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The OWASP Foundation
OWASP
http://www.owasp.org
OWASP Europe Conference 2008
Know Thyself
Dieter GollmannHamburg University of TechnologyGermany
OWASP
Thesis
To enforce (same) origin policies, you have to be able to authenticate origin.
Even when you are unable to authenticate the origin of inputs provided by others, you may still be able to authenticate your own.
Can we use an elementary security primitive “Recognizing yourself”?
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Owls to OWASP
Web application vulnerabilities like XSS or XSRF are a major security concern today.
Is there something to be learned from security mechanisms developed for mobile systems?
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Cross-site Scripting
firewall
attacker.comuntrusted zone
trusted zone
Formwith[instr.]
Malicious instructions
encoded as data
Formwith[instr.]
Pagewithinstr.
page with malicious
instructions
Reflected XSS
Pagewithinstr.
Stored XSS
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XSRF
firewall
attacker.comuntrusted zone
target system
formwithinstr.
form with malicious
instructions
user
authenticated tunnel
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Cross-site scripting (XSS)
Parties involved: Attacker, client (victim), server (‘trusted’ by client).
Attacker places malware on a page at server (stored XSS) or gets victim to include attacker’s code in a request to the server (reflected XSS).
Malware contained in page returned by server to client; executed at client with permissions of the trusted server.
Violation of an origin based security policy.
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XSRF attack
Parties involved: Attacker, user, target website (victim).
User authenticated at target (cookie, SSL session,…). User has to visit the attacker’s webpage that
contains hidden malware, e.g. in an HTML form. When the user browses this page, it automatically
submits the malware to a target site where the user has access.
Target authenticates request as coming from user; form data accepted since it comes from a legitimate user.
Evades target’s origin based security policy.
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Causes & Defences
Ultimate cause of XSS attack: The client only authenticates ‘the last hop’ of the entire page, but not the true origin of all parts of the page.
Ultimate cause of XSRF attack: The server only authenticates ‘the last hop’ of the entire page, but not the true origin of all parts of the page.
Defence 1: Do not rely on origin based policies; differentiate between code and data instead.
Defence 2: Authenticate your own requests.
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Defence 1
Filter client inputs, sanitize server outputs, escape dangerous characters.
Do you know all paths malicious code can arrive? DOM-based XSS!
Do you know how filtered input is processed further? Will a helpful component change く or ‹ to <?
Do you know about all interactions between different layers of abstraction? Addslashes and the GBK character set (simplified Chinese): 0xbf27 0xbf5c27 ( 0x27 encodes ‘ ) Chris Shiflett: http://shiflett.org/blog/2006/jan/addslashes-
versus-mysql-real-escape-string
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Interlude – Mobile IPv6
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MIPv6 Security (RFC 3775)
Mobile node has secure tunnel to home agent where it as a home address.
Mobile node moves and informs correspondent node about its new care-of-address address.
Correspondent node updates its binding cache. A malicious (mobile) node may lie about its
location; e.g. to launch a bombing attack. Secure binding update with return routability:
Challenges sent to identity and location, response binds identity to location.
Cryptographic keys are sent in the clear!
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MN
CNhome
3: MAC(Kbm;CoA, CN, BU)
Binding update protocol
HoTI
CoTI
Challenge sent to location
HoT: K0, i
CoT: K1, j
Challenge sent to home address
binds home address to
location
[RFC 3775]
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BU Protocol
1. Mobile sends two BU init messages to the correspondent, one via the home agent, the other on the direct link.
2. Correspondent constructs a key for each of the two BU init messages, returns these keys K0 and K1 independently to mobile.
3. Binding key Kbm = SHA-1(K0,K1) used by mobile to authenticate the binding update; authentication binds location (care-of-address) to identity (home address).
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Ownership of addresses
Dynamic address allocation schemes checks that a new address is still free: broadcasts query asking whether any node on the network already uses this address.
Squatting attack: Attacker falsely claims to have the address that should be allocated; prevents victim from obtaining an address in the network.
Can a node prove that it “owns” an IP address without relying on third party (home agent, CA)?
Use public key cryptography without an PKI.
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CGA: Cryptographically Generated Addresses Address owner creates a public key/
private key pair; hash of the public key as interface ID in an IPv6 address.
Mobile node signs BU information with its private key; sends the signed BU together with its public key to the correspondent.
