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Session Hijacking

An attack that steals a valid session identifier, via XSS, cookie theft, sniffing, or other means, and uses it to impersonate the authenticated user, bypassing the need for credentials.

Session hijacking takes over an existing authenticated session. The attacker obtains the session identifier (often a cookie) and uses it directly, no password needed. Theft can happen via XSS (when HttpOnly isn't set), insecure transport (no HTTPS, no Secure flag), packet capture on shared networks, malware, or other means. Defenses combine cookie hardening (Secure, HttpOnly, SameSite), TLS everywhere, short session lifetimes, and detection of anomalous session use.

Introduced in: Defensive Web Fundamentals

Examples

  • Stealing a session cookie via XSS and using it to impersonate the user.
  • Hijacking a session captured over an insecure HTTP channel.
  • Defending with Secure, HttpOnly, SameSite, and TLS everywhere.