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"a skilled backdoor-writer can defeat skilled auditors"?



On Wed, Jun 04, 2014 at 12:35:20AM +0200, rysiek wrote:
> In short several very skilled security auditors examined a small Python 
> program â?? about 100 lines of code â?? into which three bugs had been inserted by 
> the authors. There was an â??easy,â?? â??medium,â?? and â??hardâ?? backdoor. There were 
> three or four teams of auditors.
> 
> 1. One auditor found the â??easyâ?? and the â??mediumâ?? ones in about 70 minutes, and 
> then spent the rest of the day failing to find any other bugs.
> 
> 2. One team of two auditors found the â??easyâ?? bug in about five hours, and 
> spent the rest of the day failing to find any other bugs.
> 
> 3. One auditor found the â??easyâ?? bug in about four hours, and then stopped.
> 
> 4. One auditor either found no bugs or else was on a team with the third 
> auditor â?? the report is unclear.
> 
> See Chapter 7 of Yeeâ??s report for these details.
> 
> I should emphasize that that I personally consider these people to be 
> extremely skilled. One possible conclusion that could be drawn from this 
> experience is that a skilled backdoor-writer can defeat skilled auditors. This 
> hypothesis holds that only accidental bugs can be reliably detected by 
> auditors, not deliberately hidden bugs.
> 
> Anyway, as far as I understand the bugs you folks left in were accidental bugs 
> that you then deliberately didnâ??t-fix, rather than bugs that you intentionally 
> made hard-to-spot.
> 
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> https://blog.spideroak.com/20140220090004-responsibly-bringing-new-cryptography-product-market#footnote1
> 
> I have no problem believing it is thus, but can't help wondering if there are 
> any ways to mitigate it.


My mitigation would be to make auditing a default-deny rule, rather than
a default-allow.

Security auditing needs to be a holistic analysis, starting by
re-engaging with the requirements, verifying that the design is a
sensible and minimal approach to addressing the requirements, and
verifying that the implementation is a sensible, safe, auditable,
version controlled, approach to the design.

If the auditor at any point says "Well, I wouldn't have *recommended*
that you implement your JSON parsing in ad-hoc C with pointer arithmetic
and poor and misleading comments, but I can't find any *bugs* so I guess
it must be OK" then that is an immediate fail.

This is the default deny: we default to assuming the system is insecure,
and any sign that this might be true results in a failure.

Versus the current auditing method of default-allow: we run the audit,
and if no *concrete* exploits or bugs are found before the auditors run
out of time, then we trumpet that the system "has passed its audit".

Only if the design is sane, the implementation is sane, the development
team is following best practices and defensive coding strategies, with a
cryptographically and procedurally audited edit trail (immutable git
commit logs signed and committed to W/O media) in a development
environment that is safe by default rather than risky by default ...

... then you *might* have a chance of catching the intentional backdoor
inserted by the APT malware on your team member's workstation.

Current efforts in this direction fall *very* far short of the utopia I
describe.

-andy