Exploring Unable to get Local Issuer Certificate

Updated: Aug 25

"Unable to get Local Issuer Certificate” is a common SSL certificate error. It is related to the incomplete certificate chain such as (most commonly) missing the intermediate certificate. The fix is to ensure the entire certificate chain is present.


We will dive into this issue to see why this happens and how to fix it.





Understanding certificate chain

A certificate chain is an ordered list of certificates, containing an SSL/TLS server certificate, intermediate certificate, and Certificate Authority (CA) Certificates, that enable the receiver to verify that the sender and all CA's are trustworthy.


  • Root Certificate. A root certificate is a digital certificate that belongs to the issuing Certificate Authority. It comes pre-downloaded in most browsers and is stored in what is called a “trust store.” The root certificates are closely guarded by CAs.

  • Intermediate Certificate. Intermediate certificates branch off root certificates like branches of trees. They act as middle-men between the protected root certificates and the server certificates issued out to the public. There will always be at least one intermediate certificate in a chain, but there can be more than one.

  • Server Certificate. The server certificate is the one issued to the specific domain the user is needing coverage for.



We will use these files in this example.

  • CA certificate file (usually called ca.pem or cacerts.pem)

  • Intermediate certificate file (if exists, can be more than one. If you don’t know if you need an intermediate certificate, run through the steps and find out)

  • Server certificate file



How do Certificate Chains work?

When we install our TLS certificate, we also be sent an intermediate root certificate or bundle.


When a browser downloads our website’s TLS certificate upon arriving at our homepage, it begins chaining that certificate back to its root. It will begin by following the chain to the intermediate that has been installed, from there it continues tracing backwards until it arrives at a trusted root certificate.


If the certificate is valid and can be chained back to a trusted root, it will be trusted. If it can’t be chained back to a trusted root, the browser will issue a warning about the certificate.




View Certificate Chain

Use the openssl utility that can display a certificate chain. The following command:


openssl s_client -connect google.com:443 -servername google.com will display the following certificate chain:



0 s:/C=US/ST=California/L=Mountain View/O=Google Inc/CN=*.google.com

  i:/C=US/O=Google Inc/CN=Google Internet Authority G2

1 s:/C=US/O=Google Inc/CN=Google Internet Authority G2

  i:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA

2 s:/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA

 i:/C=US/O=Equifax/OU=Equifax Secure Certificate Authority


In the openssl output, the numbered lines start with the server certificate (#0) followed by the intermediate (#1) and the root (#2).


The s: indicates the certificate subject, and i: indicates the issuing certificate's subject.



Guidelines to verify the certificate chain is valid

  • Subject of each certificate matches the Issuer of the preceding certificate in the chain (except for the Entity certificate).

  • Subject and Issuer are the same for the root certificate.

If the certificates in the chain adhere to these guidelines, then the certificate chain is considered to be complete and valid.

  • The Subject of the intermediate certificate matches the Issuer of the entity certificate.

  • The Subject of the root certificate matches the Issuer of the intermediate certificate.

  • The Subject and Issuer are the same in the root certificate.



Example of a valid certificate chain

server certificate


openssl x509 -text -in entity.pem | grep -E '(Subject|Issuer):'

Issuer: C = US, O = Google Trust Services, CN = GTS CA 1O1
Subject: C = US, ST = California, L = Mountain View, O = Google LLC, CN = *.enterprise.apigee.com
        

Intermediate certificate


openssl x509 -text -in intermediate.pem  | grep -E '(Subject|Issuer):'

Issuer: OU = GlobalSign Root CA - R2, O = GlobalSign, CN = GlobalSign
Subject: C = US, O = Google Trust Services, CN = GTS CA 1O1
        

Root certificate


openssl x509 -text -in root.pem | grep -E '(Subject|Issuer):'

Issuer: OU = GlobalSign Root CA - R2, O = GlobalSign, CN = GlobalSign
Subject: OU = GlobalSign Root CA - R2, O = GlobalSign, CN = GlobalSign



Validate certificate chain with server and root Certificate


openssl verify cert.pem
cert.pem: C = Country, ST = State, O = Organization, CN = FQDN
error 20 at 0 depth lookup:unable to get local issuer certificate

We can use the following two commands to make sure that the issuer in the server certificate matches the subject in the ca certificate.

openssl x509 -in cert.pem -noout -issuer
issuer= /CN=the name of the CA

$ openssl x509 -noout -subject -in ca.pem
subject= /CN=the name of the CA

In the following case, we need to add the CAfile to verify the root certificate.

$ openssl verify -CAfile ca.pem cert.pem
cert.pem: OK


Validate certificate chain with server, intermediate, and root Certificate


$ openssl verify cert.pem
cert.pem: C = Countrycode, ST = State, O = Organization, CN = yourdomain.com
error 20 at 0 depth lookup:unable to get local issuer certificate

To complete the validation of the chain, we need to provide the CA certificate file and the intermediate certificate file when validating the server certificate file. We can do that using the parameters CAfile (to provide the CA certificate) and untrusted (to provide intermediate certificate):


$ openssl verify -CAfile ca.pem \
                 -untrusted intermediate.cert.pem \
                 cert.pem
cert.pem: OK

If we have multiple intermediate CA certficates, we can use the untrusted parameter multiple times like -untrusted intermediate1.pem -untrusted intermediate2.pem .


Related:

117 views
屏幕快照 2021-08-08 下午5.16.32.png