Executive brief
PuTTY, a widely used tool for connecting to remote servers, contains a flaw that can cause the application to crash during the initial connection phase. A malicious server or an attacker positioned between the user and the server can trigger this crash by sending specially crafted data during the security handshake. While this primarily results in a service disruption (the application closing unexpectedly), it occurs before the server's identity is verified, meaning it can be triggered by untrusted sources.
Technical details
A double-free vulnerability (CWE-415) exists in PuTTY's implementation of RSA Key Exchange (RFC 4432). The flaw occurs in the error-handling path when a server sends an unexpectedly short RSA key; the code calls a specialized free function (ssh_rsakex_freekey) that deallocates the entire structure, followed by a standard free call on the same structure. This can be triggered by a malicious server or a Man-in-the-Middle (MITM) attacker during the KEXINIT phase before host key verification. While the primary impact is a remote-triggerable crash (denial of service), the vendor notes no known path to arbitrary code execution. The issue is fixed in version 0.84.
Affected products
- PuTTY PuTTY 0.72 to 0.83
Timeline
- 2026-05-22: patched: PuTTY 0.84 released fixing the issue
- 2026-05-25: disclosed: CVE-2026-48850 published