Security: Network Attacks

Lecture Notes for CS 142
Fall 2010
John Ousterhout

  • Readings for this topic: none.
  • Building secure Web applications is very difficult today:
    • Many opportunities for attackers
    • Hard to identify all the vulnerabilities
    • Even a small mistake can compromise entire application
  • Modes of attack:
    • Attack the connection
      • Steal password
      • Hijack existing connection
    • Attack the server: code injection
    • Attack the browser: code injection
      • Directly
      • Indirectly, e.g. via server
    • Breach the browser, attack the client machine
    • Fool the user (phishing)
  • Network attacks ("man in the middle" attacks):
    • Attacker has access to network communication between browser and server.
    • Passive attacks: eavesdrop on network traffic
    • Active attacks:
      • Inject network packets
      • Modify packets
      • Block packets


  • Solution: use encryption to prevent eavesdropping and detect active attacks.
  • Key Technology: public-key encryption
    • Each principal (user, program, etc.) has two encryption keys, one public, one secret.
    • Information encrypted with one can only be decrypted with the other.
  • But, how do I find out the public key for a particular server?
    • Certificate authority: well-known, trusted server that certifies public keys.
    • Certificate: a document encrypted with the private key of a certificate authority: identifies a particular service along with its public key.
    • Service computes its keys, gives public key to certificate authority (along with proof of identity)
    • Certificate authority returns certificate for that server.
    • Server can pass along this certificate to browsers.
    • Browsers accept certificates from dozens of authorities.
    • Protocol used for secure communication between browsers and servers.
    • Browser uses certificate to verify server's identity.
    • Uses certificates and public-key encryption to pass a secret session-specific key from browser to server.
  • Why isn't HTTPS used for all Web traffic?
    • Expensive: slows down Web servers.
    • Breaks Web page caching

Problems with SSL/TLS

  • SSL stripping:
    • Common use pattern: user browses site with HTTP, upgrades to HTTPS for checkout.
    • Active network attacker interposes on communication.
    • When server returns pages with HTTPS links, attacker changes them to HTTP.
    • When browser follows those links, attacker intercepts requests, creates its own HTTPS connection to server, and forwards requests via that.
    • As a result, the attacker sees all client packets (e.g., passwords).
    • Browser provides feedback to user about whether HTTPS is in use, but most users won't notice the difference.
  • Mixed content:
    • Main page loaded with HTTPS, but some internal content loaded via HTTP (e.g. <script src="http://.../script.js">).
    • Network attacker can modify content to attack page.
    • Some browsers help to notify users:
      • IE7: displays dialog for user, doesn't show SSL lock.
      • Firefox: displays lock icon with "!"
      • Safari: no warnings
      • All browsers: no warnings for unsafe Flash .swf files
    • Common developer error: over-specified URLs:
      <embed src="">
      Instead, don't specify explicit protocols (or even site?):
      <embed src="//">
  • "Just in time" HTTPS:
    • Login page displayed with HTTP.
    • Form posted with HTTPS.
    • Appears secure but it isn't:
      • Active attack corrupts login page (send password someplace else during form post)
      • SSL stripping during form post: nothing indicates that the actual connection didn't use SSL
    • Solution: before returning HTML for login page, check for HTTPS; if page fetched via HTTP, redirect to the HTTPS version.
  • If a certificate is bad/unknown, browser issues warning dialog:
    • Most users can't understand, so they just click OK.
    • Some browsers warn repeatedly, but users will still just click through.
    • This enables various network attacks.