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Fencer Vulns
high 8.6

CVE-2018-21035 json

In Qt through 5.14.1, the WebSocket implementation accepts up to 2GB for frames and 2GB for messages.

published 2020-02-28 6 y ago modified 2026-06-17 4 mo ago also EUVD-2018-13555

cvss
8.6
version 3.0, CNA
cisa kev
Not listed
not in CISA's catalog
epss
2.3%
EPSS 2.3% chance of exploitation in 30 days, percentile 83%
public exploits
0
none known

Summary

In Qt through 5.14.1, the WebSocket implementation accepts up to 2GB for frames and 2GB for messages. Smaller limits cannot be configured. This makes it easier for attackers to cause a denial of service (memory consumption).

Patch / fix

  • qtwebsockets-opensource-src Debian 14, Debian 13, Debian 12 5.15.1-2

debian advisories DEBIAN-CVE-2018-21035

  • qt5-qtbase RHEL 8 5.12.5-6.el8
  • qt5-qtwebsockets RHEL 8 5.12.5-2.el8
  • qt5-qttools RHEL 8 5.12.5-2.el8
  • qt5-designer RHEL 8 5.12.5-2.el8
  • qt5-doctools RHEL 8 5.12.5-2.el8
  • qt5-linguist RHEL 8 5.12.5-2.el8
  • qt5-assistant RHEL 8 5.12.5-2.el8
  • qt5-qtbase-gui RHEL 8 5.12.5-6.el8

red hat advisories RHSA-2020:4690

  • qt5-qtwebsockets Rocky Linux 8 5.12.5-2.el8

rocky linux advisories RLSA-2020:4690

  • qt5-qtsvg CBL-Mariner 2.0 5.12.11-3

azure linux advisories AZL-6833

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2018-21035 updated cvss 7.5

qt5-qtwebsockets: websocket implementation allows only limited size for frames and messages therefore attacker can cause DOS

  • Red Hat Enterprise Linux 7 qt5-qtwebsockets, qt5-qtwebsockets-examples
    affected: no fix planned “Will not fix”
  • Red Hat CodeReady Linux Builder (v. 8) qt5-qtbase-static 5.12.5-6.el8, qt5-qttools-static 5.12.5-2.el8, qt5-qtbase-debuginfo 5.12.5-6.el8, qt5-qttools-debuginfo 5.12.5-2.el8, qt5-designer-debuginfo 5.12.5-2.el8, qt5-doctools-debuginfo 5.12.5-2.el8, +18 more Red Hat Enterprise Linux AppStream (v. 8) qt5-qtbase 5.12.5-6.el8, qt5-qttools 5.12.5-2.el8, qt5-designer 5.12.5-2.el8, qt5-doctools 5.12.5-2.el8, qt5-linguist 5.12.5-2.el8, qt5-assistant 5.12.5-2.el8, +14 more
    fixed: vendor fix RHSA-2020:4690

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

2.31% chance of exploitation in the next 30 days, higher than 83% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • bugreports.qt.io https://bugreports.qt.io/browse/QTBUG-70693 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.0 8.6 CVSS:3.0/AC:L/AV:N/A:H/C:N/I:N/PR:N/S:C/UI:N
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CNA 3.0 8.6 CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:N/I:N/A:P

Classification

Weakness
  • CWE-770 Allocation of Resources Without Limits or Throttling

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.