alpine receives self-signed certificate from Gmail using TLS 1.3
Starting with Fedora 29, alpine receives an (intentionally) bogus certificate from imap.gmail.com . The subject of this certificate is:
"No SNI provided; please fix your client."
Judging from other bug reports (, ), alpine is likely suffering the same fate as certain other mail clients that (a) speak TLS 1.3 and (b) do so without sending a hostname in a TLS Server Name Indication (SNI) extension.
Starting about August 2018, it appears that Google has chosen to send this special-purpose "fix your stuff" certificate to clients like alpine, ones that are new enough to have their transport layer protected with TLS 1.3, but that aren't sending a target hostname in a TLS SNI extension.
Indeed, upon inspection of dumped alpine traffic to imap.google.com , we can confirm that:
- on *both* F28 and F29, alpine does *not* send a TLS SNI extension in its Client Hello
- on F28, alpine's connections with the "/tls" parameter speak TLS 1.2, and Google sends a real certificate
- on F29, the same alpine configuration produces a TLS 1.3 connection, to which Google sends a bogus certificate
REQUEST FOR ENGINEERING: Please patch the alpine mail client to send a TLS SNI extension on encrypted IMAP connections.
For implementation, it seems reasonable to blindly copy the target server's hostname into the SNI field (although it wouldn't hurt to be able to specify the value for this field too).
Justification for this RFE can be found in the below-listed bugs, which needed the same work when Google implemented their policy.
Besides, enabling TLS SNI is preferable over less-secure options like asking users to stick with an older version of TLS or enabling the "/novalidate-cert" parameter.
This commit appears to be a fix, though I haven't tested it yet:
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This bug is long closed, but just noticed that this is resolved in Fedora 32 in alpine-2.23-2.fc32.x86_64 .