Mozilla has warned Firefox users they may be cut off from more of the web than expected – now that the browser rejects new HTTPS certificates that use the weak SHA-1 algorithm.
If you use Firefox with some antivirus products, or on a network fitted with a box that inspects traffic for malicious stuff, and visit a site that uses an old crummy SHA-1-signed SSL cert, the browser will refuse to access that website.
Firefox rejects SHA-1-signed certificates issued since the end of 2015 because the hashing algorithm is problematic: an eavesdropper could tamper with the cert to spy on you, and you'd never know, for example.
To be clear: Firefox is only supposed to snub new SHA-1 certificates, but it may end up rejecting older SHA-1 certs, too. All new certs are expected to use SHA-256 or better.
"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," explained security engineer Richard Barnes.
"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."
If this is a problem, don't panic: you can cut'n'paste
about:config into your URL bar, hit enter, and change the value of “security.pki.sha1_enforcement_level” to 0 to make SHA-1 acceptable again. Bear in mind, though, that you're trading one security problem (the inability to filter malicious traffic) against another (the inability to securely verify the integrity of the HTTPS connection).
If your security device is causing a problem, Barnes suggests updating the software to the latest iteration, since many vendors are also abandoning SHA-1. Microsoft, Google, and Facebook all moving away from SHA-1, and other tech outfits are rapidly following suit.
It has long been known that SHA-1 hashes are theoretically open to attack; in October this was proved in dramatic style with just $75,000 of cloud compute resources. Now it's about as popular as a rattlesnake in a piñata. ®