American Fuzzy Lop (AFL) is available from all good distros, or from
the upstream site: http://lcamtuf.coredump.cx/afl/

Configure to build the instumented binary:

  CC=afl-gcc ../src/configure --disable-shared --enable-static

Build the binary:

   make clean
   make afl-x509

Run the fuzzer (you may need root to change the CPU governor):

   mkdir afl-findings
   afl-fuzz -i ../src/subversion/tests/afl/afl-x509-testcase \
            -o afl-findings -M fuzzer01 \
            subversion/tests/afl/afl-x509
   afl-fuzz -i ../src/subversion/tests/afl/afl-x509-testcase \
            -o afl-findings -S fuzzer02 \
            subversion/tests/afl/afl-x509
   afl-fuzz -i ../src/subversion/tests/afl/afl-x509-testcase \
            -o afl-findings -S fuzzer03 \
            subversion/tests/afl/afl-x509

Wait, burning CPU...

Testcases that cause SEGVs are easier to debug using a separate build
without AFL instrumentation:

   gdb --args subversion/tests/afl/afl-x509 afl-findings/fuzzer01/crashes/some-test-case