To update Fedora 18 against Heartbleed, you'll need to roll your own RPMs since 18 is no longer supported. (Consider CentOS?)
Here's what worked for me, based heavily on the CentOS guide on rebuilding RPMs. Your mileage may vary. I did have things like gcc and make already installed.
$ sudo yum install rpm-build
$ mkdir -p ~/rpmbuild/{BUILD,RPMS,SOURCES,SPECS,SRPMS}
$ echo '%_topdir %(echo $HOME)/rpmbuild' > ~/.rpmmacros
$ cd /tmp
$ wget http://mirrors.kernel.org/fedora/updates/18/SRPMS/openssl-1.0.1e-37.fc18.src.rpm
$ rpm -i openssl-1.0.1e-37.fc18.src.rpm
$ cd ~/rpmbuild/SPECS
$ vim openssl.specNow, apply the patch from the Fedora bug report. The previous release number is different, of course, so I just incremented by 1.
$ rpmbuild -ba openssl.spec
$ cd ~/rpmbuild/RPMS/x86_64
$ sudo yum install openssl*.rpm
As noted on StackOverflow, you can use
yum-builddepfrom theyum-utilspackage if you need dependencies installed.