[Update: Disclosure and other points discussed in a little more detail here.]

Why memcached?

At BlackHat USA last year we spoke about attacking cloud systems, while the thinking was broadly applicable, we focused on specific providers (overview). This year, we continued in the same vein except we focused on a particular piece of software used in numerous large-scale application including many cloud services. In the realm of “software that enables cloud services”, there appears to be a handful of “go to” applications that are consistently re-used, and it’s curious that a security practitioner’s perspective has not as yet been applied to them (disclaimer: I’m not aware of parallel work).

We choose to look at memcached, a “Free & open source, high-performance, distributed memory object caching system” ¹. It’s not outwardly sexy from a security standpoint and it doesn’t have a large and exposed codebase (total LOC is a smidge over 11k). However, what’s of interest is the type of applications in which memcached is deployed. Memcached is most often used in web application to speed up page loads. Sites are almost² always dynamic and either have many clients (i.e. require horizontal scaling) or process piles of data (look to reduce processing time), or oftentimes both.  This implies that the sites that use memcached contain more interesting info than simple static sites, and are an indicator of a potentially interesting site. Prominent users of memcached include LiveJournal (memcached was originally written by Brad Fitzpatrick for LJ), Wikipedia, Flickr, YouTube and Twitter.

I won’t go into how memcached works, suffice it to say that since data tends to be read more often than written in common use cases the idea is to pre-render and store the finalised content inside the in-memory cache. When future requests ask for the page or data, it doesn’t need to be regenerated but can be simply regurgitated from the cache. Their Wiki contains more background.

go-derper

We released go-derper, a tool for playing with memcached instances. It supports three basic modes of operations:

1. Fingerprinting memcacheds to determine interesting servers
2. Extracting a (user-limited) copy of the cache
3. Writing data into the cache

The tool has minor requirements: a recent Ruby and the memcache-client gem. What follows are basic use cases.

Fingerprinting

Let’s assume you’ve scanned a hosting provider and found 239 potential targets using a basic .nse that hunts down open memcached instances³. You need to separate the wheat from the chaff and figure out which servers are potentially interesting; one way to do that is by extracting a bunch of metrics from each cache. Start small against one cache:
insurrection:demo marco$ ruby go-derper.rb -f x.x.x.x
[i] Scanning x.x.x.x
x.x.x.x:11211
==============================
memcached 1.4.5 (1064) up 54:10:01:27, sys time Wed Aug 04 10:34:36 +0200 2010, utime=369388.17, stime=520925.98
Mem: Max 1024.00 MB, max item size = 1024.00 KB
Network: curr conn 18, bytes read 44.69 TB, bytes written 695.93 GB
Cache: get 514, set 93.41b, bytes stored 825.73 MB, curr item count 1.54m, total items 1.54m, total slabs 3
Stats capabilities: (stat) slabs settings items (set) (get)


44 terabytes read from the cache in 54 days with 1.5 million items stored? This cache is used quite frequently. There’s an anomaly here in that the cache reports only 514 reads with 93 billion writes; however it’s still worth exploring if only for the size.

We can run the same fingerprint scan against multiple hosts using

ruby go-derper.rb -f host1,host2,host3,...,hostn

or, if the hosts are in a file (one per line):

ruby go-derper.rb -F file_with_target_hosts

Output is either human-readable multiline (the default), or CSV. The latter helps for quickly rearranging and sorting the output to determine potential targets, and is enabled with the “-c” switch:

ruby go-derper.rb -c csv -f host1,host2,host3,...,hostn

Lastly, the monitor mode (-m) will loop forever while retrieving certain statistics and keep track of differences between iterations, in order to determine whether the cache appears to be in active use.

Mining

Once you’ve identified a potentially interesting target, it’s time to mine that cache. The basic leach switch is “-l”:

insurrection:demo marco$ ruby go-derper.rb -l -s x.x.x.x
[w] No output directory specified, defaulting to ./output
[w] No prefix supplied, using "run1"

This will extract data from the cache in the form of a key and its value, and save the value in a file under the “./output” directory by default (if this directory doesn’t exist then the tool will exit so make sure it’s present.) This means a separate file is created for every retrieved value. Output directories and file prefixes are adjustable with “-o” and “-r” respectively, however it’s usually safe to leave these alone.

By default, go-derper fetches 10 keys per slab (see the memcached docs for a discussion on slabs; basically similar-sized entries are grouped together.) This default is intentionally low; on an actual assessment this could run into six figures. Use the “-K” switch to adjust:

ruby go-derper.rb -l -K 100 -s x.x.x.x

As mentioned, retrieved data is stored in the “./ouput” directory (or elsewhere if “-o” is used). Within this directory, each new run of the tool produces a set of files prefixed with “runN” in order to keep multiple runs separate. The files produced are:

• runN-index, an index file containing metadata about each entry retrieved
• runN-<md5>, a file containing the bytestream from a retrieved value

The mapping between key and file in which the value is stored occurs in the index file, which is useful in that potentially malicious data (keynames) aren’t used when interacting with your local filesystem APIs.

At this point, there will (hopefully) be a large number of files in your output directory, which may contain useful info. Start grepping.

What we found with a bit of field experience was that mining large caches can take some time, and repeating grep gets quite boring. The tool permits you to supply your own set of regular expressions which will be applied to each retrieved value; matches are printed to the screen and this provides a scroll-by view of bits of data that may pique your interest (things like URLs, email addresses, session IDs, strings starting with “user”, “pass” or “auth”, cookies, IP addresses etc). The “-R” switch enables this feature and takes a file containing regexes as its sole argument:

ruby go-derper.rb -l -K 100 -R regexs.txt -s x.x.x.x

Over-writing

In this blog entry I don’t cover the kinds of data we discovered (it’ll be subject to a separate entry), however it may come to pass that you discover an interesting cache entry that you’d like to overwrite. Recall entries were stored in “./output” by default, with a prefix of “runN”. If the interesting entry was stored in “output/run1-e94aae85bd3469d929727bee5009dddd”, edit the file in whatever manner you see fit and save it to your local disk. Then, tell go-derper to write the entry back into the cache with:

ruby go-derper.rb -w output/run1-e94aae85bd3469d929727bee5009dddd

This syntax is simple since go-derper will figure out the target server and key from the run’s index file.

And so?

Go-derper permits basic manipulations of a memcached instance. We haven’t covered finding open instances or the kinds of data one may come across; these will be the subject of followup posts. Below are the slides from the talk, click through to SlideShare for the downloadable PDF.

¹ http://www.memcached.org

² We’re hedging here, but we’ve not come across a static memcached site.

³ If so, you may be as surprised as we were in finding this many open instances.