Vulnerabilities > CVE-2019-15612 - Session Fixation vulnerability in Nextcloud Server

An attacker creates a false but functional session credential in order to gain or usurp access to a service. Session credentials allow users to identify themselves to a service after an initial authentication without needing to resend the authentication information (usually a username and password) with every message. If an attacker is able to forge valid session credentials they may be able to bypass authentication or piggy-back off some other authenticated user's session. This attack differs from Reuse of Session IDs and Session Sidejacking attacks in that in the latter attacks an attacker uses a previous or existing credential without modification while, in a forging attack, the attacker must create their own credential, although it may be based on previously observed credentials.

Attacks on session IDs and resource IDs take advantage of the fact that some software accepts user input without verifying its authenticity. For example, a message queuing system that allows service requesters to post messages to its queue through an open channel (such as anonymous FTP), authorization is done through checking group or role membership contained in the posted message. However, there is no proof that the message itself, the information in the message (such group or role membership), or indeed the process that wrote the message to the queue are authentic and authorized to do so. Many server side processes are vulnerable to these attacks because the server to server communications have not been analyzed from a security perspective or the processes "trust" other systems because they are behind a firewall. In a similar way servers that use easy to guess or spoofable schemes for representing digital identity can also be vulnerable. Such systems frequently use schemes without cryptography and digital signatures (or with broken cryptography). Session IDs may be guessed due to insufficient randomness, poor protection (passed in the clear), lack of integrity (unsigned), or improperly correlation with access control policy enforcement points. Exposed configuration and properties files that contain system passwords, database connection strings, and such may also give an attacker an edge to identify these identifiers. The net result is that spoofing and impersonation is possible leading to an attacker's ability to break authentication, authorization, and audit controls on the system.

This attack relies on the use of HTTP Cookies to store credentials, state information and other critical data on client systems. The first form of this attack involves accessing HTTP Cookies to mine for potentially sensitive data contained therein. The second form of this attack involves intercepting this data as it is transmitted from client to server. This intercepted information is then used by the attacker to impersonate the remote user/session. The third form is when the cookie's content is modified by the attacker before it is sent back to the server. Here the attacker seeks to convince the target server to operate on this falsified information.

In circumstances where an application holds important data client-side in tokens (cookies, URLs, data files, and so forth) that data can be manipulated. If client or server-side application components reinterpret that data as authentication tokens or data (such as store item pricing or wallet information) then even opaquely manipulating that data may bear fruit for an Attacker. In this pattern an attacker undermines the assumption that client side tokens have been adequately protected from tampering through use of encryption or obfuscation.

This attack targets predictable session ID in order to gain privileges. The attacker can predict the session ID used during a transaction to perform spoofing and session hijacking.