CVE-2021-21708Public exploitNetwork attack vector

Process crash or code execution via freed zval

Published Feb 27, 2022 · Updated Sep 17, 2024

Use after free in PHP 7.4 before 7.4.28, 8.0 before 8.0.16, and 8.1 before 8.1.3 allows attackers to crash a PHP process. php_filter_float() destroys the input zval before rejecting an integer outside configured min_range or max_range, leaving the error path to access freed memory. The condition requires code to pass an integer-form string to FILTER_VALIDATE_FLOAT with range limits; a PHP-FPM crash is demonstrated, and the CNA identifies heap overwrite and code execution as possible additional outcomes.

CVSS severity9.8
Critical
EPSS probability3.00%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use after free in PHP 7.4 before 7.4.28, 8.0 before 8.0.16, and 8.1 before 8.1.3 allows attackers to crash a PHP process. php_filter_float() destroys the input zval before rejecting an integer outside configured min_range or max_range, leaving the error path to access freed memory. The condition requires code to pass an integer-form string to FILTER_VALIDATE_FLOAT with range limits; a PHP-FPM crash is demonstrated, and the CNA identifies heap overwrite and code execution as possible additional outcomes.

The record

CVE
CVE-2021-21708
Published
Feb 27, 2022
Updated
Sep 17, 2024
Vendor
The PHP Group
Product
PHP
Classifications
CWE-416, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 27, 2022CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 2024Record 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=7.4.x <7.4.28
  2. Affected versionversion=8.0.X <8.0.16
  3. Affected versionversion=8.1.X <8.1.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The PHP Group · PHPversion=7.4.x <7.4.28; version=8.0.X <8.0.16; version=8.1.X <8.1.3

Published CVSS scores

8.2PHP GroupCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P

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.

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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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-416Use After Free
T1190Exploit Public-Facing Application

Public exploit references

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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