# CVE-2026-48850: PuTTY double free in RSA KEX

Source: https://junglewise.ai/threats/cve-2026-48850-putty-double-free-in-rsa-kex  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-25. Updated: 2026-05-26  
License: CC BY 4.0 with a link back

CVE-2026-48850 · Severity: low · CVSS 3.7 · Published 2026-05-25

PuTTY, a widely used tool for connecting to remote servers, contains a flaw that can cause the application to crash during the initial connection phase. A malicious server or an attacker positioned between the user and the server can trigger this crash by sending specially crafted data during the security handshake. While this primarily results in a service disruption (the application closing unexpectedly), it occurs before the server's identity is verified, meaning it can be triggered by untrusted sources.

Technologies: [Putty](https://junglewise.ai/threats/technologies/putty). Vendors: [Putty](https://junglewise.ai/threats/vendors/putty).

## Quick answers

**Is CVE-2026-48850 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-48850 being exploited in the wild.

**How severe is CVE-2026-48850?** Low severity, with a CVSS score of 3.7, published 25 May 2026.

**Which products and versions are affected?** Putty 0.72 to 0.83.

**Is there a fix for CVE-2026-48850?** A patch was published on 2026-05-22: PuTTY 0.84 released fixing the issue Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-48850 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-48850?** Check whether you run Putty at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A double-free vulnerability (CWE-415) exists in PuTTY's implementation of RSA Key Exchange (RFC 4432). The flaw occurs in the error-handling path when a server sends an unexpectedly short RSA key; the code calls a specialized free function (ssh_rsakex_freekey) that deallocates the entire structure, followed by a standard free call on the same structure. This can be triggered by a malicious server or a Man-in-the-Middle (MITM) attacker during the KEXINIT phase before host key verification. While the primary impact is a remote-triggerable crash (denial of service), the vendor notes no known path to arbitrary code execution. The issue is fixed in version 0.84.

## Affected products

- PuTTY PuTTY 0.72 to 0.83

## Timeline

- 2026-05-22: patched: PuTTY 0.84 released fixing the issue
- 2026-05-25: disclosed: CVE-2026-48850 published

## References

- [https://lists.tartarus.org/pipermail/putty-announce/2026/000042.html](https://lists.tartarus.org/pipermail/putty-announce/2026/000042.html)
- [https://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/rsakex-double-free.html](https://www.chiark.greenend.org.uk/~sgtatham/putty/wishlist/rsakex-double-free.html)

## Related threats

- [PuTTY assertion failure in ECDSA signature verification](https://junglewise.ai/threats/cve-2026-48852-putty-assertion-failure-in-ecdsa-signature-verification)
- [PuTTY trust sigil spoofing in Telnet and Rlogin sessions](https://junglewise.ai/threats/cve-2026-48851-putty-trust-sigil-spoofing-in-telnet-and-rlogin-sessions)
- [PuTTY Ed25519 signature malleability in eddsa_verify](https://junglewise.ai/threats/cve-2026-4115-putty-ed25519-signature-malleability-in-eddsa-verify)
- [OpenSSH and others SSH prefix truncation vulnerability (Terrapin)](https://junglewise.ai/threats/cve-2023-48795-openssh-and-others-ssh-prefix-truncation-vulnerability-terrapin)