Correspondent checks that the public verification key is linked to the IP address.
Address is “certificate” for its public key.
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Cryptographically Generated Addresses (basic idea)
subnet prefix interface ID
two reserved bits
private key
hash
public key
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Back to Web applications
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Authenticating origin
Authentication: “Know whom you are talking to.” Bad definition: Computers do not talk; too narrow
generalization from communications security. Better: Authentication binds different elements in an
IT system, e.g. identity and location. Web applications: Bind parts of a page to their origin. This may be difficult, and may not be necessary. Know thyself: Recognize the parts you have
contributed. Identification: Recognize those parts that come from
the same source. Pekka Nikander: identidem = same as before
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Authentication in Web applications
XSS defence: Server sends unpredictable one-time URLs to the client during session establishment; server can recognize these URLs as ‘its own’ and can authenticate requests as originating directly from the client. One-time URLs have to be stored in a safe place at the client. Source: Martin Johns: SessionSafe
Server initiated XSRF defence: Authenticate requests at level of the Web application (‘above’ the browser); application sends authenticators with each action: XSRFPreventionToken, e.g. HMAC(Action_Name+Secret,
SessionID); Random XSRFPreventionToken or random session cookie.
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Authentication in Web applications
Client-side only XSRF defence for HTTP layer sessions :
Proxy between browser and network marks all URLs in incoming web pages with unpredictable token; keeps database associating tokens with domains.
Proxy checks all outgoing requests: Know thyself: If a token is found, the request did not
originate in the client. Identification: Proxy then checks whether its origin matches
the domain the request is sent to. Otherwise, all authenticators (SIDs, cookies) added by the
browser are stripped from the URL. Source: Martin Johns, RequestRodeo.
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DNS rebinding
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DNS rebinding
Web browsers enforce same origin policy: Applet can only connect back to the server it was downloaded from.
To make a connection, the client’s browser needs the IP address of the server.
Authoritative DNS server resolves ‘abstract’ DNS names in its domain to ‘concrete’ IP addresses.
Client’s browser ‘trusts’ the DNS server when enforcing the same origin policy.
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Trust is Bad for Security!
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DNS rebinding attack
“Abuse trust”: Attacker creates attacker.org domain; binds this name to two IP addresses, to its own and to the target’s address.
Client downloads applet from attacker.org; applet connects to target; permitted by same origin policy.
Defence: Same origin policy using IP addressD. Dean, E.W. Felten, D.S. Wallach: Java security:
from HotJava to Netscape and beyond, 1996 IEEE Symposium on Security & Privacy.
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DNS rebinding attack
Next round: Javascript, 2001. Client visits attacker.org; attacker’s DNS server
resolves this name to attacker’s IP address with short time-to-live.
Attacker rebinds attacker.org to target’s address. Malicious script connects to attacker.org, which
now is resolved to the target’s address. Defence: Do not trust server on time-to-live; keep
time yourself and pin host name to original IP address. J. Roskind: Attacks against the Netscape
browser. in RSA Conference, April 2001.
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DNS rebinding attack
Next round: Browser plug-ins that do their own pinning.
More sophisticated authorisation system: Client browser refers to policy obtained from DNS server when deciding on connection requests.
Dangerous constellation:Communication path between plug-ins.Each plug-in has its own pinning database.
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DNS rebinding attack
Attacker may use the client’s browser as a proxy to attack the target.
Defence: Centralize security controls; one pinning database for all plug-ins; e.g., let all plug-ins use the browser’s pins.
Defence: Do not ask DNS server for the policy but the system with the IP address a DNS name is being resolved to.Similar to reverse DNS lookup.Similar to defences against bombing attacks.
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Conclusions
You cannot enforce a security policy if you cannot authenticate the attributes it refers to.
Challenge: With origin based policies, how to authenticate the location a data item came from? It might have travelled a long way.
Which end point is being authenticated? Terms like ‘the server’ or ‘the client’ are too imprecise.
Challenge: How to authenticate location without a suitable infrastructure?Know thyself and double check?
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What to look out for?
Mashups, web feeds, and syndications. With same origin policies the fun is just
starting.HTTP access control headers for cross-domain
policies.AJAX cross-domain policies.
Who will set those policies? Who will enforce those policies? Who can authenticate their attributes?