CVE-2020-6817: Bleach Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-vqhp-cxgc-6wmm
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CVSS 7.5
CWE-400 CWE-1333
Published
Advisory Summary

CVE-2020-6817 is a HIGH (CVSS 7.5) denial of service vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Bleach libraries in the PyPI ecosystem. The issue involves regular expression denial-of-service (redos) in bleach. Remediation is available by upgrading affected packages to version 3.1.4.

CVE-2020-6817 at a Glance

CVE Identifier CVE-2020-6817
GHSA Identifier GHSA-vqhp-cxgc-6wmm
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product Bleach
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

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:N/S:U/C:N/I:N/A:H).

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

How to Fix CVE-2020-6817

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: 3.1.4 
pip install bleach>=3.1.4
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-2020-6817, 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 3.1.4), 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-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. 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 `bleach.clean` behavior parsing style attributes could result in a regular expression denial of service (ReDoS). Calls to ``bleach.clean`` with an allowed tag with an allowed ``style`` attribute are vulnerable to ReDoS. For example, ``bleach.clean(..., attributes={'a': ['style']})``. ### Patches 3.1.4 ### Workarounds * do not whitelist the style attribute in `bleach.clean` calls * limit input string length ### References * https://bugzilla.mozilla.org/show_bug.cgi?id=1623633 * https://www.regular-expressions.info/redos.html * https://blog.r2c.dev/posts/finding-python-redos-bugs-at-scale-using-dlint-and-r2c/ * https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-6817 ### Credits * Reported by schwag09 of r2c ### For more information If you have any questions or comments about this advisory: * Open an issue at https://github.com/mozilla/bleach/issues * Email us at security@mozilla.org

CVE-2020-6817 Affected Bleach Versions

1 component(s)
Fixed Versions:
3.1.4
Product / Package Ecosystem Affected Versions Fixed Version
bleach
pypi/bleach
PyPI
< 3.1.4
3.1.4

References

GitHub Advisory / Wiki ADVISORY
https://github.com/mozilla/bleach/security/advisories/GHSA-vqhp-cxgc-6wmm
NVD — CVE-2020-6817 NVD
https://nvd.nist.gov/vuln/detail/CVE-2020-6817
bugzilla.mozilla.org/show_bug.cgi WEB
https://bugzilla.mozilla.org/show_bug.cgi?id=1623633
Bleach Source Repository REPOSITORY
https://github.com/mozilla/bleach
Bleach Release RELEASE
https://github.com/mozilla/bleach/releases/tag/v3.1.4
Bleach Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2020-340.yaml
snyk.io/vuln/snyk-python-bl... WEB
https://snyk.io/vuln/SNYK-PYTHON-BLEACH-561754

Frequently Asked Questions

What is CVE-2020-6817?
CVE-2020-6817 is a security vulnerability in Bleach involving regular expression denial-of-service (ReDoS) in Bleach under CWE-400: Uncontrolled Resource Consumption.
Which Bleach versions are affected?
Affected versions include releases prior to 3.1.4 across affected packages: bleach.
What version fixes CVE-2020-6817?
Upgrading to 3.1.4 (or the latest patched release for your respective branch) resolves the vulnerability.
Is bleach affected?
Yes, bleach is affected in releases prior to 3.1.4. Upgrade your dependency to 3.1.4 or later.
What is the CVSS score of CVE-2020-6817?
CVE-2020-6817 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2020-6817?
CVE-2020-6817 is classified under CWE-400: Uncontrolled Resource Consumption (The product does not properly control the allocation and maintenance of a limited resource.).
How can I check PyPI dependencies for CVE-2020-6817?
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-vqhp-cxgc-6wmm",
  "modified" : "2025-03-20T18:49:18Z",
  "published" : "2020-03-30T19:45:56Z",
  "aliases" : [ "CVE-2020-6817" ],
  "summary" : "regular expression denial-of-service (ReDoS) in Bleach",
  "details" : "### Impact\n\n`bleach.clean` behavior parsing style attributes could result in a regular expression denial of service (ReDoS).\n\nCalls to ``bleach.clean`` with an allowed tag with an allowed ``style`` attribute are vulnerable to ReDoS. For example, ``bleach.clean(..., attributes={'a': ['style']})``.\n\n### Patches\n\n3.1.4\n\n### Workarounds\n\n* do not whitelist the style attribute in `bleach.clean` calls\n\n* limit input string length\n\n### References\n\n* https://bugzilla.mozilla.org/show_bug.cgi?id=1623633\n* https://www.regular-expressions.info/redos.html\n* https://blog.r2c.dev/posts/finding-python-redos-bugs-at-scale-using-dlint-and-r2c/\n* https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-6817\n\n### Credits\n\n* Reported by schwag09 of r2c\n\n### For more information\nIf you have any questions or comments about this advisory:\n\n* Open an issue at https://github.com/mozilla/bleach/issues\n* Email us at security@mozilla.org",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "bleach"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.1.4"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/mozilla/bleach/security/advisories/GHSA-vqhp-cxgc-6wmm"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2020-6817"
  }, {
    "type" : "WEB",
    "url" : "https://bugzilla.mozilla.org/show_bug.cgi?id=1623633"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mozilla/bleach"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mozilla/bleach/releases/tag/v3.1.4"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/bleach/PYSEC-2020-340.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://snyk.io/vuln/SNYK-PYTHON-BLEACH-561754"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1333", "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2020-03-27T19:58:38Z",
    "nvd_published_at" : "2023-02-16T22:15:00Z"
  }
}