SPAM OVER IP TELEPHONY (SPIT)Identification and prevention Techniques

Felipe SantosManoj DeshpandeECE 4112 – Internetwork SecurityGeorgia Institute of Technology

Background SPAM considered one of biggest

problems in Internet SPIT is expected to become a major

issue in the next few years with increasing deployment of VoIP solutions

Potential for productivity disturbance is much greater than SPAM

Background Definition: The transmission of unsolicited

calls over Internet telephony (VoIP) “SPITTERS” will forge their identities SPITTING agent capable of placing

hundreds of simultaneous automated calls SIP is not voice only, but applies to Instant

Messaging and video as well

SPAM vs. SPIT

SPAM SPIT

User can sort through or filter messages based on content and header

VoIP is a real time protocol that does not allow grant the receiver access to the contents of the call prior to its acceptance

Email is delivered asynchronously, whenever a user decides to download/access email

Victim is interrupted instantly with the phone ringing

SPAMMER does not know for sure when or whether his message will reach the victim

A successful call guarantees that the user exists, is currently online, and will most likely receive the message soon.

SPIT Prevention Framework Goals:

Minimize false positives & negatives Minimize callee interaction in identifying

SPIT Minimize inconvenience to caller General enough to work in different

environments (work, home, etc) and cultures

SPIT Prevention Framework 5 Stage Approach:

Stage 1: no interaction w/ users Blacklist, Whitelist, Graylisting,

Circles of Trust, Pattern / AnomalyDetection

Stage 2: caller interaction Computational Puzzles, Sender

Checks, Audio CAPTCHAS (Turing Tests)

SPIT Prevention Framework 5 Stage Approach (continued):

Stage 3: feedback before call Manual authorization to receive

call and/or authenticate user Stage 4: during the call

Content analysis (not currentlyviable)

Stage 5: feedback after call Reputation System, Limited-Use

Address, Payments at Risk, Litigation

SPIT Prevention Techniques Blacklists & Whitelists

Pros: Simple implementation Effective (users in whitelist will always be allowed

through and vice versa) Cons:

Manual data gathering by user or global service required to build such lists

SPITTERS can easily spoof identity and bypass lists

SPIT Prevention Techniques Circles of Trust

Inter-domain connections are checked before a call is forwarded. Each domain control its users

Pros: Efficient Even if a user misbehaves, easy to identify user

Cons: Requires a priori inter-domain

agreements/validation Relatively complex implementation

SPIT Prevention Techniques Pattern/Anomaly Detection

Statistical analysis of a user’s calling behavior based on studies that identify “normal” call behavior.

Pros: Potentially most acurate Mature methodology

Cons: Requires monitoring agent to keep track of user

behavior Never before implemented to voice calls

SPIT Prevention Techniques Graylisting

Consists of calculating a gray level for each and every caller

Gray level determines how likely a caller is to be a SPITTER

SPIT Prevention Techniques Graylisting (continued)

Progressive Multi Gray-Leveling (PMG) Considers two levels per caller: short-term level and long-

term level Short-term level

considers the number of calls a given user places within a short period of time (i.e. 10 min)

Level changes rapidly - Prevents DoS attacks Long-term level

considers the number of calls a given user places within a long period of time (i.e. 10 hours)

Level changes slowly – prevents SPITTER from regaining calling rights

SPIT Prevention Techniques Graylisting (continued)

Progressive Multi Gray-Leveling (PMG) (continued) A threshold is established, such that if

(short-term level + long-term level) > ThresholdA user’s outgoing call is blocked

SPIT Prevention Techniques Graylisting (continued)

Pros: Effective caller limiting approach Relatively simple implementation Makes a SPITTER’s task much harder

Cons: Legitimate users can potentially have calls

blocked just for placing too many calls within a given time frame.

SPIT Prevention Techniques Computational Puzzle

Verify a caller’s “willingness” to place the call by imposing that the client solves a digital puzzle/calculation prior to call establishment

Caller must spend at least a given minimum period of time to ensure solution is not “guessed”

Pros: Limit a SPITTER’s calling rate by adding required

computational overhead to establish Cons:

Increased overhead for call establishment Could be relatively easily circumvented

SPIT Prevention Techniques Sender Check

Verify/authenticate a caller by actively consulting its domain

Equivalent of Sender Policy Framework (SPF) and Sender ID in email

Pros: Originating domain certifies its users Prevents user ID spoofing

Cons: Relies on remote domain information that may not be

correctly implemented or updated

SPIT Prevention Techniques Turing Test

Differentiate between automated computer placed calls (likely SPIT) and calls placed by human beings

Uses Audio Completely Automated Public Turing test to tell Computers and Humans Apart (CAPTCHAS)

Pros: Quickly and easily identify automated vs. human calls

Cons: Increased overhead for connection establishment Could potentially block non-SPIT automated calls (banks,

package delivery notifications, reverse 911, etc)

SPIT Prevention Techniques Consent-Based Communication

User authentication / identity verification Callee authoizes caller a priori with a previously

exchanged key or passphrase Pros:

SPIT is completely blocked, since only authorized callers can place call to user

Cons: Any new caller who wishes to contact a user must

request and receive the shared key a priori

SPIT Prevention Techniques Content Filtering

Process call content to detect SPIT as done in SPAM filters

Pros: If viable, would be the most accurate technique

Cons: Not viable / implementable. Although there exist DSP

algorithms to analyze audio data and convert audio waveforms to ASCII text, process is not real-time and call contents are not available for processing until after the call is actually placed.

SPIT Prevention Techniques Reputation System

Centralized reputation score based on user behavior and other users’ feedback

Pros: Centralized global resource to identify SPITTERS

Cons: Requires protocol standardization for feedback

framework

SPIT Prevention Techniques Payments at Risk

Require a refundable payment for each call from an unknown party. The payment is only refunded if the caller was not a SPITTER.

Pros: Increase cost / decrease profitability of SPIT

Cons: Quite unrealistic scenario, since a standardized

framework would be required for feedback and payment charging and many VoIP services are free and fully p2p

Lab VoIP Testbed

Lab Exercises Students will:

Configure and setup the VoIP testbed Establish an authenticated VoIP call and notice

a SPITTER’s inability to contact a user that requires caller authentication

Create a SPIT message Place an automated SPIT call by capturing and

replaying the SPIT message created above Place an automated SPIT call with a spoofed ID

Exercise Results User Authentication (with shared keys)

Exercise Results User Authentication (no shared keys)

Exercise Results Creating SPIT Message & Generating

Automated SPIT Call

Exercise Results Spoofing Caller ID

References J. Quittek, S. Niccolini, S. Tartarelli, and R.

Schlegel, “Prevention of Spam over IP Telephony,” NEC Technical Journal, vol. 1, no. 2, Feb., pp. 114-119, 2006.

D. Shin and C. Shim, “Voice Spam Control with Gray Leveling,” Proceedings of 2nd VoIP Security Workshop, Washington DC, June 1-2 2005.

F. Hammer et al. “Elements of Interactivity in Telephone Conversations,” Proceedings of 8th International Conference on Spoken Language Processing (ICSLP/INTERSPEECH 2004), Vol3, pp.1741-1744, Jeju Island, Korea, Oct. 2004.