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||Remote Retrieval Of Authentication Data From Internet Explorer
||13th October 2000
ACROS Security Problem Report #2000-07-22-2-PUB
Remote Retrieval Of Authentication Data From Internet Explorer
Affected System(s): Internet Explorer used in web-based systems with HTTP
Problem: Usernames and passwords can be retrieved remotely
from Internet Explorer
Solution: (see "Advisory" section)
Written: July 22, 2000
Last update: October 13, 2000
Published: October 13, 2000
Our team has analyzed how popular web browsers could be tricked to reveal the
cached username:password pairs and discovered a way how this can be done by
a remote attacker even when SSL is used to protect this data while in
transfer over insecure channels like Internet.
As a result, we have identified a weakness in Microsoft's Internet Explorer.
However, it *should not* be assumed that only this product is affected but
rather all vendors of web browsers are urged to review their products for
the identified vulnerability.
Note: We have put quite an effort into notifying these other vendors.
Unfortunately, we got very little response so we are unable to provide the
status of their products in this report.
The purpose of this report is to describe a security problem in IE's handling
of cached BASIC authentication data and also to provide a workable scenario
for exploiting this, and similar, vulnerabilities.
So you have launched your new web-based e-purchasing system and protected it
with 128-bit SSL. Your users logon to it through their browsers, providing
their usernames and passwords. Entire communication is protected with SSL so
there's no way anyone could intercept their authentication data.
Or maybe your web admin is remotely administering your web server through
some remote admin app (like IISAdmin) and you're relying on SSL to encrypt
the HTTP "Basic" authentication when it's going through the Internet.
You feel safe; you trust SSL to do its job protecting your users and your
system. Their passwords are secure.
Or are they?
We will show that it could be possible to retrieve the cached authentication
data from your user's web browser with little or no user's cooperation, even
when due care was taken to protect the communication between browser and
server with SSL. *
(*) We'll put all bugs in various SSL implementations aside and assume SSL
is working as specified.
CACHED AUTHENTICATION DATA
For the purpose of this report, we'll define "cached authentication data" as
static username:password pairs which the client application (web browser)
needs to provide to the server in order to get his requests processed. We
will only deal with username:password pairs for HTTP BASIC authentication,
which is one of the authentication mechanisms that Internet Explorer
provides. Arguably, BASIC authentication is the only browser-integrated
authentication that is supported by both Internet Explorer and Netscape
Navigator so chances are that every public web-based system that pops up
a dialog window requesting a username and password at login, is using this
type of authentication.
Our definition of cached authentication data is not to be mistaken for
"Password Caching", a feature of Internet Explorer which allows users to
store their passwords in Explorer's storage so that they are never again
asked for them (even after IE is closed and rerun). However, using this
feature users are clearly making the attack we describe below even easier
Throughout the analysis it is assumed that the attacker is capable of the
1) Listening to network traffic between client and server
2) Generating fake (spoofed) network traffic between client and server
Note: These assumptions are only a part of the assumptions stated in
the SSL Specification. SSL was developed for the purpose of protecting
against this (and much stronger) type of attacker.
For this analysis, we have set up a web server (www.test.com), installed
a valid SSL key+certificate and enabled Basic HTTP authentication. We have
also written a script (variables.cgi), which displays the contents of HTTP
authentication data (username:password) received by the browser.
Then, we opened Internet Explorer, typed "https://www.test.com/variables.cgi"
and we were presented with an authentication dialog asking us for username
and password. When these were entered correctly, the script variables.cgi was
executed which displayed our username and password.
The username and password were sent between client and server over an
encrypted SSL connection, preventing anyone listening from intercepting them.
Then, by opening the page "http://www.test.com/variables.cgi" (no SSL here!)
we could observe the authentication data being transmitted to the server over
an unencrypted link, thus making them interceptable for a network listening
The conclusion is, that even though username and password were sent to the
server over an encrypted connection, they are also sent to the server over a
subsequent, unencrypted connection if the browser establishes one. While
normally, the browser wouldn't establish such a connection (except in a badly
designed web application), this opens an opportunity for the attacker to
*make* the browser do so.
EXPLOITATION (same as in ASPR-2000-07-22-1)
For the purpose of exploitation, there are at least two ways of making the
user's browser connect to an arbitrary URL:
Social Engineering Technique
The first one is (very popular in examples) sending the user a "malicious"
e-mail message including a hyperlink to the attacker's web page, which
contains a hidden tag opening an unencrypted connection to the affected
web-based system. When the user clicks on the link in the attacker's e-mail
message, the attacker's web page is opened in the browser and the tag
causes the browser to send its authentication data to the critical web-based
system, over an unencrypted channel.
