CVE-2021-23980: Bleach Cross-Site Scripting Vulnerability

Q: What is CVE-2021-23980?

CVE-2021-23980 is a security vulnerability in Bleach involving Cross-site scripting in Bleach under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').

Q: Which Bleach versions are affected?

Affected versions include releases prior to 3.3.0 across affected packages: bleach.

Q: What version fixes CVE-2021-23980?

Upgrading to 3.3.0 (or the latest patched release for your respective branch) resolves the vulnerability.

Q: Is bleach affected?

Yes, bleach is affected in releases prior to 3.3.0. Upgrade your dependency to 3.3.0 or later.

Q: What is the CVSS score of CVE-2021-23980?

CVE-2021-23980 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.

Q: What CWE is associated with CVE-2021-23980?

CVE-2021-23980 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.).

Q: How can I check PyPI dependencies for CVE-2021-23980?

Run the dependency tree command: 'pip show bleach'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.

MODERATE

Advisory Identifier: GHSA-vv2x-vrpj-qqpq

CVSS 6.1

CWE-79

Published 2021-02-02

Advisory Summary

CVE-2021-23980 is a MODERATE (CVSS 6.1) cross-site scripting vulnerability (CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')) affecting Bleach libraries in the PyPI ecosystem. The issue involves cross-site scripting in bleach. Remediation is available by upgrading affected packages to version 3.3.0.

CVE-2021-23980 at a Glance

CVE Identifier CVE-2021-23980

GHSA Identifier GHSA-vv2x-vrpj-qqpq

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 Bleach

Package Ecosystem PyPI

Published Date 2021-02-02

Last Updated 2025-03-20

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-2021-23980

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

bleach fixed: 3.3.0

pip install bleach>=3.3.0

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 bleach

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2021-23980, inspect your installed packages using pip:

pip show bleach

Analyze the terminal output to locate any version of Bleach in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 3.3.0), your application is affected.

Note: The pip show bleach 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 Bleach, 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 A [mutation XSS](https://cure53.de/fp170.pdf) affects users calling `bleach.clean` with all of: * `svg` or `math` in the allowed tags * `p` or `br` in allowed tags * `style`, `title`, `noscript`, `script`, `textarea`, `noframes`, `iframe`, or `xmp` in allowed tags * the keyword argument `strip_comments=False` Note: none of the above tags are in the default allowed tags and `strip_comments` defaults to `True`. ### Patches Users are encouraged to upgrade to bleach v3.3.0 or greater. Note: bleach v3.3.0 introduces a breaking change to escape HTML comments by default. ### Workarounds * modify `bleach.clean` calls to at least one of: * not allow the `style`, `title`, `noscript`, `script`, `textarea`, `noframes`, `iframe`, or `xmp` tag * not allow `svg` or `math` tags * not allow `p` or `br` tags * set `strip_comments=True` * A strong [Content-Security-Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP) without `unsafe-inline` and `unsafe-eval` [`script-src`s](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src)) will also help mitigate the risk. ### References * https://bugzilla.mozilla.org/show_bug.cgi?id=1689399 * https://advisory.checkmarx.net/advisory/CX-2021-4303 * https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-23980 * https://cure53.de/fp170.pdf ### Credits * Reported by [Yaniv Nizry](https://twitter.com/ynizry) from the CxSCA AppSec group at Checkmarx * Additional eject tags not mentioned in the original advisory and the CSP mitigation line being truncated in the revised advisory reported by [Michał Bentkowski](https://twitter.com/SecurityMB) at Securitum ### For more information If you have any questions or comments about this advisory: * Open an issue at [https://github.com/mozilla/bleach/issues](https://github.com/mozilla/bleach/issues) * Email us at [security@mozilla.org](mailto:security@mozilla.org)

CVE-2021-23980 Affected Bleach Versions

1 component(s)

Fixed Versions:

3.3.0

Product / Package	Ecosystem	Affected Versions	Fixed Version
bleach pypi/bleach	PyPI	< 3.3.0	3.3.0

References

GitHub Advisory / Wiki ADVISORY

https://github.com/mozilla/bleach/security/advisories/GHSA-vv2x-vrpj-qqpq

NVD — CVE-2021-23980 NVD

https://nvd.nist.gov/vuln/detail/CVE-2021-23980

Security Fix Commit FIX COMMIT

https://github.com/mozilla/bleach/commit/79b7a3c5e56a09d1d323a5006afa59b56162eb13

advisory.checkmarx.net/advisory/cx-2021-43... WEB

https://advisory.checkmarx.net/advisory/CX-2021-4303

bugzilla.mozilla.org/show_bug.cgi WEB

https://bugzilla.mozilla.org/show_bug.cgi?id=1689399

bugzilla.mozilla.org/show_bug.cgi WEB

https://bugzilla.mozilla.org/show_bug.cgi?id=CVE-2021-23980

cure53.de/fp170.pdf WEB

https://cure53.de/fp170.pdf

Bleach Source Repository REPOSITORY

https://github.com/mozilla/bleach

Bleach Source Repository REPOSITORY

https://github.com/mozilla/bleach/blob/79b7a3c5e56a09d1d323a5006afa59b56162eb13/CHANGES#L4

Bleach Source Repository REPOSITORY

https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2021-865.yaml

pypi.org/project/bleach WEB

https://pypi.org/project/bleach

Frequently Asked Questions

What is CVE-2021-23980?

