From security@ACROS.SI Tue Oct 24 15:34:40 2000 From: ACROS Security To: BUGTRAQ@SECURITYFOCUS.COM Date: Tue, 24 Oct 2000 11:31:28 +0200 Subject: [BUGTRAQ] ASPR #2000-07-22-1: Remote Retrieval Of IIS Session Cookies From Web Browsers [The following text is in the "iso-8859-1" character set] [Your display is set for the "US-ASCII" character set] [Some characters may be displayed incorrectly] =====[BEGIN-ACROS-REPORT]===== ========================================================================= ACROS Security Problem Report #2000-07-22-1-PUB ------------------------------------------------------------------------- Remote Retrieval Of IIS Session Cookies From Web Browsers ========================================================================= PUBLIC REPORT Affected System(s): Internet Information Server (all versions) and other web servers using "unsecured" session ID cookies Problem: Session ID cookies can be retrieved remotely from web browsers Severity: High (for critical systems only) Solution: (see "Advisory" section) Written: July 22, 2000 Last update: October 23, 2000 Published: October 23, 2000 SUMMARY ======= Our team has analyzed how popular web browsers could be tricked to reveal the session ID cookies 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 Information Server. However, it *should not* be assumed that only this product is affected but rather all vendors of web servers and HTTP session management solutions are urged to review their products for the identified vulnerabilities. Note: We have put quite some 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 IIS's session management and also to provide a workable scenario for exploiting this, and similar, vulnerabilities. INTRODUCTION (same as in ASPR-2000-07-22-2) ============ So you have launched your new web-based e-banking system and protected it with 128-bit SSL. Your users logon to it through their browsers, providing their usernames and one-time passwords, then cryptographically strong random session cookies are computed and sent to their browsers for session authentication. Entire communication is protected with SSL so there's no way anyone could intercept the sensitive cookies. You feel safe; you trust SSL to do its job protecting your users and your system. Their session ID cookies are secure. Or are they? We will show that it could be possible to retrieve the session cookies 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 client scripting issues aside for the time (including cross-site scripting, which is by the way very suitable for stealing cookies). We'll also put all bugs in various SSL implementations aside and assume SSL is working as specified. SESSION ID COOKIES ================== Most "stateful" web-based systems are using session ID cookies for maintaining sessions. A session ID cookie is generated on the server in such a manner that a potential attacker could not guess (or calculate) its value. Usually (and preferably), strong cryptographic algorithms are used for this purpose (BTW, several vulnerabilities have already been identified in various session mechanisms as a result of not using them). Server only provides the user with a session ID cookie when he has proved his identity (by providing username and password, for example). Cookies are generally transmitted between browser and server in plaintext in HTTP headers. For protection against network sniffing and traffic redirection, SSL is often deployed to encrypt and authenticate the communication. Note: While it is generally clear that username:password pairs are indeed authentication data and therefore sensitive, it is many times not clear that session ID cookies are also frequently used for authentication. Numerous web-based financial systems we have seen are using some (stronger) form of authentication for initial login (like one-time passwords or SSL client certificates), while throughout the session they rely entirely on users' presentation of correct session ID cookies. Obviously, for the attacker, stealing such a cookie would mean a successful takeover of user's identity. Hence the notion that in critical systems, session ID cookies are just as sensitive as passwords (effectively they are equivalent to username:password pairs). Note: There may be other kinds of sensitive data contained in cookies (e.g. credit card data). We are confident that competent system developers will be able to extrapolate our findings to their systems in such cases and act accordingly. ANALYSIS ======== Throughout the analysis it is assumed that the attacker is capable of the following: 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 IIS 5 web server (www.test.com), installed a valid SSL key+certificate and written a script (login.asp) that sets a (native IIS) "session ID cookie" when accessed by browser. We have also written another script (cookies.asp), which displays the contents of all cookies sent by the browser. Then we opened a browser, typed "https://www.test.com/login.asp" which executed the script login.asp and our browser was "marked" with the session ID cookie. The cookie was 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.asp" (no SSL here!) we could observe session ID cookie being transmitted