CVE-2023-41893: Account takeover via auth_callback login in Home Assistant Core

Part of the Cure53 security audit of Home Assistant.

The audit team’s analyses confirmed that the redirecturi and clientid are alterable when logging in. Consequently, the code parameter utilized to fetch the accesstoken post-authentication will be sent to the URL specified in the aforementioned parameters.

Since an arbitrary URL is permitted and homeassistant.local represents the preferred, default domain likely used and trusted by many users, an attacker could leverage this weakness to manipulate a user and retrieve account access. Notably, this attack strategy is plausible if the victim has exposed their Home Assistant to the Internet, since after acquiring the victim’s accesstoken, the adversary would need to utilize it directly towards the instance to achieve any pertinent malicious actions.

To achieve this compromise attempt, the attacker must send a link with a redirecturi that they control to the victim’s own Home Assistant instance. In the eventuality the victim authenticates via the said link, the attacker would obtain code sent to the specified URL in redirecturi, which can then be leveraged to fetch an accesstoken.

An attacker could increase the efficacy of this strategy by registering a nearly identical domain to homeassistant.local, which at first glance may appear legitimate and thereby obfuscate any malicious intentions.

Nonetheless, owing to the requirements for victim interaction and Home Assistant instance exposure to the Internet, this severity rating was consequently downgraded to Low.