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certspotter/man/certspotter-script.md
Andrew Ayer 3f353ecb60 Provide guidance about determining legitimacy of a cert 
Closes: #76
2026-01-07 22:20:09 -05:00

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NAME

certspotter-script - Certificate Transparency Log Monitor (hook script)

DESCRIPTION

certspotter-script is any program that is executed by certspotter(8) when it needs to notify you about an event, such as detecting a certificate for a domain on your watch list.

Scripts are placed in the $CERTSPOTTER_CONFIG_DIR/hooks.d directory (~/.certspotter/hooks.d by default), or specified on the command line using the -script argument.

ENVIRONMENT

Event information

The following environment variables are set for all types of events:

EVENT

One of the following values, indicating the type of event:

  • discovered_cert - certspotter has discovered a certificate for a domain on your watch list.

  • malformed_cert - certspotter can't determine if a certificate matches your watch list because the certificate or the log entry is malformed.

  • error - a problem is preventing certspotter from monitoring all logs.

Additional event types may be defined in the future, so your script should be able to handle unknown values.

SUMMARY

A short human-readable string describing the event. This is the same string used in the subject line of emails sent by certspotter.

Discovered certificate information

The following environment variables are set for discovered_cert events:

WATCH_ITEM

The item from your watch list which matches this certificate. (If more than one item matches, the first one is used.)

LOG_URI

The URI of the log containing the certificate.

ENTRY_INDEX

The index of the log entry containing the certificate.

TBS_SHA256

The hex-encoded SHA-256 digest of the TBSCertificate, as defined in RFC 6962 Section 3.2. Certificates and their corresponding precertificates have the same TBS_SHA256 value. You can compute the TBS digest of a certificate using the -printhash option of the certspotter-authorize(8) command.

CERT_SHA256

The hex-encoded SHA-256 digest (sometimes called fingerprint) of the certificate or precertificate. The digest is computed over the ASN.1 DER encoding.

PUBKEY_SHA256

The hex-encoded SHA-256 digest of the certificate's Subject Public Key Info.

CERT_FILENAME

Path to a file containing the PEM-encoded certificate chain. When -no_save is used, this points to a temporary file that is automatically deleted after your script completes.

JSON_FILENAME

Path to a JSON file containing additional information about the certificate. See below for the format of the JSON file. When -no_save is used, this points to a temporary file that is automatically deleted after your script completes.

TEXT_FILENAME

Path to a text file containing information about the certificate. This file contains the same text that certspotter uses in emails. You should not attempt to parse this file because its format may change in the future. When -no_save is used, this points to a temporary file that is automatically deleted after your script completes.

NOT_BEFORE, NOT_BEFORE_UNIXTIME, NOT_BEFORE_RFC3339

The not before time of the certificate, in a human-readable format, seconds since the UNIX epoch, and RFC3339, respectively. These variables may be unset if there was a parse error, in which case VALIDITY_PARSE_ERROR is set.

NOT_AFTER, NOT_AFTER_UNIXTIME, NOT_AFTER_RFC3339

The not after (expiration) time of the certificate, in a human-readable format, seconds since the UNIX epoch, and RFC3339, respectively. These variables may be unset if there was a parse error, in which case VALIDITY_PARSE_ERROR is set.

VALIDITY_PARSE_ERROR

Error parsing not before and not after, if any. If this variable is set, then the NOT_BEFORE and NOT_AFTER family of variables are unset.

SUBJECT_DN

The distinguished name of the certificate's subject. This variable may be unset if there was a parse error, in which case SUBJECT_PARSE_ERROR is set.

SUBJECT_PARSE_ERROR

Error parsing the subject, if any. If this variable is set, then SUBJECT_DN is unset.

ISSUER_DN

The distinguished name of the certificate's issuer. This variable may be unset if there was a parse error, in which case ISSUER_PARSE_ERROR is set. Be aware that this value is controlled by the certificate authority and often contains misleading, incorrect, or unhelpful information.

ISSUER_PARSE_ERROR

Error parsing the issuer, if any. If this variable is set, then ISSUER_DN is unset.

SERIAL

The hex-encoded serial number of the certificate. Prefixed with a minus (-) sign if negative. This variable may be unset if there was a parse error, in which case SERIAL_PARSE_ERROR is set. Be aware that malicious certificate authorities can issue certificates with duplicate serial numbers, so it is not safe to use the serial number to make security decisions (such as to determine if a certificate is legitimate or not).

SERIAL_PARSE_ERROR

Error parsing the serial number, if any. If this variable is set, then SERIAL is unset.

CHAIN_ERROR

Error building or verifying the certificate chain, if any. If this variable is set, then the certificate chain in CERT_FILENAME may be incomplete or invalid.

Malformed certificate information

The following environment variables are set for malformed_cert events:

LOG_URI

The URI of the log containing the malformed certificate.

ENTRY_INDEX

The index of the log entry containing the malformed certificate.

LEAF_HASH

The base64-encoded Merkle hash of the leaf containing the malformed certificate.

PARSE_ERROR

A human-readable string describing why the certificate is malformed.

ENTRY_FILENAME

Path to a file containing the JSON log entry. The file contains a JSON object with two fields, leaf_input and extra_data, as described in RFC 6962 Section 4.6.

TEXT_FILENAME

Path to a text file containing a description of the malformed certificate. This file contains the same text that certspotter uses in emails.

Error information

The following environment variables are set for error events:

TEXT_FILENAME

Path to a text file containing a description of the error. This file contains the same text that certspotter uses in emails.

JSON FILE FORMAT

certspotter saves a JSON file for every discovered certificate and puts the path to the file in $JSON_FILENAME. When -no_save is used, the JSON file is created in a temporary location and automatically deleted after your script completes. Otherwise, it is saved in the state directory for permanent storage. Your script can read the JSON file, such as with the jq(1) command, to get additional information about the certificate which isn't appropriate for environment variables.

