Sur les sites web qui sont censés être sécurisés (c-a-d quand l'adresse commence par "https://"), Firefox doit vérifier que le certificat de sécurité présenté par le site web est valide. Si le certificat ne peut être validé, Firefox suspendra la connexion au site web et affichera ce message d'erreur : "La connexion n'est pas sécuriséeLa connexion n'est pas sécurisée" Cet article explique pourquoi le message d'erreur "SEC_ERROR_UNKNOWN_ISSUER" s'affiche sur certains sites web et comment résoudre le problème.

Que signifie ce message d'erreur ?

Au cours d'une connexion sécurisée, un site web doit présenter un certificat fourni par une autorité de certification authentifiée pour prouver que l'utilisateur est bien connecté au lien désiré et que la connexion est chiffrée. Si la page d'erreur "Your connection is not secureThis Connection is Untrusted" apparaît et que vous voyez le message "SEC_ERROR_UNKNOWN_ISSUER" après avoir cliqué sur AdvancedTechnical Details, cela signifie que le certificat a été fourni par une autorité de certification inconnue et que Firefox ne peut pas l'authentifier.

La même erreur se produit sur plusieurs sites sécurisés

Si vous rencontrez ce problème sur plusieurs sites HTTPS sans lien entre eux, cela signifie que votre connexion est interceptée et reçoit des certificats refusés par Firefox. L'interception vient souvent de logiciels analysant les connexions chiffrées ou de logiciels malveillants qui remplacent les certificats des sites légitimes par les leurs.

Antivirus Products

Generally, if your security product contains a feature to scan encrypted connections, you could try to reinstall the security product, which might trigger the software to place its certificates into the Firefox trust store again. Try the following solutions for particular security products:

Avast

In Avast security products you can disable the interception of secure connections:

1. Open the dashboard of your Avast application.
2. Go to Settings > Active Protection and click Customize next to Web Shield.
3. Uncheck the Enable HTTPS Scanning setting and confirm this by clicking OK.

More Information about this feature is available on this Avast Blog.

Bitdefender

In Bitdefender security products you can disable the interception of secure connections:

1. Open the dashboard of your Bitdefender application.
2. For the 2016 version of the Bitdefender security product click on Modules. For the 2015 version click on Protection.
3. Click on Web Protection.
4. Toggle off the Scan SSL setting.

For corporate Bitdefender products, please refer to this Bitdefender Support Center page.

ESET

In ESET security products you can try to disable and re-enable SSL/TLS protocol filtering or generally disable the interception of secure connections as described in ESET’s support article.

Kaspersky

In Kaspersky security products you can disable the interception of secure connections:

1. Open the dashboard of your Kaspersky application.
2. Click on Settings on the bottom-left.
3. Click Additional and then Network.
4. For the 2016 version of the Kaspersky security product check the Do not scan encrypted connections option and confirm this change. For the 2015 version uncheck the Scan encrypted connections option.

Family Safety settings in Windows accounts

In Microsoft Windows accounts protected by Family Safety settings, secure connections on popular websites like Google, Facebook and YouTube might be intercepted and their certificates replaced by a certificate issued by Microsoft in order to filter and record search activity.

Read this Microsoft FAQ page on how to turn off these family features for accounts. In case you want to manually install the missing certificates for affected accounts, you can refer to this Microsoft support article.

Monitoring/Filtering in corporate networks

Some traffic monitoring/filtering products used in corporate environments might intercept encrypted connections by replacing a website's certificate with their own, at the same time possibly triggering errors on secure HTTPS-sites. If you suspect this might be the case, please contact your IT department to ensure the correct configuration of Firefox to enable it working properly in such an environment.

Malware

Some forms of malware intercepting encrypted web traffic can cause this error message - refer to the article Résoudre des problèmes de Firefox causés par des logiciels malveillants on how to deal with malware problems.

The error occurs on one particular site only

In case you get this problem on one particular site only, this type of error indicates that the web server is not configured properly: The website's certificate might not have been issued by a trusted certificate authority itself and no complete certificate chain to a trusted authority was provided either (a so-called "intermediate certificate" is missing). You should contact the owner of the website and inform him of the error.

If the website allows it, you can add an exception in order to visit the site, in spite its certificate is not being trusted by default:

1. On the warning page, click AdvancedI understand the Risks.
2. Click Add Exception…. The Add Security Exception dialog will appear.
3. Read the text describing the problems with the website. You can click View… in order to closer inspect the untrusted certificate as well.
4. Click Confirm Security Exception if you are sure you want to trust the site.