EUVD-2026-31735
GHSA-rx8r-f2xx-98h9
Severity by source
AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
Primary rating from NVD · only source for this CVE.
CVSS VectorNVD
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
Lifecycle Timeline
2DescriptionCVE.org
PuTTY 0.71 before 0.84 has an assertion failure in ECDSA signature verification.
AnalysisAI
Assertion failure in ECDSA signature verification within PuTTY 0.71 through 0.83 allows a network-positioned attacker to crash the PuTTY client, producing a denial-of-service condition. The vulnerability (CWE-617: Reachable Assertion) carries a CVSS 3.7 Low score with no confidentiality or integrity impact - availability impact is limited to the client process itself. No public exploit code exists and no active exploitation has been identified; SSVC rates exploitation as none and the EPSS score of 0.04% (12th percentile) confirms minimal current exploitation probability.
Technical ContextAI
PuTTY is a widely deployed open-source SSH, Telnet, and serial client used predominantly on Windows. The vulnerability resides in the ECDSA (Elliptic Curve Digital Signature Algorithm) signature verification code path, which is invoked during SSH host key authentication when a server presents an ECDSA key. CWE-617 (Reachable Assertion) describes a condition where an assert() or equivalent check can be triggered by externally supplied data, causing abnormal process termination rather than graceful error handling. In this case, a crafted or malformed ECDSA signature value delivered by a server can satisfy the conditions required to fire the assertion, immediately terminating the PuTTY client process. The affected CPE is cpe:2.3:a:putty:putty:*:*:*:*:*:*:*:* covering the version range 0.71 up to but not including 0.84, per EUVD-2026-31735.
RemediationAI
Upgrade PuTTY to version 0.84 or later, which is the vendor-released patch per the official announcement at https://lists.tartarus.org/pipermail/putty-announce/2026/000042.html. The fix version (0.84) is confirmed by both the EUVD affected versions range and the vendor wishlist page. Where immediate upgrade is not feasible, a targeted compensating control is to configure PuTTY host key algorithms to exclude ECDSA key types (using the 'Host key algorithm preference' setting in PuTTY's SSH configuration), forcing fallback to RSA or Ed25519 - this eliminates the vulnerable code path entirely at the cost of incompatibility with ECDSA-only servers. Additionally, connecting only to SSH servers with known, pre-verified host keys (enforced via PuTTY's known-hosts list) prevents an attacker-controlled server from delivering a malicious signature.
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Same weakness CWE-617 – Reachable Assertion
View allSame technique Denial Of Service
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External POC / Exploit Code
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