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Fencer Intel
high 7.5

CVE-2018-20843 json

In libexpat in Expat before 2.2.7, XML input including XML names that contain a large number of colons could make the XML parser consume a…

published 7y ago modified 26d ago also EUVD-2018-13383

cvss
7.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
7.1%
EPSS 7.1% chance of exploitation in 30 days, percentile 94%
public exploits
0
none known

Summary

In libexpat in Expat before 2.2.7, XML input including XML names that contain a large number of colons could make the XML parser consume a high amount of RAM and CPU resources while processing (enough to be usable for denial-of-service attacks).

Patch / fix

  • Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP V3.1.5
  • Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP V3.1.5
  • Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP V3.1.5
  • expat Debian 14, Debian 13, Debian 12 2.2.6-2

debian advisories DEBIAN-CVE-2018-20843

  • expat Ubuntu 16.04 LTS 2.1.0-7ubuntu0.16.04.4
  • expat Ubuntu 18.04 LTS 2.2.5-3ubuntu0.1
  • vtk Ubuntu Pro 14.04 LTS 5.8.0-14.1ubuntu3+esm1
  • vtk Ubuntu Pro 16.04 LTS 5.10.1+dfsg-2.1ubuntu0.1~esm1
  • expat Ubuntu Pro 14.04 LTS 2.1.0-4ubuntu1.4+esm1
  • libxmltok Ubuntu Pro 16.04 LTS 1.2-3ubuntu0.16.04.1~esm2
  • libxmltok Ubuntu Pro 18.04 LTS 1.2-4ubuntu0.18.04.1~esm4
  • libxmltok Ubuntu Pro 20.04 LTS 1.2-4ubuntu0.20.04.1~esm4

ubuntu advisories UBUNTU-CVE-2018-20843

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • jbcs-httpd24-curl jboss core services 1 7.64.1-36.jbcs.el6
  • jbcs-httpd24-curl jboss core services 1 7.64.1-36.jbcs.el7
  • jbcs-httpd24-httpd jboss core services 1 2.4.37-57.jbcs.el6
  • jbcs-httpd24-httpd jboss core services 1 2.4.37-57.jbcs.el7
  • jbcs-httpd24-mod_jk jboss core services 1 1.2.48-4.redhat_1.jbcs.el6
  • jbcs-httpd24-mod_jk jboss core services 1 1.2.48-4.redhat_1.jbcs.el7
  • jbcs-httpd24-mod_md jboss core services 1 1:2.0.8-24.jbcs.el6
  • jbcs-httpd24-mod_md jboss core services 1 1:2.0.8-24.jbcs.el7

red hat advisories RHSA-2025:22871RHSA-2020:2644RHSA-2020:4484RHSA-2020:3952RHSA-2020:4846

  • expat SLE Module for Basesystem 15 SP1, SLE Module for Basesystem 15 2.2.5-3.3.1
  • expat SLES for SAP 12 SP4, SLES 12 SP4, SLED 12 SP4 +1 more 2.1.0-21.6.1

suse advisories SUSE-SU-2019:1835-1SUSE-SU-2019:1834-1

  • expat openSUSE Tumbleweed 2.4.1-1.2
  • expat openSUSE Leap 15.1, openSUSE Leap 15.0 2.2.5-lp151.3.3.1

opensuse advisories openSUSE-SU-2024:10748-1openSUSE-SU-2019:1777-1

  • mingw32-expat AlmaLinux 8 2.2.4-5.el8
  • mingw64-expat AlmaLinux 8 2.2.4-5.el8

almalinux advisories ALSA-2020:4846

  • expat Rocky Linux 8 2.2.5-4.el8

rocky linux advisories RLSA-2020:4484

  • expat Alpine 3.24, Alpine 3.23, Alpine 3.22 +15 more 2.2.7-r0

alpine advisories ALPINE-CVE-2018-20843

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

Is CVE-2018-20843 in your stack? Fencer shows which of your repositories are affected as soon as an advisory lands, and opens the fix as a pull request.

Check with Fencer

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-2018-20843 updated cvss 7.5

expat: large number of colons in input makes parser consume high amount of resources, leading to DoS

