Deny attempts to rename a file from guest users if the policy

says they may not modify existing files through FTP.

Renaming a file is effectively a way to modify it.
For instance, if a malicious party is unable to delete or overwrite
a sensitive file, they can nevertheless rename it to a hidden name
and then upload a troyan horse under the guise of the old file name.

libexec/ftpd/ftpd.c

@@ -2520,6 +2520,10 @@ renamefrom(char *name)
 {
 	struct stat st;
 
+	if (guest && noguestmod) {
+		reply(550, "Operation not permitted");
+		return (NULL);
+	}
 	if (lstat(name, &st) < 0) {
 		perror_reply(550, name);
 		return ((char *)0);