Updated, September 25th: As indicated to me by Grumpity Linden, and following testing with the Second Life Wolfpack RC viewer update 126.96.36.1999128, this issue should now be resolved.
On Friday, September 22nd, Linden Lab issued a warning to Second Life users on the Windows platform that for a period of time, they may get a Windows SmartScreen warning when installing or updating the official Second Life viewer.
As explained in the blog post, the warning from Windows SmartScreen is displayed when trying to install anything via the Internet and Microsoft is unable to verify whether the software is “safe” for installation.
The reason the warning is being generated against the official Second Life viewer was explained by Oz Linden at the Third-Party Developer meeting, with a similar explanation in the blog post. Speaking at the meeting, Oz said:
Unfortunately, our [Microsoft] code-signing key expired this week, and we had to get a new one, and that has caused Windows SmartScreen Defender to no longer believe in us. So it will warning you when trying to install any of the new viewers, including release candidates … so we’re putting out a blog post about that and putting in on the status page, so hopefully people will get the word.
In addition to this, the blog post explains why the warning will be displayed, even after the Lab has obtained a new code-signing key:
Until enough people install the application signed by the new key, it won’t have a good enough “reputation” with Microsoft to avoid the warning.
Oz further added during the TPVD meeting:
Hopefully, the number of people who need to install it [the viewer] to get Microsoft to believe in it is not too big; [but] we have no idea how long that will take.
So again, if you’re a Windows user (notably Windows 10), and you’ve downloaded one of the more recent viewers from the Lab’s web site, and you get the Windows SmartScreen warning, you can safely install the viewer.
Simply click on the MORE INFO link when the warning is first displayed. This will give you the application name and publisher’s name, together with an option to RUN ANYWAY. Click on this to install the viewer.
Remember, if the viewer has come from the Lab’s website or via the viewer’s own updater, it is safe to install.