The JSON file contains an object with the following fields:

tbs_sha256

A string containing the hex-encoded SHA-256 digest of the TBSCertificate, as defined in RFC 6962 Section 3.2. Certificates and their corresponding precertificates have the same tbs_sha256 value. You can compute the TBS digest of a certificate using the -printhash option of the certspotter-authorize(8) command.

pubkey_sha256

A string containing the hex-encoded SHA-256 digest of the certificate's Subject Public Key Info.

dns_names

An array of strings containing the DNS names for which the certificate is valid, taken from both the DNS subject alternative names (SANs) and the subject common name (CN). Internationalized domain names are encoded in Punycode.

ip_addresses

An array of strings containing the IP addresses for which the certificate is valid, taken from both the IP subject alternative names (SANs) and the subject common name (CN).

not_before

A string containing the not before time of the certificate in RFC3339 format. Null if there was an error parsing the certificate's validity.

not_after

A string containing the not after (expiration) time of the certificate in RFC3339 format. Null if there was an error parsing the certificate's validity.

Additional fields will be added in the future based on user feedback. Please open an issue at https://github.com/SSLMate/certspotter if you have a use case for another field.

DETERMINING IF A CERTIFICATE IS AUTHORIZED

You may want to compare a discovered certificate against a database of legitimate certificates to decide whether to raise an alarm about it. Beware the following caveats:

  • Since certificates may be logged to CT in the form of a precertificate, it doesn't work to compare the discovered certificate's fingerprint against a list of legitimate fingerprints.

  • Since malicious certificate authorities can issue certificates with duplicate serial numbers, it is not secure to compare the the discovered certificate's serial number against a list of legitimate serial numbers.

  • Since malicious certificate authorities can put any value in a certificate's Subject Key Identifier (SKI) extension, it is not secure to compare a discovered certificate's Subject Key Identifier against a list of legitimate Subject Key Identifiers.

  • Since malicious certificate authorities can put any value in the certificate's Issuer field, it is not secure to compare a discovered certificate's Issuer against a list of authorized Issuers.

To determine if a certificate is legitimate, you should do one of the following:

  • Compare the discovered certificate's TBS hash (as provided in the $TBS_SHA256 variable) against a list of authorized TBS hashes. The TBS hash is computed over the DER encoding of the certificate's TBSCertificate, with any SCT extension removed. You can use the -printhash option of the certspotter-authorize(8) command to compute a certificate's TBS hash.

  • Compare the discovered certificate's public key hash (as provided in the $PUBKEY_SHA256 variable) against a list of authorized public key hashes. The public key hash is computed over the DER encoding of the certificate's Subject Public Key Info. You can use the following openssl command to compute a certificate's public key hash: openssl x509 -pubkey -noout | openssl pkey -pubin -outform DER | openssl sha256

EXAMPLES

Example environment variables for a discovered_cert event:

CERT_FILENAME=/home/andrew/.certspotter/certs/3c/3cdc83b3932c194fcdf17aa2bf1abc34e8438b293c3d5c70693e175b38ff128a.pem
CERT_SHA256=3cdc83b3932c194fcdf17aa2bf1abc34e8438b293c3d5c70693e175b38ff128a
ENTRY_INDEX=6464843
EVENT=discovered_cert
ISSUER_DN=C=GB, ST=Greater Manchester, L=Salford, O=Sectigo Limited, CN=Sectigo RSA Domain Validation Secure Server CA
JSON_FILENAME=/usr2/andrew/.certspotter/certs/3c/3cdc83b3932c194fcdf17aa2bf1abc34e8438b293c3d5c70693e175b38ff128a.v1.json
LOG_URI=https://ct.cloudflare.com/logs/nimbus2024/
NOT_AFTER='2024-01-26 03:47:26 +0000 UTC'
NOT_AFTER_RFC3339=2024-01-26T03:47:26Z
NOT_AFTER_UNIXTIME=1706240846
NOT_BEFORE='2023-01-31 03:47:26 +0000 UTC'
NOT_BEFORE_RFC3339=2023-01-31T03:47:26Z
NOT_BEFORE_UNIXTIME=1675136846
PUBKEY_SHA256=33ac1d9b9e56005ccac045eac2398b3e9dd6b3f5b66ae6260f2d478c7c0d82c8
SERIAL=c170fbf3bf27481e5c351a4db6f2dc5f
SUBJECT_DN=CN=sslmate.com
SUMMARY='certificate discovered for .sslmate.com'
TBS_SHA256=2388ee81c6f45cffc73e68a35fa8921e839e20acc9a98e8e6dcaea07cbfbdef8
TEXT_FILENAME=/usr2/andrew/.certspotter/certs/3c/3cdc83b3932c194fcdf17aa2bf1abc34e8438b293c3d5c70693e175b38ff128a.txt
WATCH_ITEM=.sslmate.com

Example JSON file for a discovered certificate:

{
  "dns_names": [
    "sslmate.com",
    "www.sslmate.com"
  ],
  "ip_addresses": [],
  "not_after": "2024-01-26T03:47:26Z",
  "not_before": "2023-01-31T03:47:26Z",
  "pubkey_sha256": "33ac1d9b9e56005ccac045eac2398b3e9dd6b3f5b66ae6260f2d478c7c0d82c8",
  "tbs_sha256": "2388ee81c6f45cffc73e68a35fa8921e839e20acc9a98e8e6dcaea07cbfbdef8"
}

SEE ALSO

certspotter(8)

COPYRIGHT

Copyright (c) 2016-2025 Opsmate, Inc.

BUGS

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