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CVE-2021-21707· NVD / CVE Program· CNA php

Special characters break path parsing in XML functions

Noroxi assessment

Priority: Monitor (action score 29/100)

  • Public proof-of-concept code exists (GitHub); there is no record of exploitation in the wild.
  • Probability of exploitation in the next 30 days: 25.3% (FIRST EPSS); higher than 97.9% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: apcu-panel (AlmaLinux:8) 5.1.18-1.module_el8.6.0+2750+78feabcb; libzip (AlmaLinux:8) 1.6.1-1.module_el8.6.0+2750+78feabcb, 1.6.1-1.module_el8.6.0+3239+bedf0508; libzip-devel (AlmaLinux:8) 1.6.1-1.module_el8.6.0+2750+78feabcb, 1.6.1-1.module_el8.6.0+3239+bedf0508.

Low priority for now. Upgrade to apcu-panel (AlmaLinux:8) 5.1.18-1.module_el8.6.0+2750+78feabcb; libzip (AlmaLinux:8) 1.6.1-1.module_el8.6.0+2750+78feabcb, 1.6.1-1.module_el8.6.0+3239+bedf0508; libzip-devel (AlmaLinux:8) 1.6.1-1.module_el8.6.0+2750+78feabcb, 1.6.1-1.module_el8.6.0+3239+bedf0508 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

MediumCVSS 5.3 · v3.1—Proof of concept Fix available
Published
Nov 29, 2021
Updated
Jun 16, 2026
EPSS
25.3% · 97th percentile

Report tools

JSON

Action score

29

Monitor

Low priority for now.

CVSS
21 / 40 · 5.3 / 10
CISA KEV
0 / 30 · Not listed
EPSS
8 / 30 · 25.3%

CNA vs NVD score

NVD
5.3
CNA · php
5.3
agree

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • Proof of concept exists, no weaponized module
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-21707

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
phpphp
netappclustered data ontap
debiandebian linux
tenabletenable.sc

Using php? Add it to your stack and be the first to know about CVEs like this one. (PHP)

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When you'll hear from us

  • when a new CVE affecting this product is published
  • when a record in your stack enters KEV or gets exploit code

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Running WordPress? Scan your site free with WP Lens

  • Stack alerts: add the products and versions you run, get the new CVEs that affect them
  • Uptime monitoring: 1 site, instant email when it goes down
  • Add your domain: subdomains and takeover risk scanned every week (1 domain)
  • 10 scans an hour on the security tools (3 a day without an account), an API key and MCP
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Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • php php7.3.0 and later · before 7.3.33
  • php php7.4.0 and later · before 7.4.26
  • php php8.0.0 and later · before 8.0.13

Versions reported by the vendor

Affected version ranges reported by the assigning authority (php). Independent of NVD's CPE analysis and usually ahead of it.

  • PHP Group PHP

    • 7.3.x and later · before 7.3.33affected
    • 7.4.x and later · before 7.4.26affected
    • 8.0.X and later · before 8.0.13affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
AlmaLinux:8apcu-panelbefore 5.1.18-1.module_el8.6.0+2750+78feabcb5.1.18-1.module_el8.6.0+2750+78feabcb
AlmaLinux:8libzipbefore 1.6.1-1.module_el8.6.0+2750+78feabcb1.6.1-1.module_el8.6.0+2750+78feabcb
AlmaLinux:8libzipbefore 1.6.1-1.module_el8.6.0+3239+bedf05081.6.1-1.module_el8.6.0+3239+bedf0508
AlmaLinux:8libzip-develbefore 1.6.1-1.module_el8.6.0+2750+78feabcb1.6.1-1.module_el8.6.0+2750+78feabcb
AlmaLinux:8libzip-develbefore 1.6.1-1.module_el8.6.0+3239+bedf05081.6.1-1.module_el8.6.0+3239+bedf0508
AlmaLinux:8libzip-toolsbefore 1.6.1-1.module_el8.6.0+3239+bedf05081.6.1-1.module_el8.6.0+3239+bedf0508
AlmaLinux:8libzip-toolsbefore 1.6.1-1.module_el8.6.0+2750+78feabcb1.6.1-1.module_el8.6.0+2750+78feabcb
AlmaLinux:8phpbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-bcmathbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-clibefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-commonbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-dbabefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-dbgbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-develbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-embeddedbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-enchantbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-ffibefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-fpmbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-gdbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-gmpbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-intlbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-jsonbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-ldapbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0
AlmaLinux:8php-mbstringbefore 7.4.30-1.module_el8.7.0+3286+227f3cf07.4.30-1.module_el8.7.0+3286+227f3cf0

+96

Same product

php: all records

Other highest-scoring records for the same primary product.

