also crucial to implement limits on the number of parameters allowed per request through custom validation logic or by upgrading to newer versions of CGI.pm where these issues may be addressed. Beyond application code changes, organizations should deploy web server-level restrictions such as Apache's LimitRequestBody directive or Nginx client_max_body_size settings to provide a secondary layer of defense that catches oversized requests before they reach the Perl interpreter.
This vulnerability aligns with CWE-787 Out-of-bounds Write and CWE-400 Uncontrolled Resource Consumption, reflecting its nature as a failure in resource management due to missing input validation limits. From an offensive security perspective, it maps to MITRE ATT&CK technique T1496 Resource Hijacking, specifically subcategory DDoS: Impact Defacement or Denial of Service via resource exhaustion. Security teams should prioritize patching CGI.pm to version 5.00 or later if possible, as newer releases often include improved handling of multipart data and stricter default behaviors. For systems that cannot be immediately upgraded, implementing strict input validation at the network perimeter using firewalls or web application firewalls is essential to block requests with excessive parameter counts or body sizes before they can trigger the vulnerable code paths within the CGI module.