But surely, a serious attacker can't go relying on the user clicking a link
in his e-mail message. He needs a more effective technique.
Active Network Technique
This technique assumes the attacker has the ability to both listen to and
generate fake (spoofed) network traffic between browser and server.
We will assume a web-based system at "https://www.sensitive.com" using
HTTP BASIC authentication.
Phase 1: The HTTPS waiting phase
First, the attacker listens to the communication between the user (his
browser) and server to determine when the user connects to www.sensitive.com
on port 443 (HTTPS port). This is an indication that the user has started a
session on the sensitive server. After some amount of data is exchanged
between the two (due to encryption the attacker can't observe much more than
just the amount of exchanged data), attacker can assume that the user has
successfully authenticated to the server and his browser has cached the
Note: Actually, network traffic analysis can give pretty reliable hints
whether the HTTP authentication was successful or not, especially when the
attacker had the ability to observe a controlled session beforehand and learn
the sizes of various server's responses.
Phase 2: The HTTP waiting phase
After the "HTTPS waiting phase" is over, the user's browser has the username
and password in its memory.
Now, what the attacker would like to see is the user's browser connecting to
www.sensitive.com over (unencrypted) HTTP protocol on port 80. To force that,
he waits for the browser to send a HTTP request to ANY server, for example
Phase 3: Cached authentication data retrieval
When this happens, the attacker sends a fake response from "www.yahoo.com"
to the browser, containing the following document:
What this document does is (1) try to load "image.gif" from server
"www.sensitive.com" over unencrypted HTTP protocol (thus transmitting
authentication data for this server in cleartext) and (2) reload the page
after one second. This (second) time, the attacker lets the real
"www.yahoo.com" server answer the request so that the user gets what he
requested (user friendliness above all ;-).
Meanwhile, by sniffing the network traffic, the attacker has retrieved the
user's authentication data for the sensitive server.
Note: The file "image.gif" doesn't need to exist on "www.sensitive.com".
Note: If the "www.sensitive.com" server doesn't have port 80 (HTTP) open, the
attacker can make a fake response on its behalf, convincing the browser that
the port is open. However, in the case of Internet Explorer, there is an
easier way for making it send the authentication data over HTTP: by
"planting" a request for "http://www.sensitive.com:443/image.gif". This way,
the connection will be established to the (naturally open) port 443 and when
the server responds that it is there, the browser sends its request
(including username and password). Then, of course, the connection hangs
since a HTTP client is trying to talk to a HTTPS server, but that's no
problem since the sensitive data was already retrieved and the page will
reload in one second, that's long before a timeout would occur and possibly
alert the user.
By knowing the user's username and password, the attacker can login to the
sensitive system, assuming the user's identity.
Microsoft has issued a patch for IE, available at:
This patch changes the behavior of Internet Explorer so that it never sends
the cached authentication data over an unencrypted connection if it was
initially sent over an encrypted connection.
Users of Internet Explorer are advised to install the patch.
Managers of sensitive web-based systems using BASIC authentication are
urged to advise their users to install the patch.
Also take a look at Microsoft's Security Bulletin regarding this issue at:
and the FAQ at:
It is important to note that our limited testing only covered two web
browsers: Internet Explorer and Netscape Navigator (which isn't vulnerable).
There are many other web browsers that could be potentially affected by the
Users of web browsers can destroy cached authentication data by closing all
instances of their browsers immediately after logging out of critical
web-based systems - that's before accessing any other web site. Also, between
logging in and logging out of such system, they shouldn't visit any other web
site - not even web sites they trust.
Basically, for connecting to critical web-based systems, every user should:
1) Close all instances of the browser (if there are any) to prevent possible
2) Launch the browser
3) Log in to the system
4) Use the system
5) Log out of the system
6) Close all instances of the browser (to delete cached authentication data)
The above procedure could also protect users from various other
vulnerabilities inherent to web-based systems and should in our opinion be
used as a "best practice".
Tests were performed on:
Communicator 4.72 - not affected
Communicator 4.73 - not affected
Internet Explorer 4.01 SP2 - affected
Internet Explorer 5.01 - affected
Internet Explorer 5.5 - not affected (according to Microsoft)
We would like to acknowledge Microsoft Security Response Center for prompt
and professional response to our notification of the identified
For further details about this issue please contact:
Mr. Mitja Kolsek
SI - 2000 Maribor, Slovenia
phone: +386 41 720 908
PGP Key available at PGP.COM's key server.
PGP Fingerprint: A655 F61C 5103 F561 6D30 AAB2 2DD1 562A
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