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NAME

certspotter - Certificate Transparency Log Monitor

SYNOPSIS

certspotter [-start_at_end] [-watchlist FILENAME] [-email ADDRESS] ...

DESCRIPTION

Cert Spotter is a Certificate Transparency log monitor from SSLMate that alerts you when a SSL/TLS certificate is issued for one of your domains. Cert Spotter is easier to use than other open source CT monitors, since it does not require a database. It's also more robust, since it uses a special certificate parser that ensures it won't miss certificates.

Cert Spotter is also available as a hosted service by SSLMate, https://sslmate.com/certspotter.

You can use Cert Spotter to detect:

  • Certificates issued to attackers who have compromised your DNS and are redirecting your visitors to their malicious site.
  • Certificates issued to attackers who have taken over an abandoned sub-domain in order to serve malware under your name.
  • Certificates issued to attackers who have compromised a certificate authority and want to impersonate your site.
  • Certificates issued in violation of your corporate policy or outside of your centralized certificate procurement process.

OPTIONS

-batch_size NUMBER

: Maximum number of entries to request per call to get-entries. You should not generally need to change this. Defaults to 1000.

-email ADDRESS

: Email address to contact when a matching certificate is discovered, or an error occurs. You can specify this option more than once to email multiple addresses. Your system must have a working sendmail(1) command.

Regardless of the `-email` option, certspotter also emails any address listed
in `$CERTSPOTTER_CONFIG_DIR/email_recipients` file
(`~/.certspotter/email_recipients` by default).  (One address per line,
blank lines are ignored.)  This file is read only at startup, so you
must restart certspotter if you change it.

-healthcheck INTERVAL

: Perform a health check at the given interval (default: "24h") as described below. INTERVAL must be a decimal number followed by "h" for hours or "m" for minutes.

-logs ADDRESS

: Filename or HTTPS URL of a v2 or v3 JSON log list containing logs to monitor. The schema for this file can be found at https://www.gstatic.com/ct/log_list/v3/log_list_schema.json. Defaults to https://loglist.certspotter.org/monitor.json, which includes the union of active logs recognized by Chrome and Apple. certspotter periodically reloads the log list in case it has changed.

-no_save

: Do not save a copy of matching certificates. Note that enabling this option will cause you to receive duplicate notifications, since certspotter will have no way of knowing if you've been previously notified about a certificate.

-script COMMAND

: Command to execute when a matching certificate is found or an error occurs. See certspotter-script(8) for information about the interface to scripts.

Regardless of the `-script` option, certspotter also executes any executable
file in the `$CERTSPOTTER_CONFIG_DIR/hooks.d` directory
(`~/.certspotter/hooks.d` by default).

-start_at_end

: Start monitoring logs from the end rather than the beginning.

**WARNING**: monitoring from the beginning guarantees detection of all
certificates, but requires downloading hundreds of millions of
certificates, which takes days.

-state_dir PATH

: Directory for storing state. Defaults to $CERTSPOTTER_STATE_DIR, which is "~/.certspotter" by default.

-stdout

: Write matching certificates and errors to stdout.

-verbose

: Be verbose.

-version

: Print version and exit.

-watchlist PATH

: File containing DNS names to monitor, one per line. To monitor an entire domain namespace (including the domain itself and all sub-domains) prefix the domain name with a dot (e.g. ".example.com"). To monitor a single DNS name only, do not prefix the name with a dot.

Defaults to `$CERTSPOTTER_CONFIG_DIR/watchlist`, which is
"~/.certspotter/watchlist" by default.
Specify `-` to read the watch list from stdin.

certspotter reads the watch list only when starting up, so you must restart
certspotter if you change it.

NOTIFICATIONS

When certspotter detects a certificate matching your watchlist, or encounters an error that is preventing it from discovering certificates, it notifies you as follows:

  • Emails any address specified by the -email command line flag.

