PuTTY 0.84 Update Patches SSH Key Exchange Crash Issues and Telnet Prompt Spoofing Flaw

PuTTY 0.84 has been released with three minor security fixes that address issues that could allow remote attackers to crash the client or trick users during insecure sessions.

Although the vulnerabilities are classified as low severity, they affect core components such as SSH key exchange and Telnet session handling, making the update important for users relying on PuTTY for secure remote access.

PuTTY 0.84 Update Patches Issues

One of the key issues fixed in this release involves elliptic curve cryptography used in SSH key exchange. The flaw exists in ECDSA signature verification, where a specific mathematical condition could trigger an assertion failure and cause PuTTY to crash, as reported by Openwall.

This can be exploited by a man-in-the-middle attacker who sends a specially crafted host key and signature during the initial connection phase. Since PuTTY verifies the signature before checking the host key cache, the crash can occur before any warning is shown to the user.

While this does not lead to data compromise or code execution, it results in a denial-of-service condition. It may cause loss of unsaved session data. The issue impacts NIST curves, including P-256, P-384, and P-521, while modern curves like Ed25519 remain unaffected.

Another vulnerability affects PuTTY’s implementation of RSA key exchange (RSA KEX), a less commonly used feature. A double-free memory bug can be triggered if a malicious server sends a deliberately malformed short RSA key during the handshake process.

This flaw exists in versions 0.72 through 0.83 and occurs in an error-handling path rather than during normal operations. Despite being remotely triggerable, there is currently no evidence that the bug can be exploited beyond causing the application to crash.

However, double-free vulnerabilities are generally considered risky because they can cause memory corruption under certain conditions.

The third issue concerns PuTTY’s “trust sigil” feature, which helps users distinguish between genuine client prompts and potentially malicious prompts generated by servers.

In affected versions, when users authenticate through a proxy in a Telnet session, the trust indicator is not properly cleared afterward. As a result, all subsequent session text may appear trusted, even if a malicious server generates it.

This creates a spoofing risk: an attacker could present a fake password prompt immediately after login, potentially tricking users into revealing sensitive information, such as proxy credentials. Although this vulnerability requires Telnet, which is inherently insecure, it undermines a key visual security mechanism in PuTTY.

Additionally, the release addresses a bug tracked as CVE-2026-4115 related to EdDSA signature handling. However, the PuTTY maintainer has indicated that this issue is unlikely to pose a real-world security risk.

Users are strongly advised to upgrade to PuTTY 0.84 to mitigate these issues. Security best practices also include avoiding legacy protocols such as Telnet, turning off unused key exchange methods, such as RSA KEX, and monitoring SSH connection failures that may indicate attempted exploitation.

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