To browse the sources via GitHub: https://github.com/apache/cxf
You can fork that repository, commit changes to your fork, and submit pull requests through GitHub's pull request mechanisms.
The repository is dual-hosted, so it's also valid to use https://gitbox.apache.org/repos/asf?p=cxf.git
The source code can be checked out anonymously over HTTPs by doing:
git clone https://github.com/apache/cxf.git
git clone https://gitbox.apache.org/repos/asf/cxf.git
In addition you can use: https://fisheye.apache.org/browse/cxf