● Shell Script CGI Security

232 2026-08-05 note ♈︎ ♄︎

Shell scripts are risky for CGIThe Common Gateway Interface is a 1993 protocol for running a program per HTTP request, standardized in 2004 as RFC 3875. The server forks a process, passes the request in as environment variables and stdin, and reads headers plus a… because sandboxing backticks against injection is close to impossible.

The Minimal CGI ScriptThe whole CGI contract in one script: read the environment, print headers, print a blank line, print a body. python #!/usr/bin/env python3 import os query = os.environ.get("QUERYSTRING", "") print("Content-Type: text/plain") print() print(f"Hello from CGI. Query string: {query}") Apache or any… note explicitly warns against shell script CGI for this reason.

For the guestbookA shared guestbook on a tildeverse server lets users add entries through a controlled script rather than direct file editing. The script logs who added entries and when, preventing accidental deletion while maintaining attribution. See guestbook-add-script for the implementation, guestbook-file-permissions… system, the shell script runs locally on the tilde server, not as a web CGI, which reduces the attack surface.

If you must use shell scripts for web input, filter for known patterns rather than trying to sanitize arbitrary input:

if grep -qi "viagra\|casino\|lottery\|porn" "$TEMPFILE"; then
    echo "Entry rejected: inappropriate content detected"
    rm "$TEMPFILE"
    exit 1
fi

This is a denylist approach, not real security. Python or Perl CGI with proper parameter handling is safer.