  • Red Hat Enterprise Linux 5 expat, xmlrpc-c Red Hat Enterprise Linux 6 expat, compat-expat1 Red Hat JBoss Enterprise Application Platform 6 httpd, mod_ssl, mod_ldap, httpd-tools, httpd-manual Red Hat JBoss Enterprise Web Server 2 jbcs-httpd24, jbcs-httpd24-runtime
    affected: no fix planned “Out of support scope”
  • Red Hat OpenShift Do 1 openshiftdo/odo-init-image-rhel7 1.1.3-2
    fixed: vendor fix RHSA-2021:0949
  • Red Hat CodeReady Linux Builder (v. 8) mingw-expat 2.2.4-5.el8, mingw32-expat 2.2.4-5.el8, mingw64-expat 2.2.4-5.el8
    fixed: vendor fix RHSA-2020:4846
  • Red Hat Enterprise Linux BaseOS (v. 8) expat 2.2.5-4.el8
    fixed: vendor fix RHSA-2020:4484
  • Red Hat Enterprise Linux BaseOS AUS (v. 8.2) expat 2.2.10-1.el8_2
    fixed: vendor fix RHSA-2025:22871
  • Red Hat Enterprise Linux Client (v. 7) expat 2.1.0-12.el7 Red Hat Enterprise Linux Client Optional (v. 7) expat-devel 2.1.0-12.el7, expat-static 2.1.0-12.el7, expat-debuginfo 2.1.0-12.el7 Red Hat Enterprise Linux ComputeNode (v. 7) expat 2.1.0-12.el7 Red Hat Enterprise Linux ComputeNode Optional (v. 7) expat-devel 2.1.0-12.el7, expat-static 2.1.0-12.el7, expat-debuginfo 2.1.0-12.el7 Red Hat Enterprise Linux Server (v. 7) expat 2.1.0-12.el7 Red Hat Enterprise Linux Server Optional (v. 7) expat-static 2.1.0-12.el7, expat-debuginfo 2.1.0-12.el7
    +2 more products
    Red Hat Enterprise Linux Workstation (v. 7) expat 2.1.0-12.el7 Red Hat Enterprise Linux Workstation Optional (v. 7) expat-static 2.1.0-12.el7, expat-debuginfo 2.1.0-12.el7
    fixed: vendor fix RHSA-2020:3952
  • Red Hat JBoss Core Services
    fixed: vendor fix RHSA-2020:2646
  • Red Hat JBoss Core Services on RHEL 6 Server jbcs-httpd24-curl 7.64.1-36.jbcs.el6, jbcs-httpd24-httpd 2.4.37-57.jbcs.el6, jbcs-httpd24-mod_md 2.0.8-24.jbcs.el6, jbcs-httpd24-libcurl 7.64.1-36.jbcs.el6, jbcs-httpd24-mod_ssl 2.4.37-57.jbcs.el6, jbcs-httpd24-nghttp2 1.39.2-25.jbcs.el6, +12 more Red Hat JBoss Core Services on RHEL 7 Server jbcs-httpd24-curl 7.64.1-36.jbcs.el7, jbcs-httpd24-httpd 2.4.37-57.jbcs.el7, jbcs-httpd24-mod_md 2.0.8-24.jbcs.el7, jbcs-httpd24-libcurl 7.64.1-36.jbcs.el7, jbcs-httpd24-mod_ssl 2.4.37-57.jbcs.el7, jbcs-httpd24-nghttp2 1.39.2-25.jbcs.el7, +13 more
    fixed: vendor fix RHSA-2020:2644
show 1 more
  • Red Hat Enterprise Linux 5 firefox, xulrunner, thunderbird Red Hat Enterprise Linux 6 firefox, xulrunner, thunderbird Red Hat Enterprise Linux 7 firefox, xulrunner, thunderbird Red Hat Enterprise Linux BaseOS AUS (v. 8.2) expat 2.2.10-1.el8_2
    not affected: vulnerable code not present

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

cisa ICSA-23-348-10 updated cvss 7.5

Siemens SIMATIC S7-1500

  • Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AB0) >= 3.1.0, < 3.1.5 Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP (6ES7518-4AX00-1AC0) >= 3.1.0, < 3.1.5 Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AB0) >= 3.1.0, < 3.1.5 Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP (6ES7518-4FX00-1AC0) >= 3.1.0, < 3.1.5 Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP (6AG1518-4AX00-4AC0) >= 3.1.0, < 3.1.5
    affected: fixed in V3.1.5, mitigation support.industry.siemens.com

Facts from CISA's republication of the vendor's own advisory (CSAF); the linked advisory is the authoritative source. CISA notification · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
last yr.

Exploit prediction

7.11% chance of exploitation in the next 30 days, higher than 94% of all scored CVEs (FIRST EPSS, 2026-11-09).

Exploits and checks

  • github.com https://github.com/libexpat/libexpat/pull/262 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CISA ADP 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
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 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD 2.0 7.8 AV:N/AC:L/Au:N/C:N/I:N/A:C

Affected products

  • Siemens SIMATIC S7-1500 CPU 1518-4 PN/DP MFP
  • Siemens SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP
  • Siemens SIPLUS S7-1500 CPU 1518-4 PN/DP MFP

Classification

Weakness
  • CWE-611 Improper Restriction of XML External Entity Reference

References

Sources

Fencer Intel 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.