sslutil: print a warning when using TLS 1.0 on legacy Python Mercurial now requires TLS 1.1+ when TLS 1.1+ is supported by the client. Since we made the decision to require TLS 1.1+ when running with modern Python versions, it makes sense to do something for legacy Python versions that only support TLS 1.0. Feature parity would be to prevent TLS 1.0 connections out of the box and require a config option to enable them. However, this is extremely user hostile since Mercurial wouldn't talk to https:// by default in these installations! I can easily see how someone would do something foolish like use "--insecure" instead - and that would be worse than allowing TLS 1.0! This patch takes the compromise position of printing a warning when performing TLS 1.0 connections when running on old Python versions. While this warning is no more annoying than the CA certificate / fingerprint warnings in Mercurial 3.8, we provide a config option to disable the warning because to many people upgrading Python to make the warning go away is not an available recourse (unlike pinning fingerprints is for the CA warning). The warning appears as optional output in a lot of tests.

sslutil: require TLS 1.1+ when supported Currently, Mercurial will use TLS 1.0 or newer when connecting to remote servers, selecting the highest TLS version supported by both peers. On older Pythons, only TLS 1.0 is available. On newer Pythons, TLS 1.1 and 1.2 should be available. Security professionals recommend avoiding TLS 1.0 if possible. PCI DSS 3.1 "strongly encourages" the use of TLS 1.2. Known attacks like BEAST and POODLE exist against TLS 1.0 (although mitigations are available and properly configured servers aren't vulnerable). I asked Eric Rescorla - Mozilla's resident crypto expert - whether Mercurial should drop support for TLS 1.0. His response was "if you can get away with it." Essentially, a number of servers on the Internet don't support TLS 1.1+. This is why web browsers continue to support TLS 1.0 despite desires from security experts. This patch changes Mercurial's default behavior on modern Python versions to require TLS 1.1+, thus avoiding known security issues with TLS 1.0 and making Mercurial more secure by default. Rather than drop TLS 1.0 support wholesale, we still allow TLS 1.0 to be used if configured. This is a compromise solution - ideally we'd disallow TLS 1.0. However, since we're not sure how many Mercurial servers don't support TLS 1.1+ and we're not sure how much user inconvenience this change will bring, I think it is prudent to ship an escape hatch that still allows usage of TLS 1.0. In the default case our users get better security. In the worst case, they are no worse off than before this patch. This patch has no effect when running on Python versions that don't support TLS 1.1+. As the added test shows, connecting to a server that doesn't support TLS 1.1+ will display a warning message with a link to our wiki, where we can guide people to configure their client to allow less secure connections.

sslutil: config option to specify TLS protocol version Currently, Mercurial will use TLS 1.0 or newer when connecting to remote servers, selecting the highest TLS version supported by both peers. On older Pythons, only TLS 1.0 is available. On newer Pythons, TLS 1.1 and 1.2 should be available. Security-minded people may want to not take any risks running TLS 1.0 (or even TLS 1.1). This patch gives those people a config option to explicitly control which TLS versions Mercurial should use. By providing this option, one can require newer TLS versions before they are formally deprecated by Mercurial/Python/OpenSSL/etc and lower their security exposure. This option also provides an easy mechanism to change protocol policies in Mercurial. If there is a 0-day and TLS 1.0 is completely broken, we can act quickly without changing much code. Because setting the minimum TLS protocol is something you'll likely want to do globally, this patch introduces a global config option under [hostsecurity] for that purpose. wrapserversocket() has been taught a hidden config option to define the explicit protocol to use. This is queried in this function and not passed as an argument because I don't want to expose this dangerous option as part of the Python API. There is a risk someone could footgun themselves. But the config option is a devel option, has a warning comment, and I doubt most people are using `hg serve` to run a production HTTPS server (I would have something not Mercurial/Python handle TLS). If this is problematic, we can go back to using a custom extension in tests to coerce the server into bad behavior.

sslutil: prevent CRIME ssl.create_default_context() disables compression on the TLS channel in order to prevent CRIME. I think we should follow CPython's lead and attempt to disable channel compression in order to help prevent information leakage. Sadly, I don't think there is anything we can do on Python versions that don't have an SSLContext, as there is no way to set channel options with the limited ssl API.

sslutil: update comment about create_default_context() While ssl.create_default_context() creates a SSLContext with reasonable default options, we can't use it because it conflicts with our CA loading controls. So replace the comment with reality. (FWIW the comment was written before the existing CA loading code was in place.)

tests: use sslutil.wrapserversocket() Like the built-in HTTPS server, this code was using the ssl module directly and only using TLS 1.0. Like the built-in HTTPS server, we switch it to use sslutil.wrapserversocket() so it can follow better practices.

hgweb: use sslutil.wrapserversocket() This patch transitions the built-in HTTPS server to use sslutil for creating the server socket. As part of this transition, we implement developer-only config options to control CA loading and whether to require client certificates. This eliminates the need for the custom extension in test-https.t to define these. There is a slight change in behavior with regards to protocol selection. Before, we would always use the TLS 1.0 constant to define the protocol version. This would *only* use TLS 1.0. sslutil defaults to TLS 1.0+. So this patch improves the security of `hg serve` out of the box by allowing it to use TLS 1.1 and 1.2 (if available).