Expect-CT reports

You could get a report when a browser loaded your site with a TLS certificate that didn't meet the requirements of the browser's Certificate Transparency (CT) policy. For example Google Chrome requires all publicly-trusted TLS certificates issued after April 30, 2018 to be "CT Qualified" in order to be recognized as valid. Nowadays, it means that Chrome requires CT on all public sites, so Expect-CT could be used only as a tool to detect misconfigurations. But CT certificate configuration is almost always done by certification authorities, virtually never by the site owners, so usefulness of Expect-CT as a debugging tool is also very limited. In October 2022, Chrome removed Expect-CT in version 107. Chrome was also the only browser that had implemented the Expect-CT support.

🍌 Your browser doesn't support Expect-CT, no reports will be sent

The `Expect-CT` response header:

Expect-CT: max-age=1800, enforce, report-uri="https://flynn.has.report/report"

max-age: for how many seconds should the browser remember to send violation reports, or enforce the policy
enforce: optional, if present, the browser should refuse future connections that violate the CT policy, for max-age seconds after the reception of the Expect-CT header – using enforce doesn't make sense with max-age: 0, also keep in mind some browsers have their own CT requirements that cannot be disabled by simply omitting enforce or setting a short max-age
report-uri: where to send policy violation reports to, must use HTTPS

Test Expect-CT reporting

It would be quite a feat to get a certificate that would violate for example Chrome's CT policy so there's no "click here to generate the report" button here. Until the feature was removed, Chrome offered to send a test Expect-CT report:

Go to Chrome's Domain Security Policy debug page: chrome://net-internals/#hsts (you need to copy & paste the link)
Scroll down to Send test Expect-CT report
Enter your custom reporting endpoint address https://flynn.has.report/report (copy & paste) to the Report URI field
Hit Send and you should see that the Test report succeeded
Check your reports
You'll find one expect-ct-report test report for expect-ct-report.test host, no scts and empty certificate chains

Example Expect-CT report

This is how the full report looked like:

{
  "expect-ct-report": {
    "port": 443,
    "scts": [
      {
        "serialized_sct": "<Base64 Signed Certificate Timestamp data>",
        "source": "embedded",
        "status": "unknown",
        "version": 1
      },
      {
        "serialized_sct": "<Base64 Signed Certificate Timestamp data>",
        "source": "embedded",
        "status": "unknown",
        "version": 1
      }
    ],
    "hostname": "expect-ct-report.test",
    "date-time": "2020-05-15T23:16:25.889Z",
    "served-certificate-chain": [
      "-----BEGIN CERTIFICATE-----\n<PEM certificate data>\n-----END CERTIFICATE-----\n",
      "-----BEGIN CERTIFICATE-----\n<PEM certificate data>\n-----END CERTIFICATE-----\n"
    ],
    "effective-expiration-date": "2020-05-15T23:16:25.889Z",
    "validated-certificate-chain": [
      "-----BEGIN CERTIFICATE-----\n<PEM certificate data>\n-----END CERTIFICATE-----\n",
      "-----BEGIN CERTIFICATE-----\n<PEM certificate data>\n-----END CERTIFICATE-----\n",
      "-----BEGIN CERTIFICATE-----\n<PEM certificate data>\n-----END CERTIFICATE-----\n"
    ]
  }
}

Expect-CT reports

The Expect-CT response header:

Test Expect-CT reporting

Example Expect-CT report

Related specs & documents

The `Expect-CT` response header: