PuTTY has released version 0.84 on May 24, 2026, patching three confirmed security vulnerabilities, two involving SSH key exchange crashes and one enabling Telnet-based credential spoofing.
The disclosure was announced via the oss-security mailing list by Oracle Solaris engineer Alan Coopersmith and covers fixes present in all prior versions dating back to 0.71. All users running PuTTY 0.83 or earlier should treat this as a priority update.
The first ECDSA signature verification vulnerability has been lurking in PuTTY since version 0.71 (released March 2019).
PuTTY 0.84 Update Patches
It stems from a flawed assertion in PuTTY’s elliptic curve arithmetic code: specifically, it incorrectly treated adding two elliptic curve points with the same y-coordinate as an error condition, even though it is mathematically valid.
During SSH initial key exchange, a man-in-the-middle (MITM) attacker can substitute a crafted host key and signature, triggering this assertion failure and crashing PuTTY before the user receives any warning about an untrusted or unexpected host key.
Since PuTTY verifies the host key signature before checking against its known-hosts cache, the crash occurs silently, without user-visible alerts.
This bug only affects NIST curves in Weierstrass form: P256, P384, and P521. The widely used Ed25519 and Ed448 algorithms are fully unaffected.
The vulnerability is classified as a low-severity DoS. PuTTY runs one SSH session per process, meaning no other open sessions are impacted. However, terminal scrollback data from a prior session in the same window could be lost on a crash.
Security researcher Guido Vranken discovered and reported the bug with a working P256 proof-of-concept.
The second vulnerability, introduced in PuTTY 0.72 (July 2019) and persisting through 0.83, is a double-free memory corruption bug in PuTTY’s implementation of the RSA key exchange method defined in RFC 4432.
The code erroneously called both ssh_rsakex_freekey() which freed the entire RSA struct and then the standard free() function on the same already-freed memory.
A malicious server or MITM can deliberately trigger this by advertising RSA KEX as the only supported algorithm in its KEXINIT packet and then sending an unexpectedly short RSA key.
Since this occurs before host key verification, even unestablished sessions are at risk. While no known exploitation path achieves controllable code execution, the remote-triggerable crash qualifies it as a formal vulnerability.
The root cause traces back to a 2019 refactoring effort to build a test SSH server from PuTTY’s codebase, which introduced confusion over ownership of the RSA key struct and whether it had already been freed.
The third vulnerability, introduced in PuTTY 0.77 (May 2022), affects Telnet and Rlogin sessions routed through an authenticated network proxy.
PuTTY uses “trust sigils,” a small PuTTY icon marker displayed beside its own prompts to distinguish legitimate PuTTY-generated authentication prompts from server-spoofed ones, protecting users from credential phishing by malicious servers.
The bug caused PuTTY to fail to clear the trust sigil state after proxy authentication completed, so the entire Telnet session incorrectly inherited those trusted markers.
A MITM attacker could exploit this by sending a fake “re-enter proxy password” prompt immediately after proxy auth, one that carries the trusted PuTTY icon, before the user realizes the anomaly.
By the time the user notices that all session text is marked, credentials may already be compromised, Alan Coopersmith said.
Beyond the three patched flaws, a separate CVE-2026-4115 was issued for an EdDSA oversized S-value issue, which was fixed in this release.
However, PuTTY’s maintainer explicitly disputes its classification as a security vulnerability, providing a detailed technical rebuttal on the official PuTTY wishlist page.
Users should upgrade to PuTTY 0.84 immediately. Downloads are available at the official PuTTY website. Organizations that use Telnet-based workflows via authenticated proxies are particularly encouraged to prioritize this update.
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