diff --git a/books/free-programming-books-subjects.md b/books/free-programming-books-subjects.md
index ec0bd0edd..926ed57a0 100644
--- a/books/free-programming-books-subjects.md
+++ b/books/free-programming-books-subjects.md
@@ -949,6 +949,7 @@ Kerridge (PDF) (email address *requested*, not required)
 * [The MoonMath Manual to zk-SNARKs](https://leastauthority.com/community-matters/moonmath-manual/) - Least Authority
 * [The SELINUX Coloring Book](https://github.com/mairin/selinux-coloring-book) -  Daniel J Walsh, Máirín Duffy (PDF) (CC BY-SA)
 * [The SELinux Notebook](https://github.com/SELinuxProject/selinux-notebook) - Richard Haines, Paul Moore, SELinux Community (HTML, PDF, EPUB, azw3) (GFDL)
+* [Hacker101](https://www.hacker101.com/) - HackerOne
 
 
 ### Software Architecture