certspotter

by SSLMate

SSLMate / certspotter

Certificate Transparency Log Monitor

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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 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 that requires zero setup and provides an easy web dashboard to centrally manage your certificates. Visit https://sslmate.com/certspotter to sign up.

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.

USING CERT SPOTTER

The easiest way to use Cert Spotter is to sign up for an account at https://sslmate.com/certspotter. If you want to run Cert Spotter on your own server, follow these instructions.

Cert Spotter requires Go version 1.5 or higher.

  1. Install Cert Spotter using go get:

    go get software.sslmate.com/src/certspotter/cmd/certspotter

  2. Create a file called ~/.certspotter/watchlist listing the DNS names you want to monitor, one per line. To monitor an entire domain tree (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.

  3. Create a cron job to periodically run

    certspotter
    . See below for command line options.

Every time you run Cert Spotter, it scans all browser-recognized Certificate Transparency logs for certificates matching domains on your watch list. When Cert Spotter detects a matching certificate, it writes a report to standard out, which the Cron daemon emails to you. Make sure you are able to receive emails sent by Cron.

Cert Spotter also saves a copy of matching certificates in ~/.certspotter/certs (unless you specify the -no_save option).

When Cert Spotter has previously monitored a log, it scans the log from the previous position, to avoid downloading the same log entry more than once. (To override this behavior and scan all logs from the beginning, specify the -all_time option.)

When Cert Spotter 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 -startatend option.

You can add and remove domains on your watchlist at any time. However, the certspotter command only notifies you of certificates that were logged since adding a domain to the watchlist, unless you specify the -all_time option, which requires scanning the entirety of every log and takes many days to complete with a fast Internet connection. To examine preexisting certificates, it's better to use the Cert Spotter service https://sslmate.com/certspotter, the Cert Spotter API https://sslmate.com/certspotter/api, or a CT search engine such as https://crt.sh.

COMMAND LINE FLAGS

-watchlist FILENAME File containing identifiers to watch, one per line, as described above (use - to read from stdin). Default: ~/.certspotter/watchlist -nosave Do not save a copy of matching certificates. -startatend Start monitoring logs from the end, rather than the beginning. This significantly reduces the time to run Cert Spotter, but you will miss certificates that were added to a log before Cert Spotter started monitoring it. -alltime Scan for certificates from all time, not just those logged since the previous run of Cert Spotter. -logs FILENAMEORURL Filename of HTTPS URL of a JSON file containing logs to monitor, in the format documented at https://www.certificate-transparency.org/known-logs. Default: https://loglist.certspotter.org/monitor.json which includes the union of active logs recognized by Chrome and Apple. -state_dir PATH Directory for storing state. Default: ~/.certspotter -verbose Be verbose.

WHAT CERTIFICATES ARE DETECTED BY CERT SPOTTER?

Any certificate that is logged to a Certificate Transparency log trusted by Chromium will be detected by Cert Spotter. All certificates issued after April 30, 2018 must be logged to such a log to be trusted by Chromium.

Generally, certificate authorities will automatically submit certificates to logs so that they will work in Chromium. In addition, certificates that are discovered during Internet-wide scans are submitted to Certificate Transparency logs.

SECURITY

Cert Spotter assumes an adversarial model in which an attacker produces a certificate that is accepted by at least some clients but goes undetected because of an encoding error that prevents CT monitors from understanding it. To defend against this attack, Cert Spotter uses a special certificate parser that keeps the certificate unparsed except for the identifiers. If one of the identifiers matches a domain on your watchlist, you will be notified, even if other parts of the certificate are unparsable.

Cert Spotter takes special precautions to ensure identifiers are parsed correctly, and implements defenses against identifier-based attacks. For instance, if a DNS identifier contains a null byte, Cert Spotter interprets it as two identifiers: the complete identifier, and the identifier formed by truncating at the first null byte. For example, a certificate for example.org\0.example.com will alert the owners of both example.org and example.com. This defends against null prefix attacks http://www.thoughtcrime.org/papers/null-prefix-attacks.pdf.

SSLMate continuously monitors CT logs to make sure every certificate's identifiers can be successfully parsed, and will release updates to Cert Spotter as necessary to fix parsing failures.

Cert Spotter understands wildcard and redacted DNS names, and will alert you if a wildcard or redacted certificate might match an identifier on your watchlist. For example, a watchlist entry for sub.example.com would match certificates for *.example.com or ?.example.com.

Cert Spotter is not just a log monitor, but also a log auditor which checks that the log is obeying its append-only property. A future release of Cert Spotter will support gossiping with other log monitors to ensure the log is presenting a single view.

BygoneSSL

Cert Spotter can also notify users of bygone SSL certificates, which are SSL certificates that outlived their prior domain owner's registration into the next owners registration. To detect these certificates add a validat argument to each domain in the watchlist followed by the date the domain was registered in the following format YYYY-MM-DD. For example: example.com validat:2014-05-02

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