CVE-2021-41380Public exploitNetwork attack vector

Persistent hang via prematurely closed Zlib stream

Published Sep 17, 2021 · Updated Aug 4, 2024

Denial of service in RealVNC Viewer 6.21.406 allows remote attackers to hang the connected Viewer process via crafted RFB data. A framebuffer update that closes its Zlib stream unexpectedly traps the Viewer decoder in a tight loop that never recovers. A user must connect to the attacker's server and, for the public proof of concept, accept an unencrypted-connection warning; other Viewer connections remain available.

CVSS severity6.5
Medium
EPSS probability0.96%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in RealVNC Viewer 6.21.406 allows remote attackers to hang the connected Viewer process via crafted RFB data. A framebuffer update that closes its Zlib stream unexpectedly traps the Viewer decoder in a tight loop that never recovers. A user must connect to the attacker's server and, for the public proof of concept, accept an unencrypted-connection warning; other Viewer connections remain available.

The record

CVE
CVE-2021-41380
Published
Sep 17, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-20, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 17, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

2.6RealVNCAV:N/AC:H/Au:N/C:N/I:N/A:P
6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H
4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:N/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-20Improper Input Validation
T1499Endpoint Denial of Service

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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