Here is the latest Caml Weekly News, for the week of December 27, 2011 to January 03, 2012.
Archive: https://sympa-roc.inria.fr/wws/arc/caml-list/2011-12/msg00442.htmlContinuing the thread from last week, Gabriel Scherer asked and Fabrice Le Fessant replied:
> I played with the toplevel a bit, but was frustrated by the limitation > of one-liner input. > Even in an interactive toplevel it is nice, I think, to be able to > write multiline programs. I uploaded a new version a few minutes ago. Now, you can use "multiline true" to tell the toplevel that you will use ;; as an end of input instead of the newline (and "multiline false;;" to switch back). My patch is inspired from yours, but with minimal changes to the current code. It works for me, but I would be happy to get feed back from "multi-line users".
Archive: https://sympa-roc.inria.fr/wws/arc/caml-list/2011-12/msg00450.htmlGerd Stolpmann said and Xavier Leroy replied:
> there was recently a security alert for web services that use hash > tables to store web form parameters sent via POST (so that millions of > such parameters can be sent in a single request). It is possible to keep > the web service busy for hours with such a DoS (denial of service) > attack. The type of attack boils down to a problem in most hash table > implementations, namely that the hash functions are invertible, and it > is possible for a malicious user to construct lots of keys that all map > to the same bucket of the hash table, creating a mass collision. > > The text of the alert: > http://www.nruns.com/_downloads/advisory28122011.pdf > > I'd like to discuss this issue, because it is not restricted to the > processing of web requests, but may also occur for all other data coming > from untrusted sources. The web is only the most exposed area where this > issue exists. > > So how is Ocaml affected? The hash functions used in recent Ocaml > releases are also insecure in the above mentioned sense (currently > MurmurHash3, and even a simpler hash function in previous releases). A > quick survey of the Internet revealed at least one site that tries to > break it. Probably a good cryptographer could do it in minutes. > > A pure Hashtbl.add of the constructed keys does not yet lead to the > performance degradation, but a Hashtbl.replace, and of course > Hashtbl.find after the table is built up will. So it depends very much > of the details of the programs whether they are affected or not. > > I've just checked that Ocamlnet uses only Hashtbl.add to collect POST > parameters, so it is not directly vulnerable. But if the crafted request > is actually served by a handler, the handler would get a degraded table, > and could show in turn bad performance (again leading to DoS). > > What are possible fixes? > > 1) Avoid hash tables in contexts where security is relevant. The > alternative is Set (actually a balanced binary tree), which does not > show this problem. Highly recommended. Nothing beats guaranteed O(log n) operations. > 2) Use cryptographically secure hash functions. Hopeless: with a hash size of 30 bits, as in Caml, or even 64 bits, there are no cryptographically secure hash functions. > 3) Use "randomized" hash tables. The trick here is that there is not a > single hash function h anymore, but a family h(1)...h(n). When the hash > table is created, one of the functions is picked randomly. This makes it > impossible to craft an attack request, because you cannot predict the > function. Indeed. The optional "seed" parameter to Hashtbl.create does exactly this in the new implementation of Hashtbl (the one based on Murmur3). > So, the question is how to do 3). I see two problems here: > > a) how to define the family of hash functions. Is it e.g. sufficient to > introduce an initialization vector for the Murmurhash algorithm, and > fill it randomly? IIRC, the Web pages for the Murmur family of hashes gives some statistical evidence that this approach works. > How to get a random number that is good enough? Hmm. /dev/random is your friend on the platforms that support it. Otherwise, there's always the Random module, but Random.self_init isn't very strong.Gerd Stolpmann then replied:
> Indeed. The optional "seed" parameter to Hashtbl.create does exactly > this in the new implementation of Hashtbl (the one based on Murmur3). I see. It will be available in 3.13: val create : ?seed:int -> int -> ('a, 'b) t There is also an additional functorized interface where this seed argument exists (Hashtbl.MakeSeeded), and the hash functions seeded_hash and seeded_hash_param. Well done! Nevertheless, as we all don't know when 3.13 is ready, I'll have to find a temporary fix for Ocamlnet. Maybe just a limit for the number of POST parameters. > > How to get a random number that is good enough? > > Hmm. /dev/random is your friend on the platforms that support it. > Otherwise, there's always the Random module, but Random.self_init > isn't very strong. Well, /dev/(u)random covers most Unix platforms nowadays. If you are interested, I have a wrapper for Win32: https://godirepo.camlcity.org/svn/lib-ocamlnet2/trunk/code/src/netsys/netsys_c_win32.c Scroll down until netsys_fill_random.Richard Jones also replied and Xavier Leroy said:
> It may be worth noting that Perl solved this problem (back in 2003) by > unconditionally using a seed which is a global set to a random number > during interpreter initialization. That's how my initial reimplementation of Hashtbl worked, using the Random module to produce seeds, but I was told (correctly) that in security-sensitive applications it's better to leave the generation of random numbers under control of the programmer. For some applications Random.self_init might be good enough and for others stronger randomness is needed. Of course, you can trivially emulate Perl's behavior using the new Hashtbl implementation + the Random module.
Thanks to Alp Mestan, we now include in the Caml Weekly News the links to the recent posts from the ocamlcore planet blog at http://planet.ocamlcore.org/. (One by) Four by Nine: http://alaska-kamtchatka.blogspot.com/2011/12/one-by-four-by-nine.html Two-way bindings: http://www.nicollet.net/2011/12/two-way-bindings/ Vose's Alias Method: http://alaska-kamtchatka.blogspot.com/2011/12/voses-alias-method.html opdf: https://forge.ocamlcore.org/projects/opdf/ Packing circles into a rectangle: https://forge.ocamlcore.org/forum/forum.php?forum_id=820 Beta-release of Coq 8.4: http://coq.inria.fr/beta-release-of-coq-84
If you happen to miss a CWN, you can send me a message and I'll mail it to you, or go take a look at the archive or the RSS feed of the archives.
If you also wish to receive it every week by mail, you may subscribe online.