Grav vulnerable to Denial of Service via Improper Input Handling in 'Supported' Parameter

Endpoint: admin/config/system
Submenu: Languages
Parameter: Supported
Application: Grav v 1.7.48

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Summary

A Denial of Service (DoS) vulnerability was identified in the "Languages" submenu of the Grav admin configuration panel (/admin/config/system). Specifically, the Supported parameter fails to properly validate user input. If a malformed value is inserted—such as a single forward slash (/) or an XSS test string—it causes a fatal regular expression parsing error on the server.

This leads to application-wide failure due to the use of the preg_match() function with an improperly constructed regular expression, resulting in the following error:

preg_match(): Unknown modifier 'o' File: /system/src/Grav/Common/Language/Language.php line 244

Once triggered, the site becomes completely unavailable to all users.

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Details

• Vulnerable Endpoint: POST /admin/config/system

• Submenu: Languages

• Parameter: Supported

The application dynamically constructs a regular expression using the contents of the Supported field without escaping the input using preg_quote() or proper validation. This allows attackers to inject invalid syntax into the regex engine, crashing the application during language resolution.

Stack trace excerpt:

Whoops \ Exception \ ErrorException (E_WARNING) preg_match(): Unknown modifier 'o' /system/src/Grav/Common/Language/Language.php244

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Proof of Concept (PoC)

Payloads:

/

Steps to Reproduce:

1. Log into the Grav Admin Panel.

2. Navigate to: Configuration → System → Languages.

3. Locate the Supported field.

4. Insert one of the payloads above (e.g., a single slash /).

5. Click Save.

<img width="1897" height="639" alt="Pasted image 20250719183223" src="https://github.com/user-attachments/assets/d3a54a20-d30d-46c6-9015-722f80701cfb" />

1. Observe: All pages in the application begin throwing a fatal error and become inaccessible.

<img width="1802" height="998" alt="Pasted image 20250719175229" src="https://github.com/user-attachments/assets/b16750c2-507f-4c30-a9bb-d07fa92bb777" />

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Impact

• Application-wide Denial of Service (DoS)

• All login and admin views crash with the same error

• Potentially exploitable by:

  • Admin panel users

  • CSRF if misconfigured

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References

• CWE-1333: Improper Regular Expression

• CWE-20: Improper Input Validation

Discoverer

Marcelo Queiroz

by CVE-Hunters