Where Do All the Attacks Go?

Dinei Florencio and Cormac HerleyMicrosoft Research, Redmond

Why isn’t everyone hacked every day?

• Webroot Survey:– 90% share passwords across accounts– 41% share passwords with others– 20% use pet’s name as password

• Endless stream of new attacks every year– E.g. read LCD screens from reflections etc

• If things are so bad, how come they’re so good?

Traditional Threat Model

• Alice is a user• Charles attacks– Phishing, keyloggers, guessing, password-reuse– Malware, rootkits, – Physical side-channels, …………

• Security as good as weakest link

CharlesAliceAttacks

Charles

Problems with the threat model8. It is numerically impossible (2 billion users)• At 1000:1 ratio (i.e. 2 million attackers)• Attackers = 1/3 as many as sw developers• US undergrad gets 50x more attention from Profs

than Alice gets from Charles.• Idea that someone identifies/exploits weakest-link

does not scale.

9. Fails to explain the observations• 20% choose dog’s name as password• Avoiding Harm ≠ Security

A Threat Model that Scales

• Population of users• Population of attackers• Attacker doesn’t know you from a honeypot

• Attack when Expected{Gain} > Expected{Cost}

Attacks Internet UsersAlice(i)

AttackersCharles(j)

Attacks

• Alice(i) exerts effort ei(k) against Attack(k)

• Probability she succumbs: Pr{ei(k)}– Pr{ei(k)} monotonically decreasing with effort

• Gain to Charles(j) from Alice(i): Gi

• Cost for Attack(k), N users: Cj(N,k)

Pr{ei(k)}

ei(k)

# UsersCo

st

Charles(j) Expected Return Uj(k)

So, Charles(j) gain:(1-Pr{SP}) - (N,k)

Prob. Alice(i)succumbs

Gain fromAlice(i)

Cost of Attack(k)For N users

• Charles(j) selects Attack(k) that maximizes Uj(k)

Prob. fraud detected

Uj(k) =

Sum-of-efforts Defense

(1-Pr{SP}) Σi Pr{ei(k)} Gi - Cj(N,k)

Sum over all attacked users ofweighted efforts against Attack(k)

• Recall as ei(k) increases Pr{ei(k)} decreases• Increasing effort from users decreases return

Followed by Best-Shot Defense

(1-Pr{SP}) Σi Pr{ei(k)} Gi - Cj(N,k)

Fraud detection at Service Provider:Charles(j) must evade all detection measures

•

So, where do all the attacks go?

Average Success Rate Too Low

• Attack unprofitable if:

(1-Pr{SP}) Σi Pr{ei(k)} Gi < Cj(N,k)

• If average success = 1/N Σi Pr{ei(k)} is too low then whole attack unprofitable.

• Even if many profitable targets exist

• Similarly, if average value too low– i.e. Gi small

Attackers Collide Too Often• Recall attackers compete for vulnerable users

• Suppose Attack(k) has deterministic outcome1 if ei(k) < ε

0 otherwise• Example: brute-force using 10 popular pwds– abcdef, password, 123456, password1, etc

• Every attacker who tries succeeds in same places• If ei(k) < ε Alice(i) ends up with M attackers in acct– In general share Gi with MPr{ei(k)} other attackers

Alice(i)

Charles(j)

Pr{ei(k)} =

Attack(k) too expensive (relative to alternatives)

• Attack(k’) is cheaperUj(k) < Uj(k’) for all attackers

• Example: real-time MITM vs. pwd stealing

Fraud Detection Too High

(1-Pr{SP}) Σi Pr{ei(k)} Gi - Cj(N,k)

• Pr{SP} 1 then return 0• Example: – Alice(i)’s bank detects 99% of attempted fraud– True protection is not Alice(i)’s effort

The Free-Rider Effect

• Suppose brute-forcing is a profitable attack• All-but-one Internet users (finally) decide to

get serious and choose strong passwords– Alice(i0) continues with “abcdef”

• Profitability of brute-forcing plummets– Alice(i0)’s risk of harm 0 (w\o action on her part)

Choosing Your Dog’s Name as Password

• User chooses bank password = dog’s name• Easy money, right?

• How many users have………– Bank password = dog’s name? Say, 1%– Auto discover dog’s name? Say, 1%– Auto discover userID? Say, 1%

• How many other Charles(j) use strategy? Say, 100• Return is reduced by 108

Dog’s Name as Password• Suppose instead:– 10 mins to discover dog’s name– 10 mins to discover userID

• Thus 20 mins on average to get 1% of accts.– Compete with 10 other attackers– Bank catches 90% of attempted fraud

• At $7.25/hour acct should be worth Gi > (10x10x100/3)x7.25 = $24200

• Suppose he makes (US min wage)/10– Needs: Gi > $2420/acct

• Exercise: find profitable assumptions

Domino Effect of Acct. Escalation

• Leveraging low-value accts to high• Password re-use across accts, etc.

“One weak spot is all it takes to open secured digital doors and online accounts causing untold damage and consequences.” Ives etal 2004

Leverage Low-Value Account To High?• Is this profitable on average

• Given N webmails…– X% are contact email for bank– Y% userID can be determined automatically– Z% of banks email pwd reset link– W% the Secret Questions auto determined

• Return dramatically reduced. For example– 0.1 x 0.01 x 0.1 x 0.05 = 0.00005 (1 in 200,000)– So 5 bank accts for every million webmails

Diversity is more Important than Strength

• Password is …………– Dog’s name, cat’s name– Significant date, sports team– Written under keyboard

• How common a strategy is matters more than how secure it is

Conclusions

• Avoiding Harm ≠ Security• Internet attackers face sum-of-effort

defense• Avoiding harm is much less expensive

than being secure

• “Thinking like an attacker” doesn’t end when an attack is found.

Alice(i)

Charles(j)

“And then what?”