CVE-2026-48852: PuTTY ECDSA Signature Vulnerability

CVE-2026-48852 Overview

CVE-2026-48852 is a remotely triggerable assertion failure in PuTTY versions 0.71 through 0.83. The flaw resides in the Elliptic Curve Digital Signature Algorithm (ECDSA) signature verification path. A malformed signature from a remote SSH server can cause the PuTTY client to abort, terminating the session. The issue is tracked under [CWE-617: Reachable Assertion] and affects only availability, not confidentiality or integrity. PuTTY 0.84 contains the fix.

Critical Impact
A malicious or compromised SSH server can crash a connecting PuTTY client by sending a crafted ECDSA signature, denying interactive shell access to the affected user.

Affected Products

PuTTY 0.71 through 0.83 (Windows and Unix builds)
PuTTY-derived tools sharing the same ECDSA verification code (plink, pscp, psftp)
Third-party software embedding the affected PuTTY libraries

Discovery Timeline

2026-05-25 - CVE-2026-48852 published to NVD
2026-05-27 - Last updated in NVD database

Technical Details for CVE-2026-48852

Vulnerability Analysis

The vulnerability is a reachable assertion in PuTTY's ECDSA signature verification routine. When PuTTY parses an ECDSA signature received from an SSH server during the host key exchange, an internal consistency check fails on certain malformed inputs. Instead of returning a verification error and closing the session cleanly, the code triggers an assert() that aborts the process.

The defect is classified as [CWE-617: Reachable Assertion]. The impact is limited to denial of service against the client process. No memory corruption, code execution, or key material disclosure has been reported. Because the assertion fires before the user authenticates, an attacker controlling the server side of the TCP connection can prevent a successful SSH session.

Root Cause

The root cause is an overly strict invariant inside the ECDSA verification function. The code assumes well-formed signature scalars and aborts on values that should instead be rejected as invalid signatures. Hostile or buggy SSH servers can produce such values and reach the assertion remotely.

Attack Vector

Exploitation requires the victim to initiate an SSH connection to a server controlled by the attacker, or to a server whose traffic the attacker can intercept and modify. The attacker returns a host key signature using ECDSA with crafted scalar components. PuTTY processes the signature, hits the assertion, and terminates. The attack complexity is high because the attacker must induce the user to connect and must control the server side of the SSH handshake.

No verified proof-of-concept code has been published. The vulnerability mechanism is described in the PuTTY wishlist advisory.

Detection Methods for CVE-2026-48852

Indicators of Compromise

Unexpected PuTTY, plink, pscp, or psftp process crashes immediately after initiating an SSH connection
Windows Application Event Log entries showing putty.exe terminated with an assertion failure message
Repeated failed SSH session establishment from the same client to an external host followed by process exit

Detection Strategies

Inventory endpoints for PuTTY binaries with versions between 0.71 and 0.83 using software asset management or endpoint telemetry
Alert on PuTTY process exits that correlate with outbound SSH (TCP/22) connections to untrusted destinations
Review SSH client logs and host key prompts for connections to unfamiliar servers preceding crashes

Monitoring Recommendations

Track outbound SSH sessions from user endpoints and flag those terminating within seconds of TCP establishment
Monitor for the presence of vulnerable PuTTY versions in software inventory feeds and patch compliance reports
Correlate PuTTY crash events with the destination host reputation to identify suspicious endpoints

How to Mitigate CVE-2026-48852

Immediate Actions Required

Upgrade all PuTTY installations to version 0.84 or later, including bundled tools plink, pscp, psftp, and pageant
Identify and update third-party applications that embed PuTTY components
Restrict outbound SSH connections from user workstations to known, trusted destinations where feasible

Patch Information

The PuTTY maintainers released version 0.84 to address this issue. Download the patched build from the official PuTTY site or via package managers. Release details are available in the PuTTY announcement and the PuTTY wishlist entry.

Workarounds

Configure PuTTY's host key algorithm preference to deprioritize or disable ECDSA host keys and prefer Ed25519 or RSA where the remote server supports them
Only connect to SSH servers whose host keys are already cached and verified, reducing exposure to attacker-controlled endpoints
Use jump hosts or bastion servers with known-good SSH implementations as an intermediary for untrusted destinations

bash

# Configuration example: reorder host key algorithms in PuTTY session settings
# Connection > SSH > Host keys: move "ECDSA" below "Ed25519" and "RSA"
# Or via command line, prefer ssh.exe / OpenSSH client until patch is applied:
ssh -o HostKeyAlgorithms=ssh-ed25519,rsa-sha2-512,rsa-sha2-256 user@host