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Cross-Site Scripting (XSS)

What is XSS? A vulnerability in Web applications that lets attackers inject client-side scripts into third-party Web pages Browsers of other visitors of compromised Web page run the script – expose any data browser handles Popularity of these exploits grows and has surpassed buffer overflow exploits

Non-persistent XSS Vulnerability Web server does not properly sanitize user input but uses it as is to generate a dynamic reply (Web page) – This reply contains attackers script code Attacker can craft the URL with his script embedded in it – URL points to the target site, supplies some input + script – Entice user to click on URL – Script will steal some user info that user shares with the site, e.g. a cookie

Example Attacker User Google 1. Click here: …. 2. Send in HTTP GET as argument to Google homepage: something …. 3. Attackers script executes with Googles privs

Persistent XSS Vulnerability Data provided by attacker is stored by server and displayed to any future user – E.g. when posts to online message boards are not properly sanitized Such a script can access any content the compromised server can

Where Do Vulnerabilities Occur In server code that processes user input and dynamically renders the resulting page In client code that runs in browser and renders Web pages with data from the server – JavaScript mostly – Document Object Model (DOM) – standard model for representing HTML and XML content

Defense: Escape User Input Ensure that characters of input are treated as data, not as code – Translate any dangerous characters into another form of the same characters that cannot be interpreted as code – E.g., translate < into < Some input could be encoded into different charset – Enforce charset in each server reply so that interpretation of users input is fixed

Defense: Validate User Input Some Web sites want to allow users to input and render HTML – E.g., use HTML markup in s and online posts – Escaping doesnt help here since it would destroy HTML markup – User input must pass through the HTML policy engine to ensure it does not contain XSS

Defense: Cookie Security Because XSS can be used to steal cookies, sites cannot rely only on cookies for authentication – Tie cookies to specific IPs – HTTP Only flag in browsers allows access to cookies from HTML documents only (scripts cannot access them)

Defense: Disabling Scripts Browser-side defense – Makes some Web pages not render – Could be turned off for some sites which are trusted to be well secured against XSS

XML Randomization XSS Defense Web application randomizes XML tag prefixes before delivering a document to client – Hard for attacker to predict randomized prefixes – Cannot inject scripts into application input Noncespaces: Using Randomization to Enforce Information Flow Tracking and Thwart Cross-Site Scripting Attacks, Matthew Van Gundy and Hao Chen. In Proceedings of the 16th Annual Network and Distributed System Security Symposium (NDSS), 2009.

Insertion Vectors Tag body – review.text = attack() Node splitting – review.text = attack() Attribute value – review.contact = javascript:attack() Attribute splitting – review.contact = onclick=javascript:attack() Tag splitting – review.contact = > attack() Noncespaces: Using Randomization to Enforce Information Flow Tracking and Thwart Cross-Site Scripting Attacks, Matthew Van Gundy and Hao Chen. In Proceedings of the 16th Annual Network and Distributed System Security Symposium (NDSS), 2009.

Tag Prefix Randomization XML namespaces – User chooses a prefix for a tag – E.g. for tag: Leverage XML prefixes to annotate document with trust classes – Label of each trust class random and hard to guess by attacker Prefixes randomly chosen on each document delivery Noncespaces: Using Randomization to Enforce Information Flow Tracking and Thwart Cross-Site Scripting Attacks, Matthew Van Gundy and Hao Chen. In Proceedings of the 16th Annual Network and Distributed System Security Symposium (NDSS), 2009.

Example From Paper Noncespaces: Using Randomization to Enforce Information Flow Tracking and Thwart Cross-Site Scripting Attacks, Matthew Van Gundy and Hao Chen. In Proceedings of the 16th Annual Network and Distributed System Security Symposium (NDSS), Attack code

Trust Policy Defines tags that are trusted Defines HTML tags and operations that are allowed in untrusted content Everything else is denied Server delivers both the potentially hazardous content and the trust policy Client browser enforces policy on server-delivered content Noncespaces: Using Randomization to Enforce Information Flow Tracking and Thwart Cross-Site Scripting Attacks, Matthew Van Gundy and Hao Chen. In Proceedings of the 16th Annual Network and Distributed System Security Symposium (NDSS), 2009.

Deployment Both client and server need to be modified Easy add-on to existing software Client proxy can protect multiple clients in a network Noncespaces: Using Randomization to Enforce Information Flow Tracking and Thwart Cross-Site Scripting Attacks, Matthew Van Gundy and Hao Chen. In Proceedings of the 16th Annual Network and Distributed System Security Symposium (NDSS), 2009.

MANET Security

What Is MANET? Mobile Ad-Hoc Network – Wireless nodes – Changing topology – Possibly no trusted authority – Usually battery operated with limited CPU/memory

Security Challenges Wireless medium – Sniffing and jamming are easy, impersonation too Peers as routers – No trust in routers, may sniff, drop or fabricate data Changing topology – Routes are learned, can be manipulated by attackers No trust infrastructure or trusted entities – How to distribute keys Limited resources – Algorithms must be simple and cheap

Physical/Link Layer Attacks Sniffing: attackers can easily pick up wireless transmissions because they are broadcast at specific frequency (MAC spoofing possible too) – Frequency hopping – Directional antennas – Encryption Jamming is easy – But attacker needs powerful transmitter – Directional antennas MAC protocol misuse to monopolize shared medium – How to create a distributed protocol that detects and penalizes misbehavior?

Ad-Hoc Routing Routes are learned when needed (due to mobility) Dynamic Source Routing (DSR) – Source puts entire route in packet header Route discovery – Request messages broadcast – Intermediate nodes add themselves to the message – Reply unicast to the source with full path recorded – Nodes can cache overheard routes and may reply from cache – Link breakage results in error messages that delete routes in the network that use the broken link

Ad-Hoc Routing Ad-hoc On-Demand Distance Vector Routing – Source just specifies destination – Routers on path forward as they see fit Route discovery – Request messages broadcast – Intermediate nodes repeat the message, cache next hop to the source – Reply unicast to the source, intermediate nodes cache next hop to the destination – Intermediate node may reply from cache – When link breaks intermediate node may attempt to rediscover new route – Error messages remove routes that used the broken link

Routing Attacks Routing message flooding (DoS) Routing table overflow – Fill with bogus routes Routing cache poisoning is easy – Just fabricate requests or replies with spoofed source Fabricate false error messages

Network Layer Attacks Drop packets, modify them or replay them Delay packets Inject junk traffic Wormhole Attack – Tunnel packets to another location Blackhole Attack – Make the node part of many routes – Drop all traffic

Wormhole Attacks Attacker records traffic at one point in MANET, tunnels it (perhaps selectively) to another point and replays it Replayed traffic can arrive sooner than original traffic – This leads to an attacker node becoming part of many routes Attack works even for traffic not going over attacker nodes directly, and for encrypted traffic Wormhole attacks in wireless networks, Yih-chun Hu, Adrian Perrig, David B. Johnson, IEEE Journal on Selected Areas in Communications, 2006

Detection of Wormhole Attacks Packet leash – Information added to the packet to restrict the distance it can travel in one hop – Geographical – recipient must be close to sender. Sender records its location and time when packet is sent, recipient checks for validity. – Temporal – packet lifetime ends after certain time. Sender records the time when packet is sent, recipient checks for validity. Requires synchronized clocks Recorded information must be signed Wormhole attacks in wireless networks, Yih-chun Hu, Adrian Perrig, David B. Johnson, IEEE Journal on Selected Areas in Communications, 2006

DoS Attacks Consume node battery, CPU or memory Overflow nodes routing table Flood the node with routing messages Flood the node with data traffic Drop nodes data traffic