CVE-2021-27921: Pillow Remote Code Execution Vulnerability

HIGH
Advisory Identifier: GHSA-f4w8-cv6p-x6r5
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CVSS 7.5
CWE-400 CWE-20
Published
Advisory Summary

CVE-2021-27921 is a HIGH (CVSS 7.5) remote code execution vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Pillow libraries in the PyPI ecosystem. The issue involves pillow denial of service by uncontrolled resource consumption. Remediation is available by upgrading affected packages to version 8.1.2.

CVE-2021-27921 at a Glance

CVE Identifier CVE-2021-27921
GHSA Identifier GHSA-f4w8-cv6p-x6r5
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product Pillow
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-2021-27921

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

Pillow fixed: 8.1.2 
pip install Pillow>=8.1.2
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 Pillow

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2021-27921, inspect your installed packages using pip:

pip show Pillow

Analyze the terminal output to locate any version of Pillow in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 8.1.2), your application is affected.

Note: The pip show Pillow 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 Pillow, 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

Pillow before 8.1.2 allows attackers to cause a denial of service (memory consumption) because the reported size of a contained image is not properly checked for a BLP container, and thus an attempted memory allocation can be very large.

CVE-2021-27921 Affected Pillow Versions

1 component(s)
Fixed Versions:
8.1.2
Product / Package Ecosystem Affected Versions Fixed Version
Pillow
pypi/Pillow
PyPI
< 8.1.2
8.1.2

References

NVD — CVE-2021-27921 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-27921
Security Fix Commit FIX COMMIT
https://github.com/python-pillow/Pillow/commit/756fff33128a0b643d10518a26ad04b726dd8973
Pillow Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/pillow/PYSEC-2021-40.yaml
Pillow Source Repository REPOSITORY
https://github.com/python-pillow/Pillow
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/S7G44Z33J4BNI2DPDROHWGVG2U7ZH5JU
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TQQY6472RX4J2SUJENWDZAWKTJJGP2ML
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZTSY25UJU7NJUFHH3HWT575LT4TDFWBZ
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/S7G44Z33J4BNI2DPDROHWGVG2U7ZH5JU
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TQQY6472RX4J2SUJENWDZAWKTJJGP2ML
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/ZTSY25UJU7NJUFHH3HWT575LT4TDFWBZ
Pillow Release RELEASE
https://pillow.readthedocs.io/en/stable/releasenotes/8.1.1.html
Pillow Release RELEASE
https://pillow.readthedocs.io/en/stable/releasenotes/8.1.2.html
Pillow Security Information SECURITY
https://security.gentoo.org/glsa/202107-33

Frequently Asked Questions

What is CVE-2021-27921?
CVE-2021-27921 is a security vulnerability in Pillow involving Pillow Denial of Service by Uncontrolled Resource Consumption under CWE-400: Uncontrolled Resource Consumption.
Which Pillow versions are affected?
Affected versions include releases prior to 8.1.2 across affected packages: Pillow.
What version fixes CVE-2021-27921?
Upgrading to 8.1.2 (or the latest patched release for your respective branch) resolves the vulnerability.
Is Pillow affected?
Yes, Pillow is affected in releases prior to 8.1.2. Upgrade your dependency to 8.1.2 or later.
What is the CVSS score of CVE-2021-27921?
CVE-2021-27921 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-2021-27921?
CVE-2021-27921 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-2021-27921?
Run the dependency tree command: 'pip show Pillow'. 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-f4w8-cv6p-x6r5",
  "modified" : "2025-08-15T13:58:12Z",
  "published" : "2021-03-18T19:55:13Z",
  "aliases" : [ "CVE-2021-27921" ],
  "summary" : "Pillow Denial of Service by Uncontrolled Resource Consumption",
  "details" : "Pillow before 8.1.2 allows attackers to cause a denial of service (memory consumption) because the reported size of a contained image is not properly checked for a BLP container, and thus an attempted memory allocation can be very large.",
  "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" : "Pillow"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "8.1.2"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-27921"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/python-pillow/Pillow/commit/756fff33128a0b643d10518a26ad04b726dd8973"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/pillow/PYSEC-2021-40.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/python-pillow/Pillow"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/S7G44Z33J4BNI2DPDROHWGVG2U7ZH5JU"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TQQY6472RX4J2SUJENWDZAWKTJJGP2ML"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZTSY25UJU7NJUFHH3HWT575LT4TDFWBZ"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/S7G44Z33J4BNI2DPDROHWGVG2U7ZH5JU"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/TQQY6472RX4J2SUJENWDZAWKTJJGP2ML"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/ZTSY25UJU7NJUFHH3HWT575LT4TDFWBZ"
  }, {
    "type" : "WEB",
    "url" : "https://pillow.readthedocs.io/en/stable/releasenotes/8.1.1.html"
  }, {
    "type" : "WEB",
    "url" : "https://pillow.readthedocs.io/en/stable/releasenotes/8.1.2.html"
  }, {
    "type" : "WEB",
    "url" : "https://security.gentoo.org/glsa/202107-33"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-20", "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-03-12T23:35:22Z",
    "nvd_published_at" : "2021-03-03T09:15:00Z"
  }
}