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X.509 Certificate Linter focused on Web PKI standards and requirements.
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ZLint is a X.509 certificate linter written in Go that checks for consistency with standards (e.g. RFC 5280) and other relevant PKI requirements (e.g. CA/Browser Forum Baseline Requirements).

It can be used as a command line tool or as a library integrated into CA software.


ZLint requires Go 1.13.x or newer be installed. The command line setup instructions assume the go command is in your $PATH.

Lint Sources

Historically ZLint was focused on only RFC 5280 and v1.4.8 of the CA/Browser Forum baseline requirements. A detailed list of the original BR coverage can be found in this spreadsheet.

More recently ZLint has been restructured to make it easier to add lints based on other sources. While not complete, presently ZLint has lints sourced from:

By default ZLint will apply applicable lints from all sources but consumers may also customize which lints are used by including/exclduing specific sources.

Versioning and Releases

ZLint aims to follow semantic versioning. The addition of new lints will generally result in a MINOR version revision. Since downstream projects depend on lint results and names for policy decisions changes of this nature will result in MAJOR version revision.

Where possible we will try to make available a release candidate (RC) a week before finalizing a production ready release tag. We encourage users to test RC releases to provide feedback early enough for bugs to be addressed before the final release is made available.

Please subscribe to the ZLint Announcements mailing list to receive notifications of new releases/release candidates. This low-volumne announcements mailing list is only used for new ZLint releases and major project announcements, not questions/support/bug reports.

Command Line Usage

ZLint can be used on the command-line through a simple bundled executable ZLint as well as through ZCertificate, a more full-fledged command-line certificate parser that links against ZLint.

Example ZLint CLI usage:

go get
echo "Lint mycert.pem with all applicable lints"
zlint mycert.pem

echo "Lint mycert.pem with just the two named lints"
zlint -includeNames=e_mp_exponent_cannot_be_one,e_mp_modulus_must_be_divisible_by_8 mycert.pem

echo "List available lint sources"
zlint -list-lints-source

echo "Lint mycert.pem with all of the lints except for ETSI ESI sourced lints"
zlint -excludeSources=ETSI_ESI mycert.pem

See zlint -h for all available command line options.

Library Usage

ZLint can also be used as a library. To lint a certificate with all applicable lints is as simple as using zlint.LintCertificate with a parsed certificate:

import (

var certDER []byte = ...
parsed, _ := x509.ParseCertificate(certDER)
zlintResultSet := zlint.LintCertificate(parsed)

To lint a certificate with a subset of lints (e.g. based on lint source, or name) filter the global lint registry and use it with zlint.LintCertificateEx:

import (

var certDER []byte = ...
parsed, _ := x509.ParseCertificate(certDER)

registry, _ := lint.GlobalRegistry().Filter(lint.FilterOptions{
  ExcludeSources: []lint.LintSource{lint.EtsiEsi},
zlintResultSet := zlint.LintCertificateEx(parsed, registry)

See the zlint command's source code for an example.

Extending ZLint

For information on extending ZLint with new lints see

Zlint Users/Integrations

Pre-issuance linting is strongly recommended by the Mozilla root program. Here are some projects/CAs known to integrate with ZLint in some fashion:

Please submit a pull request to update the README if you are aware of another CA/project that uses zlint.

License and Copyright

ZMap Copyright 2020 Regents of the University of Michigan

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See LICENSE for the specific language governing permissions and limitations under the License.

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