The update, released on 22 May 2026, addresses defects that could allow a malicious server or a man-in-the-middle attacker to crash a PuTTY session or mislead a user during older, insecure remote-login workflows. The maintainers have not identified any route for code execution, but the flaws touch sensitive areas of the client, including authentication prompts and cryptographic negotiation before a trusted connection is fully established.
PuTTY remains one of the most widely used free terminal and remote access clients, particularly on Windows systems used by administrators, developers, support teams, network engineers and security practitioners. The suite supports SSH, Telnet, Rlogin, SCP and SFTP functions, with tools such as PuTTY, Plink, PSCP, PSFTP and Pageant forming part of many operational workflows. That broad deployment means even modest security fixes can carry importance for organisations that depend on the client for server administration.
The most significant fix in version 0.84 concerns a remotely triggerable double-free condition in RSA key exchange. The issue affected the less commonly used RSA key exchange method and could be provoked when a server deliberately sent an unexpectedly short key during negotiation. Because this stage happens before host-key verification, an attacker positioned between client and server could also trigger the crash by interfering with the exchange.
A double-free error occurs when software attempts to release the same memory object more than once, a class of flaw that can sometimes create exploitation opportunities. In this case, the maintainers said they were not aware of a practical method to turn the defect into code execution. The immediate risk is denial of service, causing the affected PuTTY process to terminate. Since PuTTY typically runs one SSH session per process, other running sessions would not normally be affected.
A second security fix resolves a crash in NIST ECDSA signature verification. The problem was tied to an assertion failure in elliptic curve arithmetic involving NIST curves such as P-256, P-384 and P-521. A carefully chosen host key and signature could make PuTTY fail during the initial key exchange. Ed25519 and Ed448 were not affected.
That flaw also mattered because signature verification occurs before PuTTY checks the host key against its cache. A user attempting to connect to a trusted server could therefore encounter a crash caused by an attacker substituting malicious key material before the client displayed the normal warning about an unknown or incorrect host key. The practical effect was again limited to disruption rather than compromise, but it could interrupt administrative access or erase useful scrollback data in a restarted session.
The third vulnerability involves PuTTY’s trust sigil, a visual marker used to distinguish prompts generated by PuTTY itself from text sent by a remote server. This mechanism is designed to reduce the risk of spoofed prompts that attempt to trick users into entering sensitive information, such as a private key passphrase or proxy password.
The flaw appeared in Telnet and Rlogin sessions after proxy authentication. Under certain conditions, session data could continue to be marked as trusted after the authentication phase ended. A malicious server or attacker controlling traffic could use that confusion to present a fake prompt, potentially persuading a user to re-enter a proxy password. The impact is regarded as small, especially because it depends on older protocols that lack the security model of SSH, but it reinforces why Telnet and Rlogin are unsuitable for sensitive access.
Beyond security fixes, PuTTY 0.84 adds the ability to run a specified command before starting a connection, a feature that can support workflows such as wake-on-LAN or port knocking. Unix users also receive better handling of pre-edit text for composing Unicode characters and improvements for running graphical PuTTY tools on Wayland. Additional bug fixes address SSH certificate authority configuration on Unix, proxy authentication errors and cursor blinking behaviour on Windows.
The update follows PuTTY 0.83, which added support for ML-KEM, the NIST-standardised post-quantum key exchange mechanism, alongside earlier support for NTRU Prime. That trajectory shows the project continuing to adapt to cryptographic transition pressures while maintaining compatibility with older remote-access environments still present in enterprise networks.
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