CVE-2025-9648 in CivetWeb
Summary
by MITRE • 09/29/2025
A vulnerability in the CivetWeb library's function mg_handle_form_request allows remote attackers to trigger a denial of service (DoS) condition. By sending a specially crafted HTTP POST request containing a null byte in the payload, the server enters an infinite loop during form data parsing. Multiple malicious requests will result in complete CPU exhaustion and render the service unresponsive to further requests.
This issue was fixed in commit 782e189. This issue affects only the library, standalone executable pre-built by vendor is not affected.
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Analysis
by VulDB Data Team • 10/02/2025
The vulnerability identified as CVE-2025-9648 resides within the CivetWeb library, a lightweight embedded web server library widely used in various applications for HTTP functionality. This library serves as a foundational component in numerous software systems, making the discovery of such a critical flaw particularly concerning for system administrators and security professionals. The issue manifests specifically within the mg_handle_form_request function, which processes incoming HTTP POST requests containing form data. The flaw represents a classic example of improper input validation where the library fails to properly handle null byte characters within form payloads, leading to unpredictable behavior during data processing operations.
The technical exploitation of this vulnerability occurs through the deliberate insertion of null bytes within HTTP POST request payloads, a technique that leverages the library's inadequate handling of such characters during form data parsing. When the mg_handle_form_request function encounters a null byte within the payload, it triggers an infinite loop condition in the parsing algorithm, causing the server process to consume excessive CPU resources continuously. This infinite loop represents a direct violation of the principle of bounded execution, where normal program flow should terminate after processing a finite amount of input data. The vulnerability aligns with CWE-835, which specifically addresses the issue of loops that do not contain a mechanism for termination, and demonstrates how improper input validation can lead to resource exhaustion conditions. The attack vector requires only a single malicious request to initiate the problematic behavior, but multiple requests can compound the effect, leading to complete system unresponsiveness.
The operational impact of CVE-2025-9648 extends beyond simple service disruption, as it represents a significant threat to system availability and reliability for any application utilizing the affected CivetWeb library. The infinite loop condition causes complete CPU exhaustion, making the service unavailable to legitimate users and potentially allowing attackers to perform successful denial of service attacks against critical infrastructure. This vulnerability particularly affects applications that process user-submitted form data without proper input sanitization, creating a pathway for attackers to consume system resources and render services inoperable. The fact that this issue affects only the library component, rather than the standalone executable, means that developers and system administrators must carefully audit their application dependencies and ensure they are using patched versions of the library. This vulnerability demonstrates the importance of proper input validation and the potential for seemingly benign input characters to cause catastrophic system behavior, aligning with ATT&CK technique T1499.004 for network denial of service attacks.
Mitigation strategies for CVE-2025-9648 primarily focus on updating to the patched version of the CivetWeb library, with the fix implemented in commit 782e189 providing the necessary protections against the null byte handling issue. Organizations should conduct comprehensive inventory assessments to identify all applications and systems utilizing the vulnerable library version, ensuring that all dependencies are updated to prevent exploitation. Additionally, implementing proper input validation at application level can provide defense in depth, though this approach is not a substitute for the library-level fix. Network-level protections such as rate limiting and request filtering can help reduce the impact of potential attacks by limiting the number of malicious requests that can reach the vulnerable library. Security monitoring should be enhanced to detect unusual CPU consumption patterns that might indicate exploitation attempts, while also ensuring that all development environments maintain updated library versions to prevent reintroduction of the vulnerability through new deployments.