CA Certificates

About Mercurial's handling of SSL certificates for https urls.

1. Changes in Mercurial 1.7.x

Mercurial has improved its HTTPS support in the 1.7.x series. When connecting to an HTTPS server, it will now verify the server's certificate correctly and reject the connection if the server identity can't be confirmed - but only if Certification Authorities (CAs) have been configured. As of 1.7.3, Mercurial will warn if CAs haven't been configured.

The new "certificate not verified" warning does not mean that you are less secure than before. It just informs you of how insecure you have always been.

You should fix your setup so you get the security you might expect from SSL and don't get any warnings; otherwise you might just as well stop using HTTPS.

2. Configuration of HTTPS certificate authorities

Most operating systems maintain a set of root certificates that you might decide to trust. Note that any of these authorities can approve any server identity, and any of them will thus be able to spoof any server identity.

Add the lines below to your configuration file (such as /etc/mercurial/hgrc on Unix-like systems).

2.1. Debian/Ubuntu/Gentoo/Arch Linux

On Debian, Ubuntu, Gentoo and Arch Linux you can use this global configuration:

[web]
cacerts = /etc/ssl/certs/ca-certificates.crt

2.2. Fedora/RHEL

On Fedora and RHEL you can use this global configuration:

[web]
cacerts = /etc/pki/tls/certs/ca-bundle.crt

2.3. openSUSE/SLE

Suse should already ship hg with included link to the ssl bundle, else use this global configuration:

[web]
cacerts = /etc/ssl/ca-bundle.pem

2.4. Mac OS X before 10.6

You can generate the file you need by opening Keychain Access (from /Applications/Utilities), going to the System Roots keychain, selecting everything and then choosing Export Items... from the File menu. Make sure the File Format is set to Privacy Enhanced Mail (.pem), then save it to your Desktop as Certificates. Next, in Terminal enter

sudo cp ~/Desktop/Certificates.pem /etc/hg-ca-roots.pem

then configure Mercurial as follows:

[web]
cacerts = /etc/hg-ca-roots.pem

Note that because the vendor supplied set of CA root certificates on Mac OS X is in the system keychain, you may wish to repeat these steps after installing software updates if they include changes to the root certificate list.

2.5. Mac OS X 10.6 and higher

On Mac OS X 10.6 and higher, Apple's OpenSSL (which is what Apple's Python and therefore usually Mercurial use to implement their SSL support) will look in the system keychain. Unfortunately, the SSL code in the Python core doesn't allow for this situation---it always expects you to specify a certificate bundle, and if one is specified if must contain at least one certificate. A simple way to deal with this problem is to enter (in Terminal)

openssl req -new -x509 -extensions v3_ca -keyout /dev/null -out dummycert.pem -days 3650

to generate a dummy certificate (the contents don't matter, so you can just hit return at all of the prompts), then

sudo cp dummycert.pem /etc/hg-dummy-cert.pem

and set your configuration as follows:

[web]
cacerts = /etc/hg-dummy-cert.pem

Don't download a dummy certificate someone on the Internet has created to solve this problem unless you're certain that they're trustworthy; if they kept the private key, they would be able to sign certificates that Mercurial would trust. Better just to enter the commands above.

If you have trouble getting a dummy certificate to work, see below on how to work around the issue with host certificate fingerprints.

2.6. Windows

The Windows installer for Mercurial 1.7.3 (and corresponding TortoiseHg installers) are now safe by default. They now check the validity of the identity of the server you connect to with the root certificates.

The Windows installers for Mercurial 1.7.3 (and corresponding TortoiseHg installers) contain a cacert.pem and by default configure web.cacerts in hgrc.d\paths.rc . Note that per the default settings installed, connect to repositories with self-signed certificates fail with 1.7.3. You need to adjust the default configuration for that case.

2.7. Other platforms

If your platform doesn't provide a usable CA list, you can download a cacert file from http://curl.haxx.se/docs/caextract.html or some other trusted source.

2.8. Other Certificate Authorities

If want to trust other CAs you must make sure their root certificates are in your web.cacerts. For example, in order to use certificates from by cacert.org you need to download their root certificate in PEM format and configure Mercurial to use that certificate.

2.9. Self-signed certificates

You might want to tweak your cacert file, for example by removing CAs you don't trust or by adding your own internal or self-signed CAs. Only one cacerts file can be specified at any time, so you might want to override web.cacerts in your user or repository configuration.

The root CA certificate for a server can for example be retrieved with Firefox. Browse to https://server/repo and verify that this is the repository you trust, click the lock symbol in the lower right corner, View Certificate, Details, select the certificate at the top of the Certificate Hierarchy, Export, "X.509 Certificate (PEM)" and save somewhere for example as server.pem. With other browsers on Windows you have to view the certificate at the top of the Certification Path and "Copy to File" as "Base-64 encoded X.509 (.CER)". Several such files can be concatenated into one cacerts file.

Note: Self-signed certificates generated by IIS7 might not work with OpenSSL/Python as used by Mercurial. They do apparently have different opinions on what a valid CA certificate should contain. Instead, use openssl to create your server certificate and your CA certificate.

The content of a single PEM encoded certificate can be seen with

•  openssl x509 -in server.pem -text

3. Per-repository configuration

If you want to control more explicitly who can impersonate which servers you will authenticate to and pull from you can explicitly configure the trusted CAs for each local clone.

In your local repository add the following to your .hg/hgrc:

[web]
cacerts = /path/to/server.pem

Note: This requires Mercurial 1.7.3 or later.

4. Host certificate fingerprints

With Mercurial 1.7.4 a new method for verification of server identify was introduced. The warnings given for insecure https connections will show the fingerprint of the server certificate, and when such a fingerprint is configured for a hostname in the hostfingerprints section Mercurial will verify for all connections to that server that its certificate matches the fingerprint.

For example, for https://hg.intevation.org/mercurial/crew+main/ :

[hostfingerprints]
hg.intevation.org = fa:1f:d9:48:f1:e7:74:30:38:8d:d8:58:b6:94:b8:58:28:7d:8b:d0

5. Manually bypassing security checks on self-signed certificates

This is obviously insecure.

Sometimes it may be expedient to disable security checks, for instance when dealing with hosts with self-signed certificates. This can be done by disabling the CA certificate configuration on the command line:

hg push --config web.cacerts= https://self-signed-host/repo

A --insecure option was introduced with Mercurial 1.7.5 so you can do:

hg push --insecure https://self-signed-host/repo

6. HTTP proxy support

Mercurial 1.8 will also verify certificates for HTTPS connections made with CONNECT through HTTP proxies.

7. SMTP TLS certificates

Mercurial 2.6 and later also verifies SSL/TLS certificates for SMTP.

8. See also

• http://docs.python.org/library/ssl.html#ssl-certificates

• http://curl.haxx.se/docs/sslcerts.html

• http://bugs.python.org/issue1589

• http://mercurial.selenic.com/bts/issue2407

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