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PHPApplication

CVE-2021-21703

HIGH · 7.0 CVSS v3.1 Published 2021-10-25
Fix available
A fix is available. Upgrade to 7.4.25 / 8.0.12 or later.
See remediation →
72/100
Remediation priority · Elevated
Public exploit Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
In PHP versions 7.3.x up to and including 7.3.31, 7.4.x below 7.4.25 and 8.0.x below 8.0.12, when running PHP FPM SAPI with main FPM daemon process running as root and child worker processes running as lower-privileged users, it is possible for the child processes to access memory shared with the main process and write to it, modifying it in a way that would cause the root process to conduct invalid memory reads and writes, which can be used to escalate privileges from local unprivileged user to the root user.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis
How this class of weakness works · CWE-284

The application fails to correctly restrict what a user can do or reach, so functions or resources are available to people who should not have them. Attackers simply probe for the paths where the check is missing or wrong. The fix is to enforce access-control decisions consistently on every request, evaluated against the acting user rather than assumed from context.

General guidance for the improper access control class — the official description and references above are authoritative for this specific CVE. Want a bespoke review and a reviewed fix? Ask our team →

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
PHPApplication
Affected:>= 7.3.0, <= 7.3.31>= 7.4.0, < 7.4.25>= 8.0.0, < 8.0.12
Debian LinuxOperating system
Affected:= 9.0= 10.0= 11.0
FedoraOperating system
Affected:= 33= 34= 35
Clustered Data OntapApplication
Affected:all versions
Communications Diameter Signaling RouterApplication
Affected:>= 8.0.0.0, <= 8.5.0.2

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
High
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 7.4.25 / 8.0.12 or later
Fixed in 7.4.258.0.12
Vendor patch www.openwall.com →
Recommended fix High confidence

PHP 7.4.25 or PHP 8.0.12 (or later stable releases); note PHP 7.3.x is end-of-life and should be upgraded to PHP 7.4 or 8.x

  1. 1. Identify the current PHP version installed using: php -v
  2. 2. For Debian-based systems, update package lists: apt update
  3. 3. Upgrade PHP to version 7.4.25 or later (or 8.0.12+), e.g., apt install php7.4-fpm or apt install php8.0-fpm
  4. 4. Verify the installed PHP version: php -v
  5. 5. Restart PHP-FPM service: systemctl restart php*-fpm (replace * with your version)
  6. 6. Verify the fix by checking PHP-FPM process memory isolation between root master and unprivileged workers
Caveat Minor: PHP 8.0 introduced breaking changes from PHP 7.x; test application compatibility before production deployment; PHP 7.3 is EOL and should not be used

Generated from the published advisory — verify against the referenced sources before acting.

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $1,950. Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-21703 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data