Securing Browser Cookies in Outbound SSO: Best Practices

Securing Browser Cookies in Outbound SSO: Best Practices

In an outbound Single Sign-On (SSO) scenario, a user logs into Site 1, which then authenticates access to Site 2. During this flow, authentication credentials or session tokens are often stored in browser cookies. These cookies become a critical security asset—and must be protected from interception, misuse, or cross-site attacks.

To ensure cookies are secure in transit and at rest, the following HTTP header attributes should always be applied:

Essential Cookie Security Flags

• HttpOnly
  Prevents client-side JavaScript from accessing the cookie. This mitigates the risk of XSS (Cross-Site Scripting) attacks.
  Set-Cookie: sessionId=abc123; HttpOnly
• Secure
  Ensures the cookie is only transmitted over HTTPS connections—never in plaintext over HTTP.
  Set-Cookie: sessionId=abc123; Secure
• SameSite=Strict or SameSite=Lax
  Protects against CSRF (Cross-Site Request Forgery) by restricting how cookies are sent with cross-site requests.
  Use SameSite=Strict where feasible.
  Use SameSite=None; Secure only if cross-site requests (like SSO) require it.
  Set-Cookie: sessionId=abc123; SameSite=Strict

Additional Best Practices

• Set Short Expiry Durations (Expires or Max-Age)
  Session cookies should expire quickly after inactivity to reduce risk.
  Set-Cookie: sessionId=abc123; Max-Age=3600
• Use a Strong, Random Session ID
  Avoid predictable values. Use high-entropy tokens (e.g., UUIDv4 or securely generated hashes).
• Enable HSTS (HTTP Strict Transport Security)
  Forces browsers to only use HTTPS—protecting cookies from downgrade attacks.
  Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
• Regenerate Session IDs on Login
  Prevents session fixation by issuing a fresh cookie after successful authentication.
• Monitor and Invalidate Compromised Tokens
  Use backend session tracking to detect anomalies (IP switching, multiple logins) and invalidate risky tokens.

A Note on Cross-Domain SSO Cookies

In SSO from Site 1 → Site 2, if you’re using cookies across different domains (e.g., site1.com and site2.com), the SameSite attribute must be set to None, and the Secure flag must be present:

Set-Cookie: sso_token=xyz789; Secure; SameSite=None; HttpOnly

Be very cautious: this makes the cookie accessible cross-site, so strong protections must surround its issuance, transmission, and revocation.

Final Thoughts

Cookies are deceptively simple—but a poorly secured cookie is an open invitation for attackers. Especially in SSO workflows, where trust is extended across sites, cookie hygiene is non-negotiable.

Use Secure, HttpOnly, SameSite, and HSTS—and rotate, monitor, and expire aggressively.