Mozilla Firefox Most Common Certificate errors

Some of the most common SSL/TLS certificate errors you may encounter in Firefox include:

  • SEC_ERROR_UNKNOWN_ISSUER – This means Firefox doesn’t trust the certificate authority that issued the website’s SSL certificate. This could happen if the CA is not included in Firefox’s list of trusted CAs.
  • SEC_ERROR_EXPIRED_ISSUER_CERTIFICATE – This indicates that the certificate authority’s certificate used to sign the website’s certificate has expired. This makes the website certificate invalid.
  • SEC_ERROR_UNTRUSTED_ISSUER – Similar to UNKNOWN_ISSUER, this means the issuing CA is not trusted by Firefox for some reason.
  • SEC_ERROR_EXPIRED_CERTIFICATE – The website sent an expired SSL certificate that is no longer valid.
  • SEC_ERROR_REVOKED_CERTIFICATE – The certificate has been revoked by the issuing authority, meaning it is no longer trusted.
  • SEC_ERROR_INCOMPATIBLE_SOFTWARE – Firefox can’t understand the certificate file or information for compatibility reasons.
  • SEC_ERROR_BAD_SIGNATURE – The certificate has an invalid digital signature, meaning it failed integrity checks.

The most common tends to be the UNKNOWN_ISSUER or UNTRUSTED_ISSUER errors when the CA is not in Firefox’s default trust store. Expired certificates also occur frequently on sites that don’t renew their certs on time.

there are a few potential ways to bypass the SEC_ERROR_UNKNOWN_ISSUER error in Firefox:

  1. Install the certificate authority’s root certificate:
  • Get the CA’s root certificate that issued the website’s certificate.
  • Go to Firefox Preferences > Privacy & Security > View Certificates > Authorities.
  • Import the root certificate.
  • This will make Firefox trust that CA for issuing certificates.
  1. Create an exception for the site:
  • Click “Advanced” then “Add Exception…” on the error page.
  • Confirm the security exception to permanently allow the site with that certificate.
  1. Disable certificate validation temporarily:
  • In the Firefox address bar, type about:config and hit Enter.
  • Search for security.tls.version.enable-deprecated and set the value to true.
  • Reload the site – this will disable modern certificate validation.

However bypassing the error reduces security, so it’s best to only do so temporarily or if you understand the risks with that specific certificate. Getting a proper trusted certificate is the most secure long-term solution.

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