Enforcing system crypto policies

Since Fedora 21 (Changes/CryptoPolicy) there are policies for the usage of SSL and TLS cryptographic protocols that are enforced system-wide. Each application being added in Fedora must be checked to comply with the policies. Currently the policies are restricted to applications using GnuTLS and OpenSSL, and rpmlint will warn when it detects that some action has to be taken; that detection is based on heuristics and limited to C programs, so manual inspection is recommended. Note however, that there are applications which intentionally set weaker, or custom settings on a purpose (e.g., postfix); those need not adhere to the policy. When in doubt, discuss with the Fedora security team.

C/C++ applications

  • OpenSSL applications:

    • If the application provides a configuration file that allows to modify the cipher list string, ensure that the shipped file contains "PROFILE=SYSTEM" as default. In that case no further action is required.

    • If the application doesn’t have a configuration file, ensure that there is no default cipher list specified, or that the default list is set as "PROFILE=SYSTEM". That is, check the source code for SSL_CTX_set_cipher_list(). If it is not present then nothing needs to be done (the default is used). Otherwise, if that call is present and provided a fixed string which does not contain PSK or SRP, replace the string with "PROFILE=SYSTEM", or remove the call.

  • GnuTLS applications:

    • If the application provides a configuration file that allows to modify the cipher priority string, the shipped file contains "@SYSTEM" as default. In that case no further action is required.

    • If the application doesn’t have a configuration file, ensure that it uses gnutls_set_default_priority(), or that the default priority string is "@SYSTEM". That is, check the source code for gnutls_priority_set_direct(), gnutls_priority_init(); if they are not present and gnutls_set_default_priority() is used, nothing needs to be done. Otherwise check the strings provided by the application. If it contains PSK or SRP do nothing (these applications are not currently covered by the default policy). If not, then replace gnutls_priority_set_direct() with gnutls_set_default_priority(). If gnutls_priority_init() is used instead with a fixed string, replace the string with "@SYSTEM".

Applications utilizing other cryptographic libraries do not adhere to the system wide crypto policies (note that adherence to the system-wide policies is work in progress for NSS libraries). Applications in Fedora should use one of these libraries when there is choice, and preferrably the version recommended by upstream.

Perl applications

  • IO::Socket::SSL Perl applications:

    • Check the source code for passing SSL_cipher_list argument to IO::Socket::SSL's methods like new(), start_SSL(), new_from_fd(), set_defaults(), set_client_defaults(), and set_server_defaults(). If it is not present then nothing needs to be done (the default is used). Otherwise, if that argument is present, remove the argument or change its value as described in OpenSSL section.

  • Net::SSLeay Perl applications:

    • Check the source code for CTX_set_cipher_list(), set_cipher_list(), and set_pref_cipher() subroutine calls from Net::SSLeay name space. If such a call presents, follow instructions described in the OpenSSL section.

  • LWP::UserAgent Perl applications:

    • Check the source code for passing SSL_cipher_list argument to ssl_opts() method call on a LWP::UserAgent object. If such a call presents, follow instructions described in the OpenSSL section.