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

Quick assessment

Affected
PHP Group PHP
Exploitation
Public or AI PoC available; prioritize validation
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

PHP是一种在服务器端执行的脚本语言。 PHP 存在安全漏洞，攻击者可以通过 Xml 函数空字符绕过 PHP 的访问限制，以读取或更改文件。

CVSS 5.3 · Medium EPSS 25.95% · P98
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I. Basic Information for CVE-2021-21707

Vulnerability Information
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Vulnerability Title
Special characters break path parsing in XML functions
Source: CVE Program / CVE List V5
Vulnerability Description
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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
特殊元素净化处理不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
PHP 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
PHP是一种在服务器端执行的脚本语言。 PHP 存在安全漏洞，攻击者可以通过 Xml 函数空字符绕过 PHP 的访问限制，以读取或更改文件。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
PHP Group PHP 7.3.x ~ 7.3.33 -

II. Public POCs for CVE-2021-21707

# POC Description Source Link Shenlong Link
1 Exploit for the PHP 8.1.0-dev backdoor vulnerability (CVE-2021-21707) https://github.com/useru1k/php-8.1.0-dev-exploit POC Details
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III. Intelligence Information for CVE-2021-21707

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Vendor Advisories for CVE-2021-21707 (2)

Mailing List Discussions for CVE-2021-21707 (1)

Other References for CVE-2021-21707 (2)

IV. Related Vulnerabilities

Same product: PHP
Same vendor: PHP Group
Same weakness: CWE-159

V. Comments for CVE-2021-21707

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