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NVD / CVE Program· CNA GitHub_M

CVE-2021-43818: HTML Cleaner allows crafted and SVG embedded scripts to pass through

Noroxi assessment

Priority: Monitor (action score 29/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 2.6% (FIRST EPSS); higher than 85.7% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. A user must open a link or visit a page. No special conditions are required; it is repeatable.
  • Fixed version: babel (AlmaLinux:8) 2.5.1-10.module_el8.6.0+2781+fed64c13; python-nose-docs (AlmaLinux:8) 1.3.7-31.module_el8.5.0+2569+5c5719bc, 1.3.7-31.module_el8.6.0+2781+fed64c13; python-psycopg2-doc (AlmaLinux:8) 2.7.5-7.module_el8.6.0+2781+fed64c13.

Low priority for now. Upgrade to babel (AlmaLinux:8) 2.5.1-10.module_el8.6.0+2781+fed64c13; python-nose-docs (AlmaLinux:8) 1.3.7-31.module_el8.5.0+2569+5c5719bc, 1.3.7-31.module_el8.6.0+2781+fed64c13; python-psycopg2-doc (AlmaLinux:8) 2.7.5-7.module_el8.6.0+2781+fed64c13 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

lxml is a library for processing XML and HTML in the Python language. Prior to version 4.6.5, the HTML Cleaner in lxml.html lets certain crafted script content pass through, as well as script content in SVG files embedded using data URIs. Users that employ the HTML cleaner in a security relevant context should upgrade to lxml 4.6.5 to receive a patch. There are no known workarounds available.

HighCVSS 7.1 · v3.1—No exploit Fix available
Published
Dec 13, 2021
Updated
Jun 17, 2026
EPSS
2.6% · 85th percentile

Report tools

JSON

Action score

29

Monitor

Low priority for now.

CVSS
28 / 40 · 7.1 / 10
CISA KEV
0 / 30 · Not listed
EPSS
1 / 30 · 2.6%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
7.1
CNA · GitHub_M
8.2
1.1 point gap

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:N

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-43818

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
lxmllxml
fedoraprojectfedora
debiandebian linux
netappsolidfire
netappsolidfire enterprise sds
netapphci storage node firmware
netapphci storage node
oraclecommunications cloud native core binding support function
oraclecommunications cloud native core network exposure function
oraclecommunications cloud native core policy
oraclehttp server
oraclezfs storage appliance kit

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • debian debian linux10.0
  • debian debian linux11.0
  • debian debian linux9.0
  • fedoraproject fedora34
  • fedoraproject fedora35
  • lxml lxmlbefore 4.6.5
  • netapp hci storage node firmwareall versions
  • netapp solidfireall versions
  • netapp solidfire enterprise sdsall versions
  • oracle communications cloud native core binding support function22.1.3
  • oracle communications cloud native core network exposure function22.1.1
  • oracle communications cloud native core policy22.2.0
  • oracle http server12.2.1.3.0
  • oracle http server12.2.1.4.0
  • oracle zfs storage appliance kit8.8

Versions reported by the vendor

Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.

  • lxml lxml

    • < 4.6.5affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
AlmaLinux:8babelbefore 2.5.1-10.module_el8.6.0+2781+fed64c132.5.1-10.module_el8.6.0+2781+fed64c13
AlmaLinux:8python-nose-docsbefore 1.3.7-31.module_el8.5.0+2569+5c5719bc1.3.7-31.module_el8.5.0+2569+5c5719bc
AlmaLinux:8python-nose-docsbefore 1.3.7-31.module_el8.6.0+2781+fed64c131.3.7-31.module_el8.6.0+2781+fed64c13
AlmaLinux:8python-psycopg2-docbefore 2.7.5-7.module_el8.6.0+2781+fed64c132.7.5-7.module_el8.6.0+2781+fed64c13
AlmaLinux:8python-sqlalchemy-docbefore 1.3.2-2.module_el8.6.0+2781+fed64c131.3.2-2.module_el8.6.0+2781+fed64c13
AlmaLinux:8python-sqlalchemy-docbefore 1.3.2-2.module_el8.5.0+2569+5c5719bc1.3.2-2.module_el8.5.0+2569+5c5719bc
AlmaLinux:8python2before 2.7.18-10.module_el8.6.0+2781+fed64c13.alma2.7.18-10.module_el8.6.0+2781+fed64c13.alma
AlmaLinux:8python2-attrsbefore 17.4.0-10.module_el8.6.0+2781+fed64c1317.4.0-10.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-babelbefore 2.5.1-10.module_el8.6.0+2781+fed64c132.5.1-10.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-backportsbefore 1.0-16.module_el8.6.0+2781+fed64c131.0-16.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-backports-ssl_match_hostnamebefore 3.5.0.1-12.module_el8.6.0+2781+fed64c133.5.0.1-12.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-bsonbefore 3.7.0-1.module_el8.6.0+2781+fed64c133.7.0-1.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-chardetbefore 3.0.4-10.module_el8.6.0+2781+fed64c133.0.4-10.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-coveragebefore 4.5.1-4.module_el8.6.0+2781+fed64c134.5.1-4.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-Cythonbefore 0.28.1-7.module_el8.6.0+2781+fed64c130.28.1-7.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-debugbefore 2.7.18-10.module_el8.6.0+2781+fed64c13.alma2.7.18-10.module_el8.6.0+2781+fed64c13.alma
AlmaLinux:8python2-develbefore 2.7.18-10.module_el8.6.0+2781+fed64c13.alma2.7.18-10.module_el8.6.0+2781+fed64c13.alma
AlmaLinux:8python2-dnsbefore 1.15.0-10.module_el8.6.0+2781+fed64c131.15.0-10.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-docsbefore 2.7.16-2.module_el8.6.0+2781+fed64c132.7.16-2.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-docs-infobefore 2.7.16-2.module_el8.6.0+2781+fed64c132.7.16-2.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-docutilsbefore 0.14-12.module_el8.6.0+2781+fed64c130.14-12.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-funcsigsbefore 1.0.2-13.module_el8.6.0+2781+fed64c131.0.2-13.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-idnabefore 2.5-7.module_el8.6.0+2781+fed64c132.5-7.module_el8.6.0+2781+fed64c13
AlmaLinux:8python2-ipaddressbefore 1.0.18-6.module_el8.6.0+2781+fed64c131.0.18-6.module_el8.6.0+2781+fed64c13