  • CVE-2024-4577Argument Injection in PHP-CGIKEV
    99Now
  • CVE-2019-11043Underflow in PHP-FPM can lead to RCEKEV
    99Now
  • CVE-2012-1823sapi/cgi/cgi_main.c in PHP before 5.3.12 and 5.4.x before 5.4.2, when configured as a CGI script (aka php-cgi), does not properly handle queKEV
    99Now

Published before and after this one in the same product

  • ← EarlierCVE-2021-21706Oct 4, 2021 · ZipArchive::extractTo may extract outside of destination dir
    26Monitor
  • ← EarlierCVE-2021-21703Oct 25, 2021 · PHP-FPM memory access in root process leading to privilege escalation
    28Monitor
  • Later →CVE-2021-21708Feb 27, 2022 · UAF due to php_filter_float() failing
    40Plan

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
almalinux:apcu-panel5.1.18-1.module_el8.6.0+2750+78feabcb · AlmaLinux:8Package registry (OSV)
almalinux:libzip1.6.1-1.module_el8.6.0+2750+78feabcb · AlmaLinux:8Package registry (OSV)
almalinux:libzip-devel1.6.1-1.module_el8.6.0+2750+78feabcb · AlmaLinux:8Package registry (OSV)
almalinux:libzip-tools1.6.1-1.module_el8.6.0+3239+bedf0508 · AlmaLinux:8Package registry (OSV)
almalinux:php7.4.30-1.module_el8.7.0+3286+227f3cf0 · AlmaLinux:8Package registry (OSV)
almalinux:php-bcmath7.4.30-1.module_el8.7.0+3286+227f3cf0 · AlmaLinux:8Package registry (OSV)
almalinux:php-cli7.4.30-1.module_el8.7.0+3286+227f3cf0 · AlmaLinux:8Package registry (OSV)
almalinux:php-common7.4.30-1.module_el8.7.0+3286+227f3cf0 · AlmaLinux:8Package registry (OSV)
almalinux:php-dba7.4.30-1.module_el8.7.0+3286+227f3cf0 · AlmaLinux:8Package registry (OSV)
almalinux:php-dbg7.4.30-1.module_el8.7.0+3286+227f3cf0 · AlmaLinux:8Package registry (OSV)
almalinux:php-devel7.4.30-1.module_el8.7.0+3286+227f3cf0 · AlmaLinux:8Package registry (OSV)
almalinux:php-embedded7.4.30-1.module_el8.7.0+3286+227f3cf0 · AlmaLinux:8Package registry (OSV)
debian:php7.37.3.31-1~deb10u2 · Debian:10Package registry (OSV)
debian:php7.47.4.28-1+deb11u1 · Debian:11Package registry (OSV)
opensuse:apache2-mod_php77.4.25-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
opensuse:apache2-mod_php88.0.10-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
opensuse:php77.4.25-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
opensuse:php7-embed7.4.25-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
opensuse:php7-fastcgi7.4.25-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
opensuse:php7-fpm7.4.25-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
opensuse:php7-test7.4.25-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
opensuse:php88.0.10-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
opensuse:php8-embed8.0.10-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
opensuse:php8-fastcgi8.0.10-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
opensuse:php8-fpm8.0.10-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
opensuse:php8-test8.0.10-150400.4.8.1 · openSUSE:Leap 15.4Package registry (OSV)
red hat:rh-php73-php0:7.3.33-1.el7 · Red Hat:rhel_software_collections:3::el7Package registry (OSV)
red hat:rh-php73-php-bcmath0:7.3.33-1.el7 · Red Hat:rhel_software_collections:3::el7Package registry (OSV)
red hat:rh-php73-php-cli0:7.3.33-1.el7 · Red Hat:rhel_software_collections:3::el7Package registry (OSV)
red hat:rh-php73-php-common0:7.3.33-1.el7 · Red Hat:rhel_software_collections:3::el7Package registry (OSV)

+28

Exploit status

Proof of concept available

A public proof of concept or detection signature exists. Exploit knowledge is out there; don't delay patching.

Evidence

  • Proof-of-concept repository on GitHub ×1

Noroxi does not host or link to exploit code. Metasploit module names and Nuclei template ids are shown (names, not code); Exploit-DB entries and PoC repositories are given as counts only.

Public exploit evidence
SourceKindDate
Proof-of-concept repository on GitHubstars:02025-09-08

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

  1. Published
  2. First PoC+1379 d
  3. TodayOct 24, 2026+1790 d
EPSS, last 120 days-1% in 7 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

The product’s vendor appears in a public program. Matched by name; verify the scope text in the program.

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
low · data can be read
Integrity
none
Availability
none
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality impact
Low
Integrity impact
None
Availability impact
None

Weakness class (CWE)

CWE-159 · Improper Handling of Invalid Use of Special Elements

CVSS vector

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

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (3 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records