  • Emails any address listed in the $CERTSPOTTER_CONFIG_DIR/email_recipients file (~/.certspotter/email_recipients by default). (One address per line, blank lines are ignored.) This file is read only at startup, so you must restart certspotter if you change it.

  • Executes the script specified by the -script command line flag.

  • Executes every executable file in the $CERTSPOTTER_CONFIG_DIR/hooks.d directory (~/.certspotter/hooks.d by default).

  • Writes the notification to standard out if the -stdout flag was specified.

Sending email requires a working sendmail(1) command. For details about the script interface, see certspotter-script(8).

OPERATION

certspotter continuously monitors all browser-recognized Certificate Transparency logs looking for certificates which are valid for any domain on your watch list. When certspotter detects a matching certificate, it emails you, executes a script, and/or writes a report to standard out, as described above.

certspotter also saves a copy of matching certificates in $CERTSPOTTER_STATE_DIR/certs ("~/.certspotter/certs" by default) unless you specify the -no_save option.

When certspotter has not previously monitored a log, it can either start monitoring the log from the beginning, or seek to the end of the log and start monitoring from there. Monitoring from the beginning guarantees detection of all certificates, but requires downloading hundreds of millions of certificates, which takes days. The default behavior is to monitor from the beginning. To start monitoring new logs from the end, specify the -start_at_end option.

If certspotter has previously monitored a log, it resumes monitoring the log from the previous position. This means that if you add a domain to your watch list, certspotter will not detect any certificates that were logged prior to the addition. To detect such certificates, you must delete $CERTSPOTTER_STATE_DIR/logs, which will cause certspotter to restart monitoring from the very beginning of each log (provided -start_at_end is not specified). This will cause certspotter to download hundreds of millions of certificates, which takes days. To find preexisting certificates, it's faster to use the Cert Spotter service https://sslmate.com/certspotter, SSLMate's Certificate Transparency Search API https://sslmate.com/ct_search_api, or a CT search engine such as https://crt.sh.

ERROR HANDLING

When certspotter encounters a problem with the local system (e.g. failure to write a file or execute a script), it prints a message to stderr and exits with a non-zero status.

When certspotter encounters a problem monitoring a log, it prints a message to stderr and continues running. It will try monitoring the log again later; most log errors are transient.

Every 24 hours (unless overridden by -healthcheck), certspotter performs the following health checks:

  • Ensure that the log list has been successfully retrieved at least once since the previous health check.
  • Ensure that every log has been successfully contacted at least once since the previous health check.
  • Ensure that certspotter is not falling behind monitoring any logs.

If any health check fails, certspotter notifies you by email, script, and/or standard out, as described above.

Health check failures should be rare, and you should take them seriously because it means certspotter might not detect all certificates. It might also be an indication of CT log misbehavior. Consult certspotter's stderr output for details, and if you need help, file an issue at https://github.com/SSLMate/certspotter.

EXIT STATUS

certspotter exits 0 when it receives SIGTERM or SIGINT, and non-zero when a serious error occurs.

ENVIRONMENT

CERTSPOTTER_STATE_DIR

: Directory for storing state. Overridden by -state_dir. Defaults to ~/.certspotter.

CERTSPOTTER_CONFIG_DIR

: Directory from which any configuration, such as the watch list, is read. Defaults to ~/.certspotter.

EMAIL

: Email address from which to send emails. If not set, certspotter lets sendmail pick the address.

HTTPS_PROXY

: URL of proxy server for making HTTPS requests. http://, https://, and socks5:// URLs are supported. By default, no proxy server is used.

SENDMAIL_PATH

: Path to the sendmail binary used for sending emails. Defaults to /usr/sbin/sendmail.

SEE ALSO

certspotter-script(8)

COPYRIGHT

Copyright (c) 2016-2023 Opsmate, Inc.

BUGS

Report bugs to https://github.com/SSLMate/certspotter.