CVE-2023-41896 in Home Assistant
Summary
by MITRE • 10/25/2023
Home assistant is an open source home automation. Whilst auditing the frontend code to identify hidden parameters, Cure53 detected `auth_callback=1`, which is leveraged by the WebSocket authentication logic in tandem with the `state` parameter. The state parameter contains the `hassUrl`, which is subsequently utilized to establish a WebSocket connection. This behavior permits an attacker to create a malicious Home Assistant link with a modified state parameter that forces the frontend to connect to an alternative WebSocket backend. Henceforth, the attacker can spoof any WebSocket responses and trigger cross site scripting (XSS). Since the XSS is executed on the actual Home Assistant frontend domain, it can connect to the real Home Assistant backend, which essentially represents a comprehensive takeover scenario. Permitting the site to be iframed by other origins, as discussed in GHSA-935v-rmg9-44mw, renders this exploit substantially covert since a malicious website can obfuscate the compromise strategy in the background. However, even without this, the attacker can still send the `auth_callback` link directly to the victim user. To mitigate this issue, Cure53 advises modifying the WebSocket code’s authentication flow. An optimal implementation in this regard would not trust the `hassUrl` passed in by a GET parameter. Cure53 must stipulate the significant time required of the Cure53 consultants to identify an XSS vector, despite holding full control over the WebSocket responses. In many areas, data from the WebSocket was properly sanitized, which hinders post-exploitation. The audit team eventually detected the `js_url` for custom panels, though generally, the frontend exhibited reasonable security hardening. This issue has been addressed in Home Assistant Core version 2023.8.0 and in the npm package home-assistant-js-websocket in version 8.2.0. Users are advised to upgrade. There are no known workarounds for this vulnerability.
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Analysis
by VulDB Data Team • 11/11/2023
The vulnerability identified as CVE-2023-41896 represents a critical security flaw in Home Assistant's frontend authentication mechanism that enables remote code execution through cross-site scripting attacks. This issue was discovered during a security audit by Cure53, who identified a hidden parameter `auth_callback=1` that when combined with the `state` parameter creates a dangerous authentication flow. The `state` parameter contains the `hassUrl` which is used to establish WebSocket connections, creating a pathway for attackers to manipulate the frontend's connection behavior. This vulnerability operates at the intersection of multiple security concerns including authentication bypass, session manipulation, and client-side attack vectors.
The technical implementation of this flaw stems from improper validation of WebSocket connection parameters within Home Assistant's frontend code. The authentication logic trusts the `hassUrl` value passed through the `state` parameter without sufficient sanitization or validation, allowing attackers to redirect WebSocket connections to malicious endpoints. This creates a scenario where the attacker can intercept and manipulate WebSocket communications between the frontend and backend services. The vulnerability is classified as a cross-site scripting vector under CWE-79, which specifically addresses improper neutralization of input during web page generation. The attack chain involves crafting malicious links with modified `state` parameters that force the frontend to connect to attacker-controlled WebSocket backends, enabling the execution of arbitrary JavaScript code within the legitimate Home Assistant domain context.
The operational impact of this vulnerability is severe and encompasses complete system compromise potential. Since the XSS executes within the actual Home Assistant frontend domain, attackers can establish connections to the real Home Assistant backend services, effectively gaining full control over the home automation system. This represents a sophisticated attack vector that combines authentication manipulation with client-side exploitation. The vulnerability's severity is amplified by the fact that the frontend can be iframed by external origins, as referenced in GHSA-935v-rmg9-44mw, making the compromise covert and difficult to detect. Even without iframe-based obfuscation, attackers can directly deliver malicious `auth_callback` links to victims, creating a straightforward attack path. The attack surface is particularly concerning because it leverages legitimate authentication flows, making it difficult for security monitoring systems to distinguish between normal and malicious behavior.
The mitigation strategy recommended by Cure53 focuses on redesigning the WebSocket authentication flow to eliminate trust in parameters passed through GET requests. The optimal solution involves removing reliance on the `hassUrl` parameter that originates from external sources, instead implementing a more secure authentication mechanism that validates connection endpoints through trusted internal sources. This approach aligns with the principle of least privilege and input validation best practices, addressing the core architectural flaw that enabled the vulnerability. The fix requires modifications to how WebSocket connections are established and authenticated, ensuring that connection parameters cannot be manipulated by external parties. Security researchers have noted that while the frontend demonstrated reasonable security hardening in other areas, the specific WebSocket authentication flow contained a critical design flaw that required fundamental reimplementation.
This vulnerability demonstrates the complexity of securing modern web applications that rely heavily on real-time communication protocols and authentication flows. The issue has been resolved in Home Assistant Core version 2023.8.0 and the npm package home-assistant-js-websocket version 8.2.0, requiring immediate upgrades for all affected users. The remediation process requires careful attention to the authentication flow implementation and proper parameter validation. The security implications extend beyond simple XSS exploitation, as the vulnerability allows for complete system compromise through manipulation of legitimate WebSocket connections. Organizations using Home Assistant should implement immediate patching procedures and consider additional monitoring for suspicious WebSocket connection patterns. The vulnerability serves as a reminder of the importance of validating all external inputs in authentication flows and the critical need for secure WebSocket implementation practices in home automation and IoT systems.