CVE-2021-39286: Pywb Cross-Site Scripting Vulnerability

MODERATE
Advisory Identifier: GHSA-947x-pv47-pp3q
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CVSS 6.1
CWE-79
Published
Advisory Summary

CVE-2021-39286 is a MODERATE (CVSS 6.1) cross-site scripting vulnerability (CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) affecting Pywb libraries in the PyPI ecosystem. The issue involves cross-site scripting in pywb. Remediation is available by upgrading affected packages to version 2.6.0.

CVE-2021-39286 at a Glance

CVE Identifier CVE-2021-39286
GHSA Identifier GHSA-947x-pv47-pp3q
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 Pywb
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:3.1/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-2021-39286

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

pywb fixed: 2.6.0 
pip install pywb>=2.6.0
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 pywb

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2021-39286, inspect your installed packages using pip:

pip show pywb

Analyze the terminal output to locate any version of Pywb in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.6.0), your application is affected.

Note: The pip show pywb 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 Pywb, 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

Webrecorder pywb before 2.6.0 allows XSS because it does not ensure that Jinja2 templates are autoescaped.

CVE-2021-39286 Affected Pywb Versions

1 component(s)
Fixed Versions:
2.6.0
Product / Package Ecosystem Affected Versions Fixed Version
pywb
pypi/pywb
PyPI
< 2.6.0
2.6.0

References

Security Fix Commit FIX COMMIT
https://github.com/webrecorder/pywb/commit/f7bd84cdacdd665ff73ae8d09a202f60be2ebae9
Pywb Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/pywb/PYSEC-2021-120.yaml
Pywb Source Repository REPOSITORY
https://github.com/webrecorder/pywb
Pywb Source Repository REPOSITORY
https://github.com/webrecorder/pywb/compare/v-2.5.0...v-2.6.0
GitHub Advisory / Wiki ADVISORY
https://github.com/advisories/GHSA-947x-pv47-pp3q
NVD — CVE-2021-39286 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-39286

Frequently Asked Questions

What is CVE-2021-39286?
CVE-2021-39286 is a security vulnerability in Pywb involving Cross-site scripting in pywb under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').
Which Pywb versions are affected?
Affected versions include releases prior to 2.6.0 across affected packages: pywb.
What version fixes CVE-2021-39286?
Upgrading to 2.6.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is pywb affected?
Yes, pywb is affected in releases prior to 2.6.0. Upgrade your dependency to 2.6.0 or later.
What is the CVSS score of CVE-2021-39286?
CVE-2021-39286 has a CVSS v3.1 base score of 6.1 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.
What CWE is associated with CVE-2021-39286?
CVE-2021-39286 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-2021-39286?
Run the dependency tree command: 'pip show pywb'. 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-947x-pv47-pp3q",
  "modified" : "2024-10-25T20:52:18Z",
  "published" : "2021-09-02T17:16:18Z",
  "aliases" : [ "CVE-2021-39286" ],
  "summary" : "Cross-site scripting in pywb",
  "details" : "Webrecorder pywb before 2.6.0 allows XSS because it does not ensure that Jinja2 templates are autoescaped.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/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" : "pywb"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.6.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-39286"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/webrecorder/pywb/commit/f7bd84cdacdd665ff73ae8d09a202f60be2ebae9"
  }, {
    "type" : "ADVISORY",
    "url" : "https://github.com/advisories/GHSA-947x-pv47-pp3q"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/pywb/PYSEC-2021-120.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/webrecorder/pywb"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/webrecorder/pywb/compare/v-2.5.0...v-2.6.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-08-26T15:00:34Z",
    "nvd_published_at" : "2021-08-18T18:15:00Z"
  }
}