A new cyberattack technique called “Cookie-Bite” enables attackers to bypass multi-factor authentication (MFA) and gain unauthorized access to cloud systems. Varonis Threat Labs discovered that cybercriminals use stolen browser cookies to impersonate legitimate users without needing credentials. This attack targets specific authentication cookies like ESTSAUTH and ESTSAUTHPERSISTENT, which are used by Azure Entra ID and other Microsoft services. By hijacking these cookies, attackers can bypass MFA and move freely across cloud environments, rendering traditional MFA protections ineffective.
The Cookie-Bite attack is powered by several methods to steal authentication cookies.
Cybercriminals use Adversary-in-the-Middle (AiTM) attacks, which intercept cookies in real-time, often via reverse proxy tools. Other techniques include extracting cookies from active sessions using browser memory dumping, accessing cookies through malicious browser extensions, and decrypting locally stored cookie databases. Researchers demonstrated how attackers can create custom Chrome extensions that quietly extract cookies every time users log in to Microsoft’s authentication portal, providing them with ongoing access.
Once attackers steal authentication cookies, they can inject them into their own browsers and gain immediate access to the victim’s cloud session.
This attack’s persistent nature sets it apart from traditional credential theft, as it doesn’t require stealing passwords or intercepting MFA codes. The malicious extensions continuously extract new authentication cookies each time the victim logs in. Even if passwords are changed or sessions are revoked, the attackers retain access, bypassing Conditional Access Policies (CAPs) that are usually deployed as an added security layer.
Security experts recommend several countermeasures to mitigate the Cookie-Bite attack. Organizations should monitor for unusual user behavior and suspicious sign-ins, utilize Microsoft’s Risk detection capabilities, and configure Conditional Access Policies that only allow login from compliant devices. Additionally, limiting browser extensions to an approved allowlist, implementing token protection mechanisms, and adapting security postures to address emerging threats are essential for preventing such attacks.
