Wget fails with certificate error

As part of an automated build, we run download some code from github. Minimal example:

wget github.com

Recently, the command started failing with a certificate error:

URL transformed to HTTPS due to an HSTS policy
--2017-10-05 11:43:45--  https://github.com/
Resolving github.com (github.com)... 192.30.253.112, 192.30.253.113
Connecting to github.com (github.com)|192.30.253.112|:443... connected.

ERROR: cannot verify github.com's certificate, issued by 'CN=DigiCert SHA2 Extended Validation Server CA,OU=www.digicert.com,O=DigiCert Inc,C=US':
  Unable to locally verify the issuer's authority.

I tried updating the certificate store, and wget itself:

update-ca-certificates
apt-get install wget

The error is still the same. My wget version is GNU Wget 1.17.1, and the OS is Ubuntu 16.04.3.

Solution

The answer turned out to lie somewhere in packet configuration. Unfortunately, I am unable to tell exactly why. The suspicion is some mono version installed from a ppa was messing with our cert store.