Comment by patio11 | Hacker News Reader

Comment by patio11 | Hacker News Reader

Parent Full thread

I'm going to guess it was simple spray-and-pray. (My account took its fair share of compromise attempts, too, probably because I use the same username everywhere so it's going to be in many modern dev-heavy password dumps.)

Github is, for better or worse, a high-value target these days. Compromise of a Github account will often get you credentials which you can use to compromise high-value targets, for example production machines.

(Example from the Rails world: if someone gets read access to your git repository, expect them to figure out your secret token which you use for HMACing sessions to prevent forgery. If an attacker can forge sessions, they have remote code execution on your production servers. You can assume they will go directly from code execution to root on those servers, and from root on those servers to a systemic compromise of your entire network (if they want to do that).)

Use your imagination on what a mass compromise of accounts gets you. The simplest possible example is grepping for /bitcoin/, finding the full source code and credentials for a Bitcoin exchange, and rooting them then emptying the hot wallet. That results in fairly obvious economic advantage, right? You could also use scriptable attacks to root thousands of big-n-beefy production machines on fairly trusted IP addresses, then add them to a botnet, which you'd use for spamming or various other nefarious activities.

Then, even farther down the list, you have a dedicated adversary actually go through every repository they got access to and look for something uniquely fun to do with that particular target.

If you were looking to do industrial espionage on a particular target, you'd probably pick a way that was less likely to be detected and cause a company-level security response. (Use passive recon to identify one or a few likely target email addresses, find one or a few likely passwords for them, and try them first. Heads you win, tails you just left evidence in a log of perfectly normal user behavior. You might follow up this perfectly normal user behavior with a social engineering attack.)

On the Rails note, you should be setting an ENV variable rather than hardcoding it as is the default. Same goes for any other keys/tokens/credentials that you're interacting with in your code. With these sorts of things, it's best to rely on pessimistic assumptions and take proactive steps accordingly.

Don't forget API keys in commits from users foolish enough to trust the security of private repos.

Yeah, spear phishing / social engineering makes more sense for industrial espionage.

Is there a setting I can change to turn off this "session -> code execution" feature?

You could, hypothetically, move from CookieStore to a different session store, but I am not positive that this will mitigate the vulnerability (by making it more difficult for the attacker to get write access to the session, not by making it more difficult to turn a session into code execution), and do not have a few hours to reacquaint myself with the code to give you a firmer answer there.

I am weakly confident, on the basis of some spelunking in the Rails internals in February, that if you get attacker chosen data into the session you're uniformly hosed on (at least) Rails 2 and Rails 3.

And Rails 4. The difference with Rails 4 is that the session is encrypted as well as HMAC'd, but it's still the case that as along as you have the token (or can otherwise craft a session) you're hosed.

if you get attacker chosen data into the session you're uniformly hosed on (at least) Rails 2 and Rails 3.

I wonder if you could expand on this? I'm not too knowledgeable about Rails or security, so I'd wonder how that works?

Long story short: If a cookie is valid (HMAC signature matches what you'd expect given the cookie contents and your secret key), Rails (Rails 2) or Rack middleware (Rails 3) will attempt to use Marshal.load(txt) on the cookie contents to populate your session hash. You might naively assume that this creates a map of strings into other strings, but surprise, Marshal.load(txt) can create arbitrary object payloads and, through a code path which the margins of this comment are just slightly too thin to contain, allow you to do remote code execution.

See: http://www.exploit-db.com/exploits/27527/

This can be done in an entirely automated fashion and requires no knowledge of the application other than the session's secret key.

As patio11 mentioned, moving off the CookieStore is a step in the right direction, with the understanding of course that you guard whatever you move to with your life. That means the db with the ActiveRecord store, or memcached if you're putting it in the cache store with the cache set to memcached. With memcached, be careful not to leave the port open to the internet if you're managing it on your own (it's designed to be used inside a trusted network, just like redis).

In theory, an additional precaution one could take: change the default session serializer to `JSON`. By using `JSON` instead of `Marshal`, you're limited to storing strings, hashes and arrays in the session, but that's a good thing. The remote code execution vulnerability takes advantage of `Marshal` deserializing the session and loading objects deep within the Rails process in order to run arbitrary code. Take away that ability by using `JSON` instead of `Marshal` and you should no longer have to worry about this particular attack vector.

After a brief spelunk of the Rails codebase (and not getting very far, my laptop bricked itself today) there doesn't seem to be an easy way to do this, apart from finding the instance of `ActiveSupport::MessageVerifier` in your rails process and then running something like `verifier.instance_variable_set(:@serializer, JSON)`.

Reply on news.ycombinator.com