to the server over an unencrypted link, thus making it interceptable for a network listening attacker. The conclusion is that even though a session cookie was sent to the browser over an encrypted connection (and would be normally - in a real-life system - sent back to the server over an encrypted connection too), it is also sent to the same server over an 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* any user's browser do so. EXPLOITATION (same as in ASPR-2000-07-22-2) ============ For the purpose of exploitation, there are at least two ways of making the user's browser connect to an arbitrary URL: Malicious E-mail 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 session ID cookie 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 while his browser still holds the session ID cookie. 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 session ID cookies for session 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 the amount of exchanged data), attacker can assume that the user has successfully authenticated to the server and his browser has received the session ID cookie. 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 session ID cookie 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 "http://www.yahoo.com/index.html". 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 session ID cookie 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 session ID cookie for the current session. 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. Identity theft -------------- By knowing the user's session ID cookie, the attacker can hijack his current session, assuming his identity. ADVISORY ======== Microsoft has issued a patch for IIS, available at: http://www.microsoft.com/Downloads/Release.asp?ReleaseID=25233 (IIS 4.0) http://www.microsoft.com/Downloads/Release.asp?ReleaseID=25232 (IIS 5.0) This patch makes it possible for IIS to mark its session cookies as "secure" thus preventing them from being sent over unencrypted connections. Administrators of IIS-based critical systems (e.g. web-banking) are advised to install the patch and configure the appropriate registry options accordingly. It is important to note that our limited testing only covered one web server. There are many other web servers and various session management server add-ons that could be potentially affected by the identified vulnerability. WORKAROUND ========== Users of web browsers can destroy session ID cookies 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 JavaScript attacks 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 session cookies) 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". TESTED SYSTEMS ============== Tests were performed on: Internet Information Server 3.0 - not tested, but its session ID generation is so weak that it is definitely not usable for critical systems even if it's not affected Internet Information Server 4.0 - affected Internet Information Services 5.0 - affected It is important to note that both Internet Explorer and Netscape Communicator *refuse* to send cookies that are marked as "secure" over unencrypted connection - which is a good thing and in our opinion the correct behavior even though RCF 2109 is not sufficiently specific about it. We haven't tested other browsers but have notified their vendors. ACKNOWLEDGMENTS =============== We would like to acknowledge Microsoft Security Response Center for prompt and professional response to our notification of the identified vulnerability. SUPPORT ======= For further details about this issue please contact: Mr. Mitja Kolsek ACROS, d.o.o. Stantetova 4 SI - 2000 Maribor, Slovenia phone: +386 41 720 908 e-mail: mitja.kolsek@acros.si web: http://www.acros.si PGP Key available at PGP.COM's key server. PGP Fingerprint: A655 F61C 5103 F561 6D30 AAB2 2DD1 562A DISTRIBUTION ============ This report was sent to: - ACROS client mailing list - ASPR Mailing List (see http://www.acros.si/aspr_mailing_list.html) - BugTraq mailing list - NTBugTraq mailing list - Win2KSecAdvice mailing list - SI-CERT This distribution is governed by our ASPR Notification And Publishing Policy (http://www.acros.si/aspr_policy.html). Find our other reports on our ASPR page (http://www.acros.si/aspr.html). DISCLAIMER ========== The information in this report is purely informational and meant only for the purpose of education and protection. ACROS, d.o.o. shall in no event be liable for any damage whatsoever, direct or implied, arising from use or spread of this information. All identifiers (hostnames, IP addresses, company names, individual names etc.) used in examples and exploits are used only for explanatory purposes and have no connection with any real host, company or individual. In no event should it be assumed that use of these names means specific hosts, companies or individuals are vulnerable to any attacks nor does it mean that they consent to being used in any vulnerability tests. The use of information in this report is entirely at user's risk. COPYRIGHT ========= (c) 2000 ACROS, d.o.o., Slovenia. Forwarding and publishing of this document is permitted providing all information between marks "[BEGIN-ACROS-REPORT]" and "[END-ACROS-REPORT]" remains unchanged. =====[END-ACROS-REPORT]=====