What is CVE-2021-42785?

CVE-2021-42785 is a critical-severity vulnerability in Glavsoft Llc Tightvnc Viewer, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2021-11-23.

How severe is CVE-2021-42785?

Critical severity. CVSS v3 base score is 9.8 out of 10.

Buffer overflow in Glavsoft Llc Tightvnc Viewer

CVE-2021-42785

Buffer Overflow vulnerability in tvnviewer.exe of TightVNC Viewer allows a remote attacker to execute arbitrary instructions via a crafted FramebufferUpdate packet from a VNC server.

Vulnerability class: Buffer Overflow

Published 2021-11-23 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.023 (83.0th percentile), scored 2026-09-30.

Low probability of exploitation in the next 30 days, but still tracked. 83rd percentile — 83.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.023 · newest: 0.023 · change: +0.001

Affected products

Glavsoft Llc Tightvnc Viewer — versions 2.8.59
Tightvnc

Weakness classification (CWE)

CWE-120 — Buffer Copy without Checking Size of Input (Classic Buffer Overflow)

References

www.tightvnc.com/whatsnew.php (Release Notes, Vendor Advisory)

Frequently asked questions

What is CVE-2021-42785?: CVE-2021-42785 is a critical-severity vulnerability in Glavsoft Llc Tightvnc Viewer, classified under Buffer Copy without Checking Size of Input (Classic Buffer Overflow). CVSS score: 9.8/10. Published 2021-11-23.
How severe is CVE-2021-42785?: Critical severity. CVSS v3 base score is 9.8 out of 10.