CVE-2021-43818: Lxml Lxml's HTML Cleaner Allows Crafted Vulnerability

MODERATE
Advisory Identifier: GHSA-55x5-fj6c-h6m8
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CVSS 8.2
CWE-79 CWE-74
Published
Advisory Summary

CVE-2021-43818 is a MODERATE (CVSS 8.2) lxml's html cleaner allows crafted vulnerability (CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) affecting Lxml libraries in the PyPI ecosystem. The issue involves lxml's html cleaner allows crafted and svg embedded scripts to pass through. Remediation is available by upgrading affected packages to version 4.6.5.

CVE-2021-43818 at a Glance

CVE Identifier CVE-2021-43818
GHSA Identifier GHSA-55x5-fj6c-h6m8
Severity MODERATE
CVSS v3.1 Base Score 8.2
CWE Classification CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Affected Product Lxml
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 8.2

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction Required (UI:R)
Scope Changed (S:C)
Confidentiality Low (C:L)
Integrity High (I:H)
Availability None (A:N)

How to Fix CVE-2021-43818

To remediate this vulnerability, upgrade all affected Lxml dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

lxml fixed: 4.6.5 
pip install lxml>=4.6.5
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show lxml

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2021-43818, inspect your installed packages using pip:

pip show lxml

Analyze the terminal output to locate any version of Lxml in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 4.6.5), your application is affected.

Note: The pip show lxml command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting'), where the product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. In Lxml, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Impact 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. ### Patches The issue has been resolved in lxml 4.6.5. ### Workarounds None. ### References The issues are tracked under the report IDs GHSL-2021-1037 and GHSL-2021-1038.

CVE-2021-43818 Affected Lxml Versions

1 component(s)
Fixed Versions:
4.6.5
Product / Package Ecosystem Affected Versions Fixed Version
lxml
pypi/lxml
PyPI
< 4.6.5
4.6.5

References

GitHub Advisory / Wiki ADVISORY
https://github.com/lxml/lxml/security/advisories/GHSA-55x5-fj6c-h6m8
NVD — CVE-2021-43818 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-43818
Security Fix Commit FIX COMMIT
https://github.com/lxml/lxml/commit/12fa9669007180a7bb87d990c375cf91ca5b664a
Security Fix Commit FIX COMMIT
https://github.com/lxml/lxml/commit/a3eacbc0dcf1de1c822ec29fb7d090a4b1712a9c#diff-59130575b4fb2932c957db2922977d7d89afb0b2085357db1a14615a2fcad776
Security Fix Commit FIX COMMIT
https://github.com/lxml/lxml/commit/f2330237440df7e8f39c3ad1b1aa8852be3b27c0
Lxml Security Information SECURITY
https://www.oracle.com/security-alerts/cpujul2022.html
Lxml Security Information SECURITY
https://www.oracle.com/security-alerts/cpuapr2022.html
Lxml Security Information SECURITY
https://www.debian.org/security/2022/dsa-5043
Lxml Security Information SECURITY
https://security.netapp.com/advisory/ntap-20220107-0005
Lxml Security Information SECURITY
https://security.gentoo.org/glsa/202208-06
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/ZQ4SPKJX3RRJK4UWA6FXCRHD2TVRQI44
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/WZGNET2A4WGLSUXLBFYKNC5PXHQMI3I7
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/V2XMOM5PFT6U5AAXY6EFNT5JZCKKHK2V
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TUIS2KE3HZ2AAQKXFLTJFZPP2IFHJTC7
lists.debian.org/debian-lts-announce... WEB
https://lists.debian.org/debian-lts-announce/2021/12/msg00037.html
Lxml Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/lxml/PYSEC-2021-852.yaml
Lxml Source Repository REPOSITORY
https://github.com/lxml/lxml

Frequently Asked Questions

What is CVE-2021-43818?
CVE-2021-43818 is a security vulnerability in Lxml involving lxml's HTML Cleaner allows crafted and SVG embedded scripts to pass through under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').
Which Lxml versions are affected?
Affected versions include releases prior to 4.6.5 across affected packages: lxml.
What version fixes CVE-2021-43818?
Upgrading to 4.6.5 (or the latest patched release for your respective branch) resolves the vulnerability.
Is lxml affected?
Yes, lxml is affected in releases prior to 4.6.5. Upgrade your dependency to 4.6.5 or later.
What is the CVSS score of CVE-2021-43818?
CVE-2021-43818 has a CVSS v3.1 base score of 8.2 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:N.
What CWE is associated with CVE-2021-43818?
CVE-2021-43818 is classified under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.).
How can I check PyPI dependencies for CVE-2021-43818?
Run the dependency tree command: 'pip show lxml'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-55x5-fj6c-h6m8",
  "modified" : "2024-09-30T16:50:30Z",
  "published" : "2021-12-13T18:14:36Z",
  "aliases" : [ "CVE-2021-43818" ],
  "summary" : "lxml's HTML Cleaner allows crafted and SVG embedded scripts to pass through",
  "details" : "### Impact\nThe HTML Cleaner in lxml.html lets certain crafted script content pass through, as well as script content in SVG files embedded using data URIs.\n\nUsers that employ the HTML cleaner in a security relevant context should upgrade to lxml 4.6.5.\n\n### Patches\nThe issue has been resolved in lxml 4.6.5.\n\n### Workarounds\nNone.\n\n### References\nThe issues are tracked under the report IDs GHSL-2021-1037 and GHSL-2021-1038.\n",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:H/A:N"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:H/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "lxml"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.6.5"
      } ]
    } ]
  } ],
  "references" : [ {
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    "url" : "https://github.com/lxml/lxml/security/advisories/GHSA-55x5-fj6c-h6m8"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-43818"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/lxml/lxml/commit/12fa9669007180a7bb87d990c375cf91ca5b664a"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/lxml/lxml/commit/a3eacbc0dcf1de1c822ec29fb7d090a4b1712a9c#diff-59130575b4fb2932c957db2922977d7d89afb0b2085357db1a14615a2fcad776"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/lxml/lxml/commit/f2330237440df7e8f39c3ad1b1aa8852be3b27c0"
  }, {
    "type" : "WEB",
    "url" : "https://www.oracle.com/security-alerts/cpujul2022.html"
  }, {
    "type" : "WEB",
    "url" : "https://www.oracle.com/security-alerts/cpuapr2022.html"
  }, {
    "type" : "WEB",
    "url" : "https://www.debian.org/security/2022/dsa-5043"
  }, {
    "type" : "WEB",
    "url" : "https://security.netapp.com/advisory/ntap-20220107-0005"
  }, {
    "type" : "WEB",
    "url" : "https://security.gentoo.org/glsa/202208-06"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/ZQ4SPKJX3RRJK4UWA6FXCRHD2TVRQI44"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/WZGNET2A4WGLSUXLBFYKNC5PXHQMI3I7"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/V2XMOM5PFT6U5AAXY6EFNT5JZCKKHK2V"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TUIS2KE3HZ2AAQKXFLTJFZPP2IFHJTC7"
  }, {
    "type" : "WEB",
    "url" : "https://lists.debian.org/debian-lts-announce/2021/12/msg00037.html"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/lxml/PYSEC-2021-852.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/lxml/lxml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-74", "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-12-13T18:12:54Z",
    "nvd_published_at" : "2021-12-13T18:15:00Z"
  }
}