Login to WordPress or PHP application fails without any error messages, when ModSecurity is enabled in Plesk

Symptoms

• The Log In button for WordPress, Typo3 or other CMS in Plesk is not working: it redirects to application login page with empty credentials fields.

• Login to PHP application using POST request fails without any error messages.

Cause

Login mechanism is breaking by ModSecurity Tradeoff/Thorough modes, enabled in Plesk > Tools & Settings > Web Application Firewall > Settings.  These modes include analysis of POST data that lead to changing sending packets.

Resolution

Log into Plesk and apply one of the following solutions:

• Switch ModSecurity mode to Fast on Plesk > Tools & Settings > Web Application Firewall (ModSecurity) > Settings: this mode does not include analysis of POST data.

• Disable ModSecurity for a domain via Plesk > Domains > example.com > Web Application Firewall.

• Forbid ModSecurity to access request bodies (Windows Server only):

  1. Connect to the server via RDP.

  2. Open the file C:\Program Files\ModSecurity IIS\modsecurity.conf in a text editor and set the SecRequestBodyAccess setting to the Off value:

     CONFIG_TEXT: SecRequestBodyAccess Off

  3. Restart IIS webserver via a command prompt:

     C:\> iisreset