ZLint is a X.509 certificate linter written in Go that checks for consistency with RFC 5280 and the CA/Browser Forum Baseline Requirements (v.1.4.8).
A detailed list of BR coverage can be found here: https://docs.google.com/spreadsheets/d/1ywp0op9mkTaggigpdF2YMTubepowJ50KQBhc_b00e-Y.
ZLint requires Go 1.12.x or newer be
installed. The command line setup instructions assume the go command is in
your $PATH.
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.
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 github.com/zmap/zlint/cmd/zlint
zlint mycert.pem
ZLint can also be used as a library:
import (
"github.com/zmap/zcrypto/x509"
"github.com/zmap/zlint"
)
parsed, err := x509.ParseCertificate(raw)
if err != nil {
// The certificate could not be parsed. Either error or halt.
log.Fatalf("could not parse certificate: %s", err)
}
zlintResultSet := zlint.LintCertificate(parsed)See https://github.com/zmap/zlint/blob/master/cmd/zlint/main.go for an example.
Generating Lint Scaffolding. The scaffolding for a new lints can be created
by running ./newLint.sh <lint_name> <structName>. Lint names are generally of
the form e_subject_common_name_not_from_san where the first letter is one of:
e, w, or n (error, warning, or notice respectively). Struct names
following Go conventions, e.g., subjectCommonNameNotFromSAN. Example:
./newLint.sh e_subject_common_name_not_from_san subjectCommonNameNotFromSAN.
This will generate a new lint in the lints directory with the necessary
fields filled out.
Choosing a Lint Result Level. When choosing what lints.LintStatus your new
lint should return (e.g. Notice,Warn, Error, or Fatal) the following
general guidance may help. Error should be used for clear violations of RFC/BR
MUST or MUST NOT requirements and include strong citations. Warn should be
used for violations of RFC/BR SHOULD or SHOULD NOT requirements and again
should include strong citations. Notice should be used for more general "FYI"
statements that violate non-codified community standards or for cases where
citations are unclear. Lastly Fatal should be used when there is an
unresolvable error in zlint, zcrypto or some other part of the certificate
processing.
Scoping a Lint. Lints are executed in three steps. First, the ZLint
framework determines whether a certificate falls within the scope of a given
lint by calling CheckApplies. This is often used to scope lints to only check
subscriber, intermediate CA, or root CAs. This function commonly calls one of a
select number of helper functions: IsCA, IsSubscriber, IsExtInCert, or
DNSNamesExist. Example:
func (l *caCRLSignNotSet) CheckApplies(c *x509.Certificate) bool {
return c.IsCA && util.IsExtInCert(c, util.KeyUsageOID)
}Next, the framework determines whether the certificate was issued after the
effective date of a Lint by checking whether the certificate was issued prior
to the lint's EffectiveDate. You'll also need to fill out the source and
description of what the lint is checking. We encourage you to copy text
directly from the BR or RFC here. Example:
func init() {
RegisterLint(&Lint{
Name: "e_ca_country_name_missing",
Description: "Root and Subordinate CA certificates MUST have a countryName present in subject information",
Citation: "BRs: 7.1.2.1",
EffectiveDate: util.CABEffectiveDate,
Test: &caCountryNameMissing{},
})
}The meat of the lint is contained within the RunTest function, which is
passed x509.Certificate. Note: This is an X.509 object from
ZCrypto not the Go standard library. Lints
should perform their described test and then return a ResultStruct that
contains a Result and optionally a Details string, e.g.,
ResultStruct{Result: Pass}. If you encounter a situation in which you
typically would return a Go error object, instead return
ResultStruct{Result: Fatal}.
Example:
func (l *caCRLSignNotSet) RunTest(c *x509.Certificate) *ResultStruct {
if c.KeyUsage&x509.KeyUsageCRLSign != 0 {
return &ResultStruct{Result: Pass}
}
return &ResultStruct{Result: Error}
}Creating Tests. Every lint should also have two corresponding tests for a
success and failure condition. We have typically generated test certificates
using Go (see https://golang.org/pkg/crypto/x509/#CreateCertificate for
details), but OpenSSL could also be used. Test certificates should be placed in
testlint/testCerts and called from the test file created by newLint.sh.
Prepend the PEM with the output of openssl x509 -text.
Example:
func TestBasicConstNotCritical(t *testing.T) {
// Only need to change these two values and the lint name
inputPath := "../testlint/testCerts/caBasicConstNotCrit.pem"
expected := Error
out, _ := Lints["e_basic_constraints_not_critical"].ExecuteTest(ReadCertificate(inputPath))
if out.Result != expected {
t.Errorf("%s: expected %s, got %s", inputPath, expected, out.Status)
}
}ZLint maintains a map of
top-level-domains
and their validity periods that is referenced by linters. As ICANN adds and
removes TLDs this map need to be updated. To do so, ensure the
zlint-gtld-update command is installed and in your $PATH and run go generate:
go get github.com/zmap/zlint/cmd/zlint-gtld-update
go generate github.com/zmap/zlint/...
Pre-issuance linting is strongly recommended by the Mozilla root program. Here are some projects/CAs known to integrate with ZLint in some fashion:
- CFSSL
- Sectigo and crt.sh
- Digicert
- EJBCA
- Government of Spain, FNMT
- Globalsign
- GoDaddy
- Izenpe
- Let's Encrypt and Boulder
- Siemens
- QuoVadis
Please submit a pull request to update the README if you are aware of another CA/project that uses zlint.
ZMap Copyright 2019 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 http://www.apache.org/licenses/LICENSE-2.0
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.