CVE-2021-21707 | Special characters break path parsing in XML functions

Exp

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.

Published: 2021-11-29 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-21707 is rated Exploit Available (59.6/100): CVSS Medium severity, with medium exploitation likelihood (EPSS 0.56%). 1 public exploit reference(s) are indexed (Exploit-DB). Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2021-21707

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2021-21707

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2025-12-30 0.81% 0.56% -0.25%
2 2025-12-28 0.58% 0.81% +0.23%
3 2025-12-27 — 0.58% —

Full EPSS history (27 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-21707

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
5.3 3.1 MEDIUM
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:L)
Some sensitive info could get out, but not a total data dump.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:N)
Service keeps running; no real outage angle.
 3.9 1.4 [email protected]
5.3 3.1 MEDIUM
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:L)
Some sensitive info could get out, but not a total data dump.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:N)
Service keeps running; no real outage angle.
 3.9 1.4 [email protected]
5.0 2.0 MEDIUM
AV:N/AC:L/Au:N/C:P/I:N/A:N Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:P)
Partial confidentiality impact.
Integrity impact (I:N)
No integrity impact.
Availability impact (A:N)
No availability impact.
 10.0 2.9 [email protected]

Weakness enumeration for CVE-2021-21707

OS Trackers for CVE-2021-21707

vendor priority summary link
alpine medium CVE-2021-21707: 3 source package rows (php7, php8, php81); 6 state rows across 4 repos (3.17-community, 3.18-community, 3.19-community, edge-community); fixed 6, open 0. https://security.alpinelinux.org/vuln/CVE-2021-21707
debian not yet assigned CVE-2021-21707 not yet assigned priority: Debian including 1 source packages (php7.4), 1 status rows across 1 suites (bullseye): resolved 1. https://security-tracker.debian.org/tracker/CVE-2021-21707
redhat medium — https://access.redhat.com/security/cve/CVE-2021-21707
suse medium CVE-2021-21707 severity moderate: SUSE including 682 source package names (8-4.1:apache2-mod_php8-8.0.10-150400.4.8.1, 8-4.1:php8-8.0.10-150400.4.8.1, …), 2198 product×package rows across 51 product lines (Container bci/php, Container bci/php-apache, … (51 product lines)): Fixed 2198. https://www.suse.com/security/cve/CVE-2021-21707/
ubuntu low CVE-2021-21707 low priority: Ubuntu including 6 source packages (php5, php7.0, php7.2, php7.4, php8.0, php8.1), 91 status rows across 16 suites (bionic, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): DNE 76, released 6, needs-triage 3, not-affected 3, needed 2, ignored 1. https://ubuntu.com/security/CVE-2021-21707

Affected software / configurations for CVE-2021-21707

Vendor Product Version Raw CPE
php php >= 7.3.0, < 7.3.33 cpe:2.3:a:php:php:*:*:*:*:*:*:*:*
php php >= 7.4.0, < 7.4.26 cpe:2.3:a:php:php:*:*:*:*:*:*:*:*
php php >= 8.0.0, < 8.0.13 cpe:2.3:a:php:php:*:*:*:*:*:*:*:*
netapp clustered_data_ontap — cpe:2.3:a:netapp:clustered_data_ontap:-:*:*:*:*:*:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*
debian debian_linux 11.0 cpe:2.3:o:debian:debian_linux:11.0:*:*:*:*:*:*:*
tenable tenable.sc < 5.21.0 cpe:2.3:a:tenable:tenable.sc:*:*:*:*:*:*:*:*

References for CVE-2021-21707

URL Tags
https://bugs.php.net/bug.php?id=79971 Exploit Issue Tracking Patch Release Notes Vendor Advisory
https://lists.debian.org/debian-lts-announce/2022/12/msg00030.html Issue Tracking Mailing List
https://security.netapp.com/advisory/ntap-20211223-0005/ Third Party Advisory
https://www.debian.org/security/2022/dsa-5082 Third Party Advisory
https://www.tenable.com/security/tns-2022-09 Patch Release Notes Third Party Advisory
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