CVE-2026-42859
CVE-2026-42859 is a high-severity vulnerability with a CVSS 4.0 base score of 8.1. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-120.
Key facts
- Severity: High (CVSS 4.0 base score 8.1)
- EPSS exploit prediction: 1% (42nd percentile)
- Actively exploited: Not listed in CISA KEV
- EU (EUVD) id: EUVD-2026-29167
- Weakness: CWE-120
- Published:
- Last modified:
Description
Neat VNC is a VNC server library. Prior to 0.9.6, a pre-authentication stack buffer overflow exists in neatvnc in the RSA-AES security type handler. An unauthenticated remote attacker who can reach the VNC listening socket can send a crafted security type 5 (RSA-AES) or security type 129 (RSA-AES-256) handshake with an oversized client RSA public key, causing rsa_aes_send_challenge in src/auth/rsa-aes.c to overflow a 1024-byte on-stack buffer when encrypting the server challenge. This results in at least a denial of service via server crash. This vulnerability is fixed in 0.9.6.
Frequently asked questions
- What is CVE-2026-42859?
- Neat VNC is a VNC server library. Prior to 0.9.6, a pre-authentication stack buffer overflow exists in neatvnc in the RSA-AES security type handler. An unauthenticated remote attacker who can reach the VNC listening socket can send a crafted security type 5 (RSA-AES) or security type 129 (RSA-AES-256) handshake with an oversized client RSA public key, causing rsa_aes_send_challenge in src/auth/rsa-aes.c to overflow a 1024-byte on-stack buffer when encrypting the server challenge. This results in at least a denial of service via server crash. This vulnerability is fixed in 0.9.6.
- How severe is CVE-2026-42859?
- CVE-2026-42859 has a CVSS 4.0 base score of 8.1, rated high severity.
- Is CVE-2026-42859 being actively exploited?
- It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (42nd percentile), an estimate of the probability of exploitation in the next 30 days.
- How do I fix CVE-2026-42859?
- Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
- Does CVE-2026-42859 have an EU (EUVD) identifier?
- Yes. CVE-2026-42859 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-29167.
- When was CVE-2026-42859 published?
- CVE-2026-42859 was published on 2026-05-11 and last updated on 2026-06-17.
References
- https://github.com/any1/neatvnc/commit/1f6cd6b75cc167fed3a19a9d1552a1f662f6b337
- https://github.com/any1/neatvnc/security/advisories/GHSA-567c-gpv8-qh9h
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