CVE-2026-120632 in CGI
Summary
by MITRE • 11/04/2026
CGI versions before 5.00 for Perl allow CPU exhaustion via a backtracking regex on the XForms multipart/related Content-Type.
When a POST Content-Type contains multipart/related, the boundary and start parameters are extracted with an unanchored pattern that puts .+ on either side of the boundary capture, which backtracks in cubic time on a header that does not match. No request body is needed.
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Analysis
by VulDB Data Team • 11/04/2026
The Common Gateway Interface implementation for Perl prior to version 5.00 contains a critical regular expression vulnerability that enables remote denial-of-service attacks through CPU exhaustion. This flaw specifically targets the parsing logic used when processing HTTP POST requests with a Content-Type header set to multipart/related. The underlying issue stems from an unanchored regex pattern designed to extract boundary and start parameters from the MIME headers of incoming requests. By placing greedy quantifiers such as .+ on both sides of the target capture group, the regular expression engine is forced into catastrophic backtracking when it encounters input that does not match the expected format. This behavior transforms a simple header parsing operation into an exponential computational burden for the server process handling the request.
From a technical perspective, the vulnerability exploits the inherent inefficiency of certain regex engines when faced with ambiguous patterns and long non-matching strings. When a malicious actor submits a crafted HTTP POST request containing a multipart/related Content-Type header with carefully constructed boundary values that do not align with the expected structure, the Perl CGI module attempts to match the unanchored pattern against the input data. Because the regex engine cannot immediately determine that the string will fail to match due to the greedy quantifiers and lack of anchoring, it proceeds to explore numerous potential matching paths before ultimately failing. This process results in cubic time complexity relative to the length of the header content, causing the server thread or process associated with handling the request to consume nearly all available CPU resources for an extended period. Notably, this exploitation does not require a request body, allowing attackers to trigger the condition using only malformed headers, which simplifies the attack vector significantly and increases its potential impact on service availability.
The operational impact of this vulnerability is severe in terms of system stability and resource management. Since CGI processes are often spawned per request or managed by web servers that may limit concurrent connections based on CPU usage, a single crafted request can effectively take down an entire server instance if the process becomes unresponsive due to high CPU load. This constitutes a classic denial-of-service scenario where availability is compromised without any need for authentication or complex payload delivery mechanisms beyond standard HTTP headers. The lack of requirement for a body means that automated scanning tools and opportunistic attackers can easily identify and exploit this weakness across multiple systems, leading to widespread service disruptions in environments relying on older Perl CGI implementations.
This vulnerability aligns with CWE-400, which describes uncontrolled resource consumption, specifically manifesting as CPU exhaustion through inefficient algorithmic complexity. In the context of offensive security frameworks such as MITRE ATT&CK, this behavior is consistent with techniques used to degrade service availability by overwhelming processing capabilities, often categorized under resource hijacking or denial-of-service tactics that target application layer resources rather than network bandwidth. The specific mechanism involving regex backtracking highlights the importance of input validation and safe parsing practices in web server components.
To mitigate this risk, organizations must upgrade their Perl CGI modules to version 5.00 or later, where the regular expression patterns have been corrected to prevent catastrophic backtracking by using anchored matches or more efficient parsing logic. For systems that cannot be immediately patched, implementing a Web Application Firewall with rules designed to detect and block excessively long Content-Type headers or malformed multipart boundaries can provide an effective compensating control. Additionally, configuring web servers to enforce strict limits on header size and request processing time can help contain the impact of such attacks by terminating hung processes before they exhaust system resources. Regular auditing of third-party modules for known regex vulnerabilities is also recommended as part of a comprehensive application security strategy.