Cookie Tampering: How Attackers Modify Cookies to Break Into Web Apps (And How You Can Prevent It)

Jer Catallo Posted on May 30 Cookie Tampering: How Attackers Modify Cookies to Break Into Web Apps (And How You Can Prevent It) # cybersecurity # tutorial # todayilearned # security Cookies are a common way for web apps to remember who you are and what you can access. The server sets a cookie, sends it to your browser, and trusts it on every request. The problem starts when developers put sensitive data like login status or role flags directly in cookies without any protection. An attacker who can read or modify that cookie can just change the values and gain access they should not have. This blog covers three common cookie tampering techniques: plain-text manipulation, hashed cookie bypass, and base64 decoding. Each one shows a different way developers get this wrong, and what the right fix looks like. Ethical Considerations This demonstration is for educational purposes only. Cookie tampering against systems you do not own or have explicit permission to test is illegal in most jurisdictions. Always practice these techniques in isolated lab environments or authorized penetration testing engagements. Unauthorized access to computer systems violates laws like the Computer Fraud and Abuse Act (CFAA) in the United States and similar legislation worldwide. Step 1: Plain-Text Cookie Manipulation Some apps set cookies with values like logged_in=true and admin=false in plain text. The server sends these to your browser and then trusts them on every request with no further checks. The server sets these cookies after login: Set-Cookie: logged_in=true; Max-Age=3600; Path=/ Set-Cookie: admin=false; Max-Age=3600; Path=/ Enter fullscreen mode Exit fullscreen mode Send a baseline request with no cookies to see the default response: curl http://<target-ip>/cookie-test Enter fullscreen mode Exit fullscreen mode Response: Not Logged In Now send the cookies exactly as the server set them: curl -H "Cookie: logged_in=true; admin=false" http://<target-ip>/cookie-test Enter f