Pi3Web ISAPI DoS vulnerability Discovered by: Hamid Ebadi CSIRT Team Member Amirkabir University CSIRT Laboratory (APA Laboratory) autcert@aut.ac.ir Introduction Pi3Web is a free, multithreaded, highly configurable and extensible HTTP server and development environment for cross platform internet server development and deployment. Pi3web is vulnerable to a denial of service (DoS) vulnerability whenever an invalid ISAPI module is requested from server. Vulnerable version Pi3Web <=2.0.3 Vulnerability By requesting the following URL from pi3web the server crashes: http://WEB_SITE/isapi/users.txt EnhPi3.exe -Bad Image The application or DLL c:\Pi3Web\Isapi\users.txt is not a valid Windows image. Please check this against your installation diskette The vulnerability is caused. The crash is due to insufficient checks for incoming requests. Whenever a file in ISAPI directory, which is not a valid DLL is requested, the server tries to load it into memory as a DLL library and a crash happens. Workaround Before an official patch is released, use one of the following workarounds to mitigate the problem: 1. Disable ISAPI mapping in server configuration in Server Admin > Mapping Tab. 2. Delete the users.txt, install.daf and readme.daf in ISAPI folder. Credit This vulnerability has been discovered by Hamid Ebadi from Amirkabir university CSIRT laboratory. autcert@aut.ac.ir https://www.ircert.cc