CVE-2021-34585 in V2 Web Server
Summary
by MITRE • 10/26/2021
In the CODESYS V2 web server prior to V1.1.9.22 crafted web server requests can trigger a parser error. Since the parser result is not checked under all conditions, a pointer dereference with an invalid address can occur. This leads to a denial of service situation.
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Analysis
by VulDB Data Team • 08/15/2025
The vulnerability identified as CVE-2021-34585 affects the CODESYS V2 web server software, specifically impacting versions prior to V1.1.9.22. This issue represents a critical security flaw that stems from inadequate input validation within the web server's request parsing mechanism. The vulnerability manifests when crafted malicious web server requests are processed by the affected system, creating a scenario where the parser encounters an error condition that is not properly handled. The root cause lies in the software's failure to validate parser results under all operational conditions, creating a dangerous path where invalid memory access can occur.
The technical implementation of this vulnerability involves a classic pointer dereference error that occurs when the web server's parser encounters malformed input and fails to properly validate the parsing outcome. When the parser processes the crafted requests, it generates an error state that should be caught and handled appropriately, but instead the system proceeds to attempt a pointer dereference using an invalid memory address. This behavior aligns with CWE-476 which describes NULL pointer dereference vulnerabilities, where a program attempts to access memory through a pointer that has not been properly initialized or validated. The improper handling of the parser error creates a condition where the system attempts to access memory at an address that does not contain valid data, resulting in an immediate system crash or denial of service condition.
The operational impact of this vulnerability is severe and directly affects the availability of the affected CODESYS V2 web server instances. When exploited, the vulnerability results in a denial of service scenario that can completely disrupt web server operations and prevent legitimate users from accessing services. This type of attack can be particularly damaging in industrial control systems and automation environments where CODESYS V2 is commonly deployed, as it can lead to operational downtime and potential safety risks. The vulnerability operates at the application layer and can be exploited remotely without requiring authentication, making it particularly dangerous in environments where network exposure is high. From an ATT&CK framework perspective, this vulnerability maps to T1499.004 which covers network denial of service attacks, and potentially T1595.001 for reconnaissance activities that might precede exploitation attempts.
Organizations affected by this vulnerability should prioritize immediate remediation through the application of the vendor-provided patch or upgrade to version V1.1.9.22 or later. The patch addresses the core issue by implementing proper validation of parser results and ensuring that error conditions are handled gracefully without attempting invalid memory operations. Additional mitigations should include network segmentation to limit exposure of affected systems, implementation of web application firewalls to filter suspicious requests, and monitoring for unusual traffic patterns that might indicate exploitation attempts. Security teams should also consider implementing intrusion detection systems that can identify the specific malformed request patterns associated with this vulnerability. The vulnerability demonstrates the importance of proper input validation and error handling in server applications, particularly in industrial control environments where availability is critical and traditional security controls may be insufficient.