Add security advisory for previous commit.

security-advisories/2015-03-05-0-c++-addl-cpu-amplification.md

+Problem
+=======
+
+CPU usage amplification attack, similar to previous vulnerability
+[2015-03-02-2][1].
+
+Discovered by
+=============
+
+David Renshaw <david@sandstorm.io>
+
+Announced
+=========
+
+2015-03-05
+
+Impact
+======
+
+- Remotely cause a peer to execute a tight `for` loop counting from 0 to
+  2^29, possibly repeatedly, by sending it a small message. This could enable
+  a DoS attack by consuming CPU resources.
+
+Fixed in
+========
+
+- git commit [80149744bdafa3ad4eedc83f8ab675e27baee868][0]
+- release 0.5.1.2:
+  - Unix: https://capnproto.org/capnproto-c++-0.5.1.2.tar.gz
+  - Windows: https://capnproto.org/capnproto-c++-win32-0.5.1.2.zip
+- release 0.4.1.1:
+  - Unix: https://capnproto.org/capnproto-c++-0.4.1.2.tar.gz
+- release 0.6 (future)
+
+[0]: https://github.com/sandstorm-io/capnproto/commit/80149744bdafa3ad4eedc83f8ab675e27baee868
+
+Details
+=======
+
+Advisory [2015-03-02-2][1] described a bug allowing a remote attacker to
+consume excessive CPU time or other resources usin a specially-crafted message.
+The present advisory is simply another case of the same bug which was initially
+missed.
+
+The new case occurs only if the application invokes the `totalSize()` method
+on an object reader.
+
+The new case is somewhat less severe, in that it only spins in a tight `for`
+loop that doesn't call any application code. Only CPU time is possibly
+consumed, not RAM or other resources. However, it is still possible to create
+significant delays for the receiver with a specially-crafted message.
+
+[1]: https://github.com/sandstorm-io/capnproto/blob/master/security-advisories/2015-03-02-2-all-cpu-amplification.md
+
+Preventative measures
+=====================
+
+Our fuzz test actually covered this case, but we didn't notice the problem
+because the loop actually completes in less than a second. We've added a new
+test case which is more demanding, and will make sure that when we do extended
+testing with American Fuzzy Lop, we treat unexpectedly long run times as
+failures.