According to the plan we published earlier for deprecating SHA-1, on January 1, 2016, Firefox 43 began rejecting new certificates signed with the SHA-1 digest algorithm. For Firefox users with unfiltered access to the Internet, this change probably went unnoticed, since there simply aren’t that many new SHA-1 certs being used. However, for Firefox users who are behind certain “man-in-the-middle” devices (including some security scanners and antivirus products), this change removed their ability to access HTTPS web sites. When a user tries to connect to an HTTPS site, the man-in-the-middle device sends Firefox a new SHA-1 certificate instead of the server’s real certificate. Since Firefox rejects new SHA-1 certificates, it can’t connect to the server.
How to tell if you’re affected
If you can access this article in Firefox, you’re fine. If you’re reading this in another browser, see if you can load the security blog (or any other HTTPS link) in Firefox. Click “Advanced”, and if you see the error code “SEC_ERROR_CERT_SIGNATURE_ALGORITHM_DISABLED”, then you’re affected.
What to do if you’re affected
The easiest thing to do is to install the newest version of Firefox. You will need to do this manually, using an unaffected copy of Firefox or a different browser, since we only provide Firefox updates over HTTPS.
If you want to avoid reinstalling, advanced users can fix their local copy of Firefox by going to about:config and changing the value of “security.pki.sha1_enforcement_level” to 0 (which will accept all SHA-1 certificates).
You should also make sure that any systems you have that might be doing man-in-the-middle are up to date, for example, some anti-virus software or security scanning devices. Some vendors have removed the use of SHA-1 in recent updates.
Commitment to deprecate SHA-1
We are still committed to removing support for SHA-1 certificates from Firefox. The latest version of Firefox re-enables support for SHA-1 certificates to ensure that we can get updates to users behind man-in-the-middle devices, and enable us to better evaluate how many users might be affected. Vendors of TLS man-in-the-middle systems should be working to update their products to use newer digest algorithms.