Web Browser
Privacy and Security
Part I
Today’s Topics

Trusted Paths

Context-Sensitive Certificate Verification
(optional paper)
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Trusted Paths

Trusted paths are used to help users
ensure that they are communicating with
whom they think they are
• Ex. Ctrl-Alt-Del in Windows systems cannot
be intercepted

Trusted paths for Web are difficult because
• From remote server to browser to user
• Trivial to make fake UIs that look legit
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Example Attack #1
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Example Attack #1
Is this from eBay?
No trusted path, hard to tell
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Example Attack #2
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Example Attack #2
Is this from eBay?
No trusted path, hard to tell
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Example Attack #3
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Example Attack #3
Is this from eBay?
No trusted path to real
eBay to verify
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
One Idea: Dynamic Security Skins



User remembers one image
• Shown in a trusted window
User remembers one password
• Ease of use
• Sites get hashed password only
Uses Secure Remote Password w/ server
• Generated using a shared secret
Dhamija and Tygar, The Battle Against Phishing:
Dynamic Security Skins, SOUPS 2005
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
How to Show Trusted Path

Static security indicators
• Ex. Secure window uses a certain color border
• Ex. Secure window uses lock icon
• Rejected, too predictable and easy to spoof

Custom security indicator
• Ex. One indicator per site
• Ex. One indicator per user
• Rejected, too much effort
• (Also too much to remember)
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Dynamic Security Skins
• In theory, lots of images
should make it hard to spoof
• Trusted path to password window
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Dynamic Security Skins
• A unique pattern is generated by
each web site (visual hash)
• Trusted path from password entry
to web site
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Another Idea: Tokens

Two factor authentication
• Something you have
• Usually cryptographic

SecureID

Smart cards

Random cryptographic tokens

Scratch cards
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
A Third Idea: Mobile Phones

Everyone’s got a mobile phone

Client side certificates
• Private keys generated/stored on phone
• New key for each phone

Keys linked to domain names

Key generated upon new connection

Bluetooth from phone to PC

Very few server modifications
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Discussion of Trusted Path

“[O]n each launch of Firefox, paint the
Firefox interface with a nonintrusive,
randomly generated pattern. Because sites
wouldn’t be able to replicate this pattern,
users would know when they were viewing
[a] spoofed UI”

Other ideas for trusted paths?

Other barriers to adoption?
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Today’s Topics

Trusted Paths

Context-Sensitive Certificate Verification
(optional paper)
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Certificates

A secure way of binding a public key with
an identity
• Ex. Amazon sends its certificate via https
• Makes it easier to encrypt communications

How to know if this certificate is legitimate?
• Certificate is also signed by a well-known
certificate authority (CA)
• Certificates of these CAs often included
in web browser
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Self-Signed Certificates

Some sites use self-signed certificates
• Want to avoid monetary and overhead costs
• Often leads to security alerts like below
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Why Certificate Verification Fails
1.
Browser may not know public key of the CA
that issued the server’s certificate
• Internal web server (only by members of the
organization) (significant annual fee)
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Why Certificate Verification Fails
1.
Browser may not know public key of the CA
that issued the server’s certificate
• Internal web server (only by members of the
organization) (significant annual fee)
• Own CA: public key installed in browser (no
verification errors), but large number of users /
user owned computers means high maint
2.
Issuer’s or the server’s certificate may be
expired
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Why Certificate Verification Fails
3.
Common name of certificate does not
match server’s fully qualified domain name
♦ Mistake, ex. s3.acme.com vs s10.acme.com
♦ Might be attacker using his own identity with a
CA generated certificate (difficult)
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Aside: Phishing Attack
Signed certificate from
Equifax / Geotrust
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Why Certificate Verification Fails
3.
Common name of certificate does not
match server’s fully qualified domain name
♦ Mistake, ex. s3.acme.com vs s10.acme.com
♦ Might be attacker using his own identity with a
CA generated certificate (easy, but expensive)
♦ Might be attacker using a stolen certificate
(along with the private key) (difficult)
♦ Or might be self-signed certificate (easy)
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Why Certificate Verification Fails
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Why Certificate Verification Fails
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Discussion
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Context-Sensitive Certificate Verification

Clarify relationship between user and
server’s (non verified) certificate
• Not giving the user override mechanisms

Distribute signed certificates of internal
servers out of band

Use typically unused certificate fields:
• CA’s contact information (field: issuer
alternative name)
• CA administrator’s name, address, telephone
and fax numbers, and work hours.
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Context Sensitive Certificate Verification
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
If you said you are
an internal member…
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
If you said you are
an external member…
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Specific Passwords Warnings

Helps prevent eavesdropping

Allow overriding

Existing version:
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Specific Passwords Warnings
Is this an
important account?
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Specific Passwords Warnings
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Discussion

Thoughts so far on designs?
• Context-sensitive Certificate Verification
• Specific Password Warnings
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
User Studies

Computer literate users (CLU)

Evaluate:
• Likelihood of successful attack in
representative security-sensitive Web apps
• Possibility of “foolproofing” browsers, so they
can be used securely even by untrained CLUs
• Can education about the relevant security
principles, attacks, and tools improve the
security of how users browse the Web?
 Note: This last hypothesis is not covered in this
presentation
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Study’s Design

17 male participants (Pitt CS seniors)

Two studies:
• Unmodified browser (IE)
• Modified Mozilla Firebird 0.6.1 with
CSCV and SPW

No feedback given between these two
studies
• (Note: ordering not randomized)
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Study’s Design

Visit three fictional but realistic sites
• Students given password protected accounts

Site1: “maintained by Pitt”
• Monitor reward points (do well in exams, etc)
• HTTPS + Certificate issued by internal CA

Site2: “e-merchant not affiliated with Pitt”
• Spend reward points on books, CDs, etc.
• HTTPS + bogus certificate

Site3: “users’ Web email accounts”
• HTTP only (no certificate)
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Study’s Design
User’s Action
Access to a site despite lack of
security
Simply did not visit the site
insecurely
Correctly obtained and installed
the issuing CA’s certificate
Choosing not to access to 2nd
and 3rd site insecurely
Score
(points)
0
50
100
100
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Study’s Results

Guesses?
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Study’s Results

With current Web browsers, the mentioned
attacks are alarmingly likely to succeed
• More often than not, users’ behavior defeats
the existing Web security mechanisms.
• “um, another of those pop-ups.”
• “I always just click yes when I see these popups.”
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Study’s Results

CSCV blocked MITM attacks against
HTTPS-based applications completely

SPW greatly reduced the insecure
transmission of passwords in an HTTPbased application

Although untrained, users had little trouble
using CSCV and SPW
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Discussion

Thoughts on results?
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/
Discussion

Possible novelty effects
• People might change behavior after getting
used to new messages

Behavior outside of lab study
• People might still not go find person to verify
Usable Privacy and Security • Carnegie Mellon University • Spring 2007 • Cranor/Hong • http://cups.cs.cmu.edu/courses/ups-sp06/