Browsers have plans to remove trust of all legacy Symantec SSL/TLS certificates issued under the Symantec infrastructure. Websites secured with these legacy Symantec SSL/TLS certificates trigger security warnings in browsers and application relying on its root store.:
More detail from Google. Mozilla, and Apple:
Google security blog
Mozilla security blog
Apple information about distrusting Symantec certificate authorities
Symantec TLS/SSL certificates affected by browser distrust are based on their issuance date. Refer to this table and check your certificate inventory for certificates at risk of potential distrust.
Also distrusted - Certificates issued from the legacy Symantec root hierarchy on or after December 1, 2017
For uninterrupted business continuity, some Managed PKI for SSL customers continue to issue certificates from the legacy Symantec root hierarchy after the December 1, 2017 switch to the DigiCert hierarchy. These certificates are already distrusted by Google Chrome and will be distrusted by Apple on July 20, 2018. Other browser vendors have not yet announced distrust plans.
To identify impacted certificates: