CVE-2018-20021

LibVNC before commit c3115350eb8bb635d0fdb4dbbb0d0541f38ed19c contains a CWE-835: Infinite loop vulnerability in VNC client code. Vulnerability allows attacker to consume excessive amount of resources like CPU and RAM

Published: 2018-12-19 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2018-20021 is rated Moderate Risk (59.3/100): CVSS High severity, with medium exploitation likelihood (EPSS 2.55%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2018-20021

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2025-11-21 3.04% 2.55% -0.49%
2 2025-11-18 2.64% 3.04% +0.40%
3 2025-03-30 — 2.64% —

Full EPSS history (12 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2018-20021

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.5 3.0 HIGH
CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]
7.8 2.0 HIGH
AV:N/AC:L/Au:N/C:N/I:N/A:C Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:N)
No integrity impact.
Availability impact (A:C)
Complete availability impact.
 10.0 6.9 [email protected]

Weakness enumeration for CVE-2018-20021

OS Trackers for CVE-2018-20021

vendor priority summary link
alpine high CVE-2018-20021: 1 source package rows (libvncserver); 4 state rows across 2 repos (3.22-community, edge-community); fixed 0, open 4. https://security.alpinelinux.org/vuln/CVE-2018-20021
debian not yet assigned CVE-2018-20021 not yet assigned priority: Debian including 4 source packages (libvncserver, ssvnc, tightvnc, veyon), 20 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 20. https://security-tracker.debian.org/tracker/CVE-2018-20021
gentoo normal CVE-2018-20021: 2 GLSA(s) (201908-05, 202006-06), 2 atom(s) (net-libs/libvncserver, net-misc/ssvnc); latest impact normal. https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2018-20021
redhat low — https://access.redhat.com/security/cve/CVE-2018-20021
suse medium CVE-2018-20021 severity moderate: SUSE including 22 source package names (LibVNCServer-0.9.1-160.6.1, LibVNCServer-devel-0.9.1-160.6.1, …), 86 product×package rows across 54 product lines (SUSE Enterprise Storage 4, SUSE Linux Enterprise High Performance Computing 12 SP5, … (54 product lines)): Fixed 86. https://www.suse.com/security/cve/CVE-2018-20021/
ubuntu medium CVE-2018-20021 medium priority: Ubuntu including 5 source packages (italc, libvncserver, ssvnc, tightvnc, x11vnc), 84 status rows across 18 suites (bionic, cosmic, disco, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): not-affected 41, DNE 14, released 12, ignored 9, needs-triage 7, needed 1. https://ubuntu.com/security/CVE-2018-20021

Affected software / configurations for CVE-2018-20021

Vendor Product Version Raw CPE
libvnc_project libvncserver < 0.9.12 cpe:2.3:a:libvnc_project:libvncserver:*:*:*:*:*:*:*:*
canonical ubuntu_linux 14.04 cpe:2.3:o:canonical:ubuntu_linux:14.04:*:*:*:lts:*:*:*
canonical ubuntu_linux 16.04 cpe:2.3:o:canonical:ubuntu_linux:16.04:*:*:*:lts:*:*:*
canonical ubuntu_linux 18.04 cpe:2.3:o:canonical:ubuntu_linux:18.04:*:*:*:lts:*:*:*
canonical ubuntu_linux 18.10 cpe:2.3:o:canonical:ubuntu_linux:18.10:*:*:*:*:*:*:*
debian debian_linux 8.0 cpe:2.3:o:debian:debian_linux:8.0:*:*:*:*:*:*:*
debian debian_linux 9.0 cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*

References for CVE-2018-20021

cvelogic Threat Intelligence