Merge branch 'adding_crime_security' into 'master'

Adding  CRIME Vulnerability to Security doc

Added an additional page to the security section that goes over current status and fixes for the TLS Protocol CRIME Vulnerability. Also added its respective link at the security README. 

Any suggestions appreciated @JobV @sytses 

See merge request !2102

doc/security/README.md

@@ -6,4 +6,5 @@
 - [Information exclusivity](information_exclusivity.md)
 - [Reset your root password](reset_root_password.md)
 - [User File Uploads](user_file_uploads.md)
+- [How we manage the CRIME vulnerability](crime_vulnerability.md)
 - [Enforce Two-Factor authentication](two_factor_authentication.md)

doc/security/crime_vulnerability.md

+# How we manage the TLS protocol CRIME vulnerability
+
+> CRIME ("Compression Ratio Info-leak Made Easy") is a security exploit against
+secret web cookies over connections using the HTTPS and SPDY protocols that also
+use data compression. When used to recover the content of secret
+authentication cookies, it allows an attacker to perform session hijacking on an
+authenticated web session, allowing the launching of further attacks.
+([CRIME](https://en.wikipedia.org/w/index.php?title=CRIME&oldid=692423806))
+
+### Description
+
+The TLS Protocol CRIME Vulnerability affects compression over HTTPS, therefore
+it warns against using SSL Compression (for example gzip) or SPDY which
+optionally uses compression as well.
+
+GitLab supports both gzip and [SPDY][ngx-spdy] and mitigates the CRIME
+vulnerability by deactivating gzip when HTTPS is enabled. You can see the
+sources of the files in question:
+
+* [Source installation NGINX file][source-nginx]
+* [Omnibus installation NGINX file][omnibus-nginx]
+
+Although SPDY is enabled in Omnibus installations, CRIME relies on compression
+(the 'C') and the default compression level in NGINX's SPDY module is 0
+(no compression).
+
+### Nessus
+
+The Nessus scanner, [reports a possible CRIME vulnerability][nessus] in GitLab
+similar to the following format:
+
+```
+Description
+
+This remote service has one of two configurations that are known to be required for the CRIME attack:
+SSL/TLS compression is enabled.
+TLS advertises the SPDY protocol earlier than version 4.
+
+...
+
+Output
+
+The following configuration indicates that the remote service may be vulnerable to the CRIME attack:
+SPDY support earlier than version 4 is advertised.
+```
+
+From the report above it is important to note that Nessus is only checking if
+TLS advertises the SPDY protocol earlier than version 4, it does not perform an
+attack nor does it check if compression is enabled. With just this approach, it
+cannot tell that SPDY's compression is disabled and not subject to the CRIME
+vulnerability.
+
+### References
+
+* Nginx ["Module ngx_http_spdy_module"][ngx-spdy]
+* Tenable Network Security, Inc. ["Transport Layer Security (TLS) Protocol CRIME Vulnerability"][nessus]
+* Wikipedia contributors, ["CRIME"][wiki-crime] Wikipedia, The Free Encyclopedia
+
+[source-nginx]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/support/nginx/gitlab-ssl
+[omnibus-nginx]: https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/files/gitlab-cookbooks/gitlab/templates/default/nginx-gitlab-http.conf.erb
+[ngx-spdy]: http://nginx.org/en/docs/http/ngx_http_spdy_module.html
+[nessus]: https://www.tenable.com/plugins/index.php?view=single&id=62565
+[wiki-crime]: https://en.wikipedia.org/wiki/CRIME