+96

Same product

lxml: all records

Other highest-scoring records for the same primary product.

  • CVE-2026-49825lxml: javascript: URL bypass in Cleaner via xlink:href
    32Monitor
  • CVE-2022-2309NULL Pointer Dereference in lxml/lxml
    31Monitor
  • CVE-2026-41066lxml: Default configuration of iterparse() and ETCompatXMLParser() allows XXE to local files
    30Monitor

Published before and after this one in the same product

  • ← EarlierCVE-2018-19787Dec 2, 2018 · An issue was discovered in lxml before 4.2.5.
    25Monitor
  • ← EarlierCVE-2020-27783Dec 3, 2020 · A XSS vulnerability was discovered in python-lxml's clean module.
    25Monitor
  • ← EarlierCVE-2021-28957Mar 21, 2021 · An XSS vulnerability was discovered in python-lxml's clean module versions before 4.6.3.
    25Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
almalinux:python393.9.7-1.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
almalinux:python39-attrs20.3.0-2.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
almalinux:python39-cffi1.14.3-2.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
almalinux:python39-chardet3.0.4-19.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
almalinux:python39-cryptography3.3.1-2.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
almalinux:python39-Cython0.29.21-5.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
almalinux:python39-debug3.9.7-1.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
almalinux:python39-devel3.9.7-1.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
almalinux:python39-idle3.9.7-1.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
almalinux:python39-idna2.10-3.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
almalinux:python39-iniconfig1.1.1-2.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
almalinux:python39-PyMySQL0.10.1-2.module_el8.6.0+2780+a40f65e1 · AlmaLinux:8Package registry (OSV)
alpine:py3-lxml4.6.5-r0 · Alpine:v3.16Package registry (OSV)
debian:lxml4.7.1-1 · Debian:12Package registry (OSV)
opensuse:python-lxml4.7.1-3.7.1 · openSUSE:Leap 15.3Package registry (OSV)
PyPI:lxml4.6.5Package registry (OSV)
red hat:mod_wsgi0:4.7.1-4.module+el8.4.0+9822+20bf1249 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:numpy0:1.19.4-3.module+el8.5.0+12204+54860423 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:numpy-debugsource0:1.19.4-3.module+el8.5.0+12204+54860423 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:python-cffi0:1.14.3-2.module+el8.4.0+9822+20bf1249 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:python-cffi-debugsource0:1.14.3-2.module+el8.4.0+9822+20bf1249 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:python-chardet0:3.0.4-19.module+el8.4.0+9822+20bf1249 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:python-cryptography0:3.3.1-2.module+el8.4.0+9822+20bf1249 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:python-PyMySQL0:0.10.1-2.module+el8.4.0+9822+20bf1249 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:PyYAML0:5.4.1-1.module+el8.5.0+10613+59a13ec4 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:PyYAML-debugsource0:5.4.1-1.module+el8.5.0+10613+59a13ec4 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:rh-python38-python-lxml0:4.4.1-8.el7 · Red Hat:rhel_software_collections:3::el7Package registry (OSV)
red hat:rh-python38-python-lxml-debuginfo0:4.4.1-8.el7 · Red Hat:rhel_software_collections:3::el7Package registry (OSV)
rocky linux:Cython0:0.29.21-5.module+el8.4.0+574+843c4898 · Rocky Linux:8Package registry (OSV)
rocky linux:mod_wsgi0:4.7.1-4.module+el8.4.0+574+843c4898 · Rocky Linux:8Package registry (OSV)

+22

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • A user must open a link or visit a page.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
low · data can be read
Integrity
low · data or configuration can be modified
Availability
low · the service can be disrupted

Impact can extend beyond the vulnerable component (scope changes).

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
Required
Scope
Changed
Confidentiality impact
Low
Integrity impact
Low
Availability impact
Low

Weakness class (CWE)

CWE-74 · Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:L

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (13 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records