mailing list archives
privacy problems with HTTP cache-control
From: mbp () LINUXCARE COM (Martin Pool)
Date: Wed, 29 Mar 2000 15:19:21 +1000
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.
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
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
The privacy implications of cookies have been 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.
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
mbp () linuxcare com, http://www.linuxcare.com/
Linuxcare. Support for the revolution.
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