diff --git a/Doc/lib/libcgi.tex b/Doc/lib/libcgi.tex
index 055f66c..add322d 100644
--- a/Doc/lib/libcgi.tex
+++ b/Doc/lib/libcgi.tex
@@ -598,7 +598,9 @@
 usually not set to a very useful value in a CGI script.
 
 \item When reading or writing external files, make sure they can be read
-or written by every user on the system.
+or written by the userid under which your CGI script will be running:
+this is typically the userid under which the web server is running, or some
+explicitly specified userid for a web server's \samp{suexec} feature.
 
 \item Don't try to give a CGI script a set-uid mode.  This doesn't work on
 most systems, and is a security liability as well.