diff --git a/src/xz/file_io.c b/src/xz/file_io.c
index 28280293..78fbdf72 100644
--- a/src/xz/file_io.c
+++ b/src/xz/file_io.c
@@ -199,6 +199,7 @@ io_sandbox_enter(int src_fd)
 			CAP_EVENT, CAP_FCNTL, CAP_LOOKUP, CAP_READ, CAP_SEEK)))
 		goto error;
 
+	// If not reading from stdin, remove all capabilities from it.
 	if (src_fd != STDIN_FILENO && cap_rights_limit(
 			STDIN_FILENO, cap_rights_clear(&rights)))
 		goto error;