PuTTY CVE-2026-48850

The vulnerability requires that RSA key exchange be negotiated during the SSH handshake - meaning either the server offers RSA KEX and PuTTY accepts it, or PuTTY's KEX algorithm preference list includes RSA methods (which is common in default PuTTY configurations). CVSS PR:N confirms no authentication is required from the attacker's perspective: the attacker operates as the server (or MITM), not as an authenticated SSH client. CVSS AC:H is the primary limiting factor, indicating that triggering the double free reliably requires conditions beyond a simple connect-and-send, such as specific heap state or protocol sequencing. CVSS UI:N means no victim interaction beyond initiating the SSH connection is needed. The scope is limited to the PuTTY client process itself (S:U), so exploitation cannot directly pivot to other processes or users on the system. Exploitation is further limited to PuTTY versions 0.72-0.83 specifically; other SSH clients are not affected by this CVE.

Multiple signals converge on a low real-world risk assessment. The CVSS 3.7 Low score reflects high attack complexity (AC:H), no confidentiality or integrity impact (C:N/I:N), and only limited availability impact (A:L). EPSS at 0.04% (12th percentile) is very low, indicating the broader security community does not expect widespread exploitation in the near term. SSVC analysis confirms exploitation status as none, the attack is not automatable, and technical impact is partial - all pointing away from prioritized emergency response. There is no CISA KEV listing and no public exploit code identified at time of analysis. One notable data inconsistency: the tag 'Information Disclosure' conflicts with the CVSS C:N (no confidentiality impact) rating; this discrepancy is not resolved in the available data and may reflect a tagging error or a secondary, unscored behavior described in the vendor wishlist page. Defenders should not treat this as a high-priority emergency but should schedule remediation as part of normal patch cycles for internet-facing or sensitive SSH client deployments.

A user running vulnerable PuTTY (0.72-0.83) connects to an attacker-controlled or compromised SSH server. The server negotiates RSA key exchange, and upon processing the crafted RSA KEX response, PuTTY triggers the double free condition, causing a crash or potential heap corruption. The AC:H CVSS rating indicates this requires specific conditions - likely precise heap layout or server-side sequencing - making reliable exploitation difficult but not impossible in targeted scenarios.