Industry News

Session Fixation Flaw Keeps Cookies Alive for Major Services after Logout

A new flaw in cookie handling that makes log-ins persistent has been discovered by security researcher Rishi Narang.

When a user logs into an account, the server sends a cookie – a small piece of text – that holds his session ID and tells the server he successfully passed authentication and should be served content without a further log-in prompt when navigating between pages.

Cookies are set to expire, either when they reach their validity date, or when the user logs out. The new discovery, however, reveals that a number of websites such as Yahoo, LinkedIn and Twitter still keep the cookie/session ID for an authenticated session valid even if they have expired or the  user has logged out of his account.

According to the researcher’s report, old cookies for these services can be simply added to the browser and they become valid immediately, even if they are expired or nulled via logout.

“…These cookies are days (sometimes months) old. As a result, someone can successfully access accounts that belong to individuals from different global locations. Even if they would have logged-in/logged out many a times, theirs cookie would still be valid,” reads the blog post.

The situation is even worse for Yahoo users. Earlier this year, a spam message redirected users to a malicious page where they had their cookies stolen. Most have been advised to simply log out of Yahoo services to render the stolen cookies useless for the attacker. If today’s report is true, some of the unauthorized account usage reports may still be the result of the cookie harvesting campaign in January, although those cookies should have gone rotten quite a while ago.

About the author


Bogdan Botezatu is living his second childhood at Bitdefender as senior e-threat analyst. When he is not documenting sophisticated strains of malware or writing removal tools, he teaches extreme sports such as surfing the web without protection or rodeo with wild Trojan horses. He believes that most things in life can be beat with strong heuristics and that antimalware research is like working for a secret agency: you need to stay focused at all times, but you get all the glory when you catch the bad guys.