How severe is CVE-2021-21426?

CVE-2021-21426 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-21426?

CVE-2021-21426 affects the following packages: openmage/magento-lts (Packagist), openmage/magento-lts (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-21426?

Update openmage/magento-lts to 19.4.13 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21426 is resolved across your whole dependency graph.

How do I detect CVE-2021-21426 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for openmage/magento-lts, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21426 if there is no patch (or I can't update yet)?

Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.

Is CVE-2021-21426 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21426 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21426?

CVE-2021-21426 has an EPSS (Exploit Prediction Scoring System) score of 1.2%, placing it in the 67th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21426?

CVE-2021-21426 is classified as Deserialization of Untrusted Data (CWE-502). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-21426 published, and has it been updated?

CVE-2021-21426 was published on April 21, 2021 and was last updated on August 7, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘

🐘 Packagist

Not in CISA KEV

CRITICAL severity

CVE-2021-21426 — magento-lts

CRITICAL

CVE-2021-21426 is a critical-severity (CVSS 9.8) Deserialization of Untrusted Data vulnerability in openmage/magento-lts. A fix is available for openmage/magento-lts — see the affected versions and patch details below.

Fixes a bug in Zend Framework's Stream HTTP Wrapper

Also known asGHSA-m496-x567-f98c

Published

Apr 21, 2021

Updated

Aug 7, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Oct 11, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs67th percentile — riskier than 67% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-21426 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

2 pkgs affected

🐘openmage/magento-lts🐘openmage/magento-lts

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

Description

Impact

CVE-2021-3007: Backport of Zend_Http_Response_Stream, added certain type checking as a way to prevent exploitation. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3007

This vulnerability is caused by the unsecured deserialization of an object. In versions higher than Zend Framework 3.0.0, the attacker abuses the Zend3 feature that loads classes from objects in order to upload and execute malicious code in the server. The code can be uploaded using the “callback” parameter, which in this case inserts a malicious code instead of the “callbackOptions” array.

Patches

Has the problem been patched? What versions should users upgrade to? v20.0.9 v19.4.13

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`openmage/magento-lts`	all versions	19.4.13`composer require openmage/magento-lts:^19.4.13`
🐘Packagist	`openmage/magento-lts`	≥ 20.0.0&&< 20.0.9	20.0.9`composer require openmage/magento-lts:^20.0.9`

Affected Products

1 product · 2 configurations

Application

magentoopenmage

≥ 20.0.0 && < 20.0.9

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for openmage/magento-lts, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update openmage/magento-lts to 19.4.13 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21426 is resolved across your whole dependency graph.
Workarounds
Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.

Frequently Asked Questions

### Impact CVE-2021-3007: Backport of Zend_Http_Response_Stream, added certain type checking as a way to prevent exploitation. https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3007 This vulnerability is caused by the unsecured deserialization of an object. In versions higher than Zend Framework 3.0.0, the attacker abuses the Zend3 feature that loads classes from objects in order to upload and execute malicious code in the server. The code can be uploaded using the “callback” parameter, which in this case inserts a malicious code instead of the “callbackOptions” array. ### Patches _Ha

O3 Security · Impact-Aware SCA

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CVSS Score

9.8/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

High

Raw vector

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

CWE Classification

CWE-502Deserialization of Untrusted Data

Weaponization

No known public exploit tooling found.

OpenSSF Scorecard

Package health score

magento-lts

6.2/10

Maintained10/10

Code-Review10/10

Branch-Protection2/10

Assessed Oct 5, 2026

Timeline

Disclosed

Apr 21, 2021

Last updated

Aug 7, 2026

Identifiers

CVE-2021-21426

GHSA-m496-x567-f98c

References

Advisorygithub.com Advisorynvd.nist.gov

Related vulnerabilities

This CVE is critical severity; 1 of 8 other CVEs affecting openmage/magento-lts is rated as high or higher (top: critical).

CVE-2026-42458CVE-2026-42155CVE-2026-42207CVE-2026-40488CVE-2026-40098CVE-2026-25525CVE-2026-25524CVE-2026-25523

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedAug 7, 2026

CVE-2021-21426 — magento-lts

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Impact

Patches

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Frequently Asked Questions

Is CVE-2021-21426 in your dependencies?