CVE-2021-21408Network attack vector

Sandbox escape via dynamic static class access

Published Jan 10, 2022 · Updated Apr 23, 2025

Sandbox escape in Smarty before 3.1.43 and 4.0.0 through 4.0.2 allows remote authenticated users to invoke restricted PHP methods. The template parser compiled variable-based static class references without applying a nonempty static_classes security-policy allowlist. Template-author capability and an enabled security policy are required; successful exploitation executes a PHP static method forbidden by policy.

CVSS severity8.8
High
EPSS probability2.22%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Sandbox escape in Smarty before 3.1.43 and 4.0.0 through 4.0.2 allows remote authenticated users to invoke restricted PHP methods. The template parser compiled variable-based static class references without applying a nonempty static_classes security-policy allowlist. Template-author capability and an enabled security policy are required; successful exploitation executes a PHP static method forbidden by policy.

The record

CVE
CVE-2021-21408
Published
Jan 10, 2022
Updated
Apr 23, 2025
Vendor
Smarty
Product
Smarty
Classifications
CWE-20
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jan 10, 2022CVE publishedPublication date reported by the CVE source.
  2. Apr 23, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 3.1.43
  2. Affected versionversion=>= 4.0.0, < 4.0.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Smarty · Smartyversion=< 3.1.43; version=>= 4.0.0, < 4.0.3

Published CVSS scores

8.8GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
6.5NIST NVDAV:N/AC:L/Au:S/C:P/I:P/A:P
8.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-20Improper Input Validation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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