CVE-2018-19787: Lxml Improper Neutralization Of Input During Vulnerability

MODERATE
Advisory Identifier: GHSA-xp26-p53h-6h2p
Share
CVSS 6.1
CWE-79
Published
Advisory Summary

CVE-2018-19787 is a MODERATE (CVSS 6.1) improper neutralization of input during vulnerability (CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) affecting Lxml libraries in the PyPI ecosystem. The issue involves improper neutralization of input during web page generation in lxml. Remediation is available by upgrading affected packages to version 4.2.5.

CVE-2018-19787 at a Glance

CVE Identifier CVE-2018-19787
GHSA Identifier GHSA-xp26-p53h-6h2p
Severity MODERATE
CVSS v3.1 Base Score 6.1
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: 6.1

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/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 Low (I:L)
Availability None (A:N)

How to Fix CVE-2018-19787

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: 4.2.5 
pip install lxml>=4.2.5
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-2018-19787, 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 4.2.5), 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

An issue was discovered in lxml before 4.2.5. lxml/html/clean.py in the lxml.html.clean module does not remove javascript: URLs that use escaping, allowing a remote attacker to conduct XSS attacks, as demonstrated by "j a v a s c r i p t:" in Internet Explorer. This is a similar issue to CVE-2014-3146.

CVE-2018-19787 Affected Lxml Versions

1 component(s)
Fixed Versions:
4.2.5
Product / Package Ecosystem Affected Versions Fixed Version
lxml
pypi/lxml
PyPI
< 4.2.5
4.2.5

References

NVD — CVE-2018-19787 NVD
https://nvd.nist.gov/vuln/detail/CVE-2018-19787
Security Fix Commit FIX COMMIT
https://github.com/lxml/lxml/commit/6be1d081b49c97cfd7b3fbd934a193b668629109
GitHub Advisory / Wiki ADVISORY
https://github.com/advisories/GHSA-xp26-p53h-6h2p
Lxml Source Repository REPOSITORY
https://github.com/lxml/lxml
Lxml Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/lxml/PYSEC-2018-12.yaml
lists.debian.org/debian-lts-announce... WEB
https://lists.debian.org/debian-lts-announce/2018/12/msg00001.html
lists.debian.org/debian-lts-announce... WEB
https://lists.debian.org/debian-lts-announce/2020/11/msg00044.html
usn.ubuntu.com/3841-1 WEB
https://usn.ubuntu.com/3841-1
usn.ubuntu.com/3841-2 WEB
https://usn.ubuntu.com/3841-2

Frequently Asked Questions

What is CVE-2018-19787?
CVE-2018-19787 is a security vulnerability in Lxml involving Improper Neutralization of Input During Web Page Generation in LXML 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 4.2.5 across affected packages: lxml.
What version fixes CVE-2018-19787?
Upgrading to 4.2.5 (or the latest patched release for your respective branch) resolves the vulnerability.
Is lxml affected?
Yes, lxml is affected in releases prior to 4.2.5. Upgrade your dependency to 4.2.5 or later.
What is the CVSS score of CVE-2018-19787?
CVE-2018-19787 has a CVSS v3.1 base score of 6.1 (MODERATE) with vector string CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.
What CWE is associated with CVE-2018-19787?
CVE-2018-19787 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-2018-19787?
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-xp26-p53h-6h2p",
  "modified" : "2025-12-20T05:24:54Z",
  "published" : "2022-05-13T01:13:21Z",
  "aliases" : [ "CVE-2018-19787" ],
  "summary" : "Improper Neutralization of Input During Web Page Generation in LXML",
  "details" : "An issue was discovered in lxml before 4.2.5. lxml/html/clean.py in the lxml.html.clean module does not remove javascript: URLs that use escaping, allowing a remote attacker to conduct XSS attacks, as demonstrated by \"j a v a s c r i p t:\" in Internet Explorer. This is a similar issue to CVE-2014-3146.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/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:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "lxml"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.2.5"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2018-19787"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/lxml/lxml/commit/6be1d081b49c97cfd7b3fbd934a193b668629109"
  }, {
    "type" : "ADVISORY",
    "url" : "https://github.com/advisories/GHSA-xp26-p53h-6h2p"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/lxml/lxml"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/lxml/PYSEC-2018-12.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://lists.debian.org/debian-lts-announce/2018/12/msg00001.html"
  }, {
    "type" : "WEB",
    "url" : "https://lists.debian.org/debian-lts-announce/2020/11/msg00044.html"
  }, {
    "type" : "WEB",
    "url" : "https://usn.ubuntu.com/3841-1"
  }, {
    "type" : "WEB",
    "url" : "https://usn.ubuntu.com/3841-2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2022-06-28T23:26:59Z",
    "nvd_published_at" : "2018-12-02T10:29:00Z"
  }
}