CVE-2017-20205 in Source SDK
Summary
by MITRE • 10/15/2025
Valve's Source SDK (source-sdk-2013)'s ragdoll model parsing logic contains a stack-based buffer overflow vulnerability.The tokenizer function `nexttoken` copies characters from an input string into a fixed-size stack buffer without performing bounds checks. When `ParseKeyValue` processes a collisionpair rule longer than the destination buffer (256 bytes), an overflow of the stack buffer `szToken` can occur and overwrite the function return address. A remote attacker can trigger the vulnerable code by supplying a specially crafted ragdoll model which causes the oversized collisionpair rule to be parsed, resulting in remote code execution on affected clients or servers. Valve has addressed this issue in many of their Source games, but independently-developed games must manually apply patch.
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Analysis
by VulDB Data Team • 10/17/2025
The vulnerability identified as CVE-2017-20205 represents a critical stack-based buffer overflow within Valve's Source SDK 2013 implementation, specifically affecting the ragdoll model parsing functionality. This flaw exists in the tokenizer function known as `nexttoken` which lacks proper bounds checking when copying data from input strings into fixed-size stack buffers. The vulnerability manifests when processing collisionpair rules within ragdoll model files, where the destination buffer `szToken` has a fixed size of 256 bytes. When an attacker provides a maliciously crafted ragdoll model containing a collisionpair rule exceeding this buffer limit, the overflow occurs during the parsing process initiated by the `ParseKeyValue` function. The technical nature of this vulnerability places it squarely within the CWE-121 category of stack-based buffer overflow conditions, where insufficient bounds checking allows an attacker to overwrite adjacent stack memory including the function return address.
The operational impact of this vulnerability extends across all Source engine-based games that utilize the affected SDK components, creating a significant risk for both client and server environments. A remote attacker capable of delivering a specially crafted ragdoll model file can exploit this weakness to achieve arbitrary code execution on affected systems, potentially leading to complete system compromise. The attack vector requires the victim to load or process the malicious model file, which could occur through various means including multiplayer game sessions, mod downloads, or server-side content delivery. This vulnerability is particularly concerning because it affects not only Valve's own games but also numerous independently developed titles that rely on the Source SDK, meaning the attack surface encompasses a broad range of applications. The exploitation process follows established patterns consistent with the ATT&CK framework's technique T1059 for command and scripting interpreter, as the overflow enables execution of arbitrary code on target systems.
Mitigation strategies for this vulnerability require immediate patch application to all affected Source engine implementations, with manual remediation necessary for independently developed games that have not received official updates from Valve. The most effective approach involves implementing proper bounds checking in the `nexttoken` function to prevent copying data that exceeds the allocated buffer size of 256 bytes. Security-conscious developers should also implement input validation measures that restrict the length of collisionpair rules during model parsing, ensuring that any oversized inputs are either rejected or truncated before processing. Additionally, runtime protections such as stack canaries and address space layout randomization should be considered as supplementary defenses, though these are not sufficient to prevent this specific vulnerability on their own. Organizations maintaining Source engine-based applications must also establish robust content validation procedures for user-generated models and ensure that all third-party content undergoes thorough security screening before deployment. The vulnerability highlights the importance of maintaining up-to-date SDK versions and implementing comprehensive security testing protocols for game engines that process external data files.