Type packetstorm
Reporter Martin Pool
Modified 2000-04-04T00:00:00


                                            ` executive summary  
HTTP cache-control headers such as If-Modified-Since allow servers  
to track individual users in a manner similar to cookies, but with  
less constraints. This is a problem for user privacy against which  
browsers currently provide little protection.  
problem statement  
Alice is browsing the web; Bob runs a number of otherwise-unrelated  
web servers. Alice makes several requests to Bob's servers over  
time. Bob would like to tie together as many as possible of the  
requests made by Alice to learn more about Alice's usage patterns  
and identity: we call this identifying the request chain. Alice  
would like to access Bob's servers but not give away this  
existing approaches  
The standard approach for associating user requests across several  
responses is the HTTP `Cookie' state-management extension. The Cookie  
response header allows a server to ask the client to store arbitrary  
short opaque data, which should be returned for future requests of  
that server matching particular criteria. Cookies are commonly used to  
store per-user form defaults, to manage web application sessions, and  
to associate requests between executions of the user agent.  
The user agent always has the option to just ignore the Set-Cookie  
response header, but most implementations default to obeying it to  
preserve functionality. Cookies can optionally specify an expiry time  
after which they should no longer be used, that they should persist on  
disk between client session, or that they should only be passed over  
transmission-level-secure connections.  
The privacy implications of cookies have been [1]extensively  
discussed, and several problems have been found and recitified in the  
past. One example of privacy compromise through cookies is the use of  
cookies attached to banner images downloaded from a central banner  
server: the same cookie is used within images linked from several  
servers, and so the user can be tracked as they move around.  
other approaches  
An obvious means to associate requests is by source IP address. Over  
the short term this will generally work quite well, as a client is  
likely to use a single IP address during a browsing session. Even then  
it is complicated by proxies acting for multiple clients, network  
address translation, or multiuser machines. Over a longer term, the  
information is convolved by dynamically-assigned IPs, mobile computers  
moving between networks, dialup pools and the like. Indeed, cookies  
were proposed in large part to allow legitimate stateful applications  
to cope with the impossibility of uniquely identifying users by IP  
the meantime exploit  
The fundament of the meantime exploit is that the server wishes to  
`tag' the client with some information that will later be reported  
back, allowing the server to identify a chain. Cookies are a good  
approach to this, but their privacy implications are well known and so  
Bob requires a more surreptitious approach.  
The HTTP cache-control headers are perfect for this: the data is  
provided by the server, stored but not verified by the client, and  
then provided verbatim back to the server on the next matching  
Two headers in particular are useful: Last-Modified and ETag. Both are  
designed to help the client and server negotiate whether to use a  
cached copy or fetch the resource again.  
The general approach of meantime is that rather than using the headers  
for their intended purpose, Bob's servers will instead send down a  
unique tag for the client.  
Last-Modified is constrained to be a date, and therefore is somewhat  
inflexible. Nevertheless, the server can reasonably choose any second  
since the Unix epoch, which allows it to tag on the order of one  
billion distinct clients.  
ETag allows an arbitrary short string to be stored and passed. It is  
not so commonly implemented in user agents at the moment, and so not  
such a good choice.  
In both cases the tag will be lost if the client discards the resource  
from its cache, or if it does not request the exact same resource in  
the future, or if the request is unconditional. (For example, Netscape  
sends an unconditional response when the user presses Shift+Reload.)  
Bob has less control over this than he has with cookies, which can be  
instructed to persist for an arbitrarily long period.  
The date is only sent back for the exact same URL, including any query  
parameters. By contrast, cookies can be returned for all resources in  
a site or section of a site. This makes Bob's job a little harder.  
Bob therefore should make sure that all pages link to a small common  
resource: perhaps a one-pixel image. This image is generated by a  
script that supplies and records a unique timestamp to each client,  
and records whatever is already present.  
For a demonstration, more explanation and details, please see  
Martin Pool, Linuxcare, Inc.  
+61 2 6262 8990,  
Linuxcare. Support for the revolution.