CVE-2016-2563 in PuTTY
Summary
by MITRE
Stack-based buffer overflow in the SCP command-line utility in PuTTY before 0.67 and KiTTY 0.66.6.3 and earlier allows remote servers to cause a denial of service (stack memory corruption) or execute arbitrary code via a crafted SCP-SINK file-size response to an SCP download request.
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Analysis
by VulDB Data Team • 01/05/2025
The vulnerability identified as CVE-2016-2563 represents a critical stack-based buffer overflow affecting the SCP command-line utility within PuTTY and KiTTY software suites. This flaw exists in versions prior to 0.67 for PuTTY and 0.66.6.3 for KiTTY, creating a significant security risk for users who rely on these tools for secure file transfers. The vulnerability specifically targets the handling of SCP-SINK file-size responses during download operations, demonstrating how seemingly benign network interactions can be weaponized for malicious purposes.
The technical implementation of this vulnerability stems from improper input validation within the SCP protocol implementation. When a remote server crafts a malicious SCP-SINK response containing an oversized file-size field, the client application fails to properly bounds-check the incoming data before copying it into a fixed-size stack buffer. This classic buffer overflow scenario occurs because the application assumes the file size information will remain within predetermined limits, without adequate sanitization or size verification mechanisms. The flaw manifests during the download request process when the SCP utility attempts to parse the server's response, leading to stack memory corruption that can result in program termination or arbitrary code execution.
The operational impact of this vulnerability extends beyond simple denial of service to encompass potential remote code execution capabilities, making it particularly dangerous for environments where users regularly connect to untrusted servers. Attackers can exploit this weakness by setting up malicious SSH servers that respond to SCP download requests with crafted file-size values designed to overflow the stack buffer. The consequences include complete system compromise when successful, as the overflow can be manipulated to overwrite return addresses and function pointers, effectively allowing attackers to redirect execution flow. This vulnerability directly aligns with CWE-121, which describes stack-based buffer overflow conditions, and represents a prime example of how protocol implementation flaws can create persistent security weaknesses.
From a threat modeling perspective, this vulnerability demonstrates the importance of input validation in network protocols and highlights the risks associated with client-side applications that process untrusted server responses. The attack vector requires a malicious server to be present in the network, making it a server-side compromise scenario rather than a direct client-side attack, though the implications remain severe for any user who connects to compromised servers. Organizations should consider implementing network segmentation and monitoring for unusual SCP traffic patterns as part of their defensive strategy. Mitigation efforts include immediate upgrades to PuTTY version 0.67 or later and KiTTY version 0.66.6.4 or higher, along with network-level controls such as firewall rules that restrict SCP traffic to trusted sources and monitoring for anomalous file-size values in SCP communications. The vulnerability also underscores the necessity of applying security patches promptly and maintaining updated security tooling to protect against similar implementation flaws in other SSH client applications.