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PHPApplication

CVE-2021-21707

MEDIUM · 5.3 CVSS v3.1 Published 2021-11-29
Fix available
A fix is available. Upgrade to 5.21.0 / 7.3.33 or later.
See remediation →
62/100
Remediation priority · Elevated
Public exploit Remotely reachable No privileges 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 below 7.3.33, 7.4.x below 7.4.26 and 8.0.x below 8.0.13, certain XML parsing functions, like simplexml_load_file(), URL-decode the filename passed to them. If that filename contains URL-encoded NUL character, this may cause the function to interpret this as the end of the filename, thus interpreting the filename differently from what the user intended, which may lead it to reading a different file than intended.

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

A detailed technical summary for this CVE is being prepared.

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, < 7.3.33>= 7.4.0, < 7.4.26>= 8.0.0, < 8.0.13
Debian LinuxOperating system
Affected:= 10.0= 11.0
Clustered Data OntapApplication
Affected:all versions
Tenable.scApplication
Affected:< 5.21.0

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
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
None
Availability
None

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

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 5.21.0 / 7.3.33 / 7.4.26 or later
Fixed in 5.21.07.3.337.4.26
Vendor patch bugs.php.net →
Recommended fix High confidence

PHP 7.3.33+, PHP 7.4.26+, or PHP 8.0.13+ (depending on your major.minor branch)

  1. 1. Identify the currently installed PHP version using 'php -v' or 'phpinfo()'
  2. 2. If running PHP 7.3.x, upgrade to PHP 7.3.33 or later
  3. 3. If running PHP 7.4.x, upgrade to PHP 7.4.26 or later
  4. 4. If running PHP 8.0.x, upgrade to PHP 8.0.13 or later
  5. 5. For Debian systems, run 'apt update && apt upgrade php*' or install from the Debian security repository
  6. 6. Restart any web servers (Apache, Nginx) or PHP-FPM services after upgrading
  7. 7. Verify the fix by confirming the PHP version meets the minimum fixed version requirements
Caveat Minor PHP version upgrades typically have low risk but test thoroughly in staging before production deployment

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 $1,600. 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-2021-21707 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