3.15 What known problems are there with anti-virus software?

Anti-virus and firewall software can block Emacs from starting subprocesses and opening network connections. Most such products have an Advanced mode where they will prompt you rather than silently blocking. In some cases the “scan all files” or “auto protect” option of anti-virus programs has caused failures running shell related commands within Emacs. See Why is nothing happening when I enter shell commands?.