CVE-2021-23980 is a security vulnerability in Bleach involving Cross-site scripting in Bleach under CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting').

Which Bleach versions are affected?

Affected versions include releases prior to 3.3.0 across affected packages: bleach.

What version fixes CVE-2021-23980?

Upgrading to 3.3.0 (or the latest patched release for your respective branch) resolves the vulnerability.

Is bleach affected?

Yes, bleach is affected in releases prior to 3.3.0. Upgrade your dependency to 3.3.0 or later.

What is the CVSS score of CVE-2021-23980?

CVE-2021-23980 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-2021-23980?

CVE-2021-23980 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-2021-23980?

Run the dependency tree command: 'pip show bleach'. 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-vv2x-vrpj-qqpq",
  "modified" : "2025-03-20T18:50:31Z",
  "published" : "2021-02-02T17:58:40Z",
  "aliases" : [ "CVE-2021-23980" ],
  "summary" : "Cross-site scripting in Bleach",
  "details" : "### Impact                                                                                                                                                                                                                                                    \n\nA [mutation XSS](https://cure53.de/fp170.pdf) affects users calling `bleach.clean` with all of:\n\n* `svg` or `math` in the allowed tags\n* `p` or `br` in allowed tags\n* `style`, `title`, `noscript`, `script`, `textarea`, `noframes`, `iframe`, or `xmp` in allowed tags\n* the keyword argument `strip_comments=False`\n\nNote: none of the above tags are in the default allowed tags and `strip_comments` defaults to `True`.\n\n### Patches\n\nUsers are encouraged to upgrade to bleach v3.3.0 or greater.\n\nNote: bleach v3.3.0 introduces a breaking change to escape HTML comments by default.\n\n### Workarounds\n\n* modify `bleach.clean` calls to at least one of:\n  * not allow the `style`, `title`, `noscript`, `script`, `textarea`, `noframes`, `iframe`, or `xmp` tag\n  * not allow `svg` or `math` tags\n  * not allow `p` or `br` tags\n  * set `strip_comments=True`\n\n* A strong [Content-Security-Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP) without `unsafe-inline` and `unsafe-eval` [`script-src`s](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/script-src)) will also help mitigate the risk.\n\n### References\n\n* https://bugzilla.mozilla.org/show_bug.cgi?id=1689399\n* https://advisory.checkmarx.net/advisory/CX-2021-4303\n* https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-23980\n* https://cure53.de/fp170.pdf\n\n### Credits\n\n* Reported by [Yaniv Nizry](https://twitter.com/ynizry) from the CxSCA AppSec group at Checkmarx\n* Additional eject tags not mentioned in the original advisory and the CSP mitigation line being truncated in the revised advisory reported by [Michał Bentkowski](https://twitter.com/SecurityMB) at Securitum\n\n### For more information\n\nIf you have any questions or comments about this advisory:\n\n* Open an issue at [https://github.com/mozilla/bleach/issues](https://github.com/mozilla/bleach/issues)\n* Email us at [security@mozilla.org](mailto:security@mozilla.org)",
  "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:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "bleach"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.3.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/mozilla/bleach/security/advisories/GHSA-vv2x-vrpj-qqpq"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-23980"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mozilla/bleach/commit/79b7a3c5e56a09d1d323a5006afa59b56162eb13"
  }, {
    "type" : "WEB",
    "url" : "https://advisory.checkmarx.net/advisory/CX-2021-4303"
  }, {
    "type" : "WEB",
    "url" : "https://bugzilla.mozilla.org/show_bug.cgi?id=1689399"
  }, {
    "type" : "WEB",
    "url" : "https://bugzilla.mozilla.org/show_bug.cgi?id=CVE-2021-23980"
  }, {
    "type" : "WEB",
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  }, {
    "type" : "PACKAGE",
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  }, {
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  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2021-865.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://pypi.org/project/bleach"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
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    "github_reviewed_at" : "2021-02-02T15:54:20Z",
    "nvd_published_at" : "2023-02-16T22:15:00Z"
  }
}

CVSS V3

6.1 MODERATE

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)

Vector String

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

Advisory Metadata

Source ID: GHSA-vv2x-vrpj-qqpq
Schema Version: 1.4.0
Published: 2021-02-02 17:58
Last Modified: 2025-03-20 18:50
Database Record File: advisories/github-reviewed/2021/02/GHSA-vv2x-vrpj-qqpq/GHSA-vv2x-vrpj-qqpq.json