Detection Guidance

This vulnerability involves a non-privileged user injecting arbitrary JavaScript via the Client header during authentication in Jellyfin versions prior to 10.11.9.

To detect potential exploitation attempts on your network or system, you can monitor HTTP requests to the Jellyfin AuthenticateByName endpoint and inspect the Client header for suspicious or unexpected HTML or JavaScript content.

For example, using command-line tools like tcpdump or tshark to capture traffic and grep or jq to filter headers could help identify malicious payloads.

• Use tcpdump to capture HTTP traffic on the Jellyfin server port (default 8096): tcpdump -A -s 0 'tcp port 8096 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
• Filter captured traffic for AuthenticateByName requests and inspect the Client header for suspicious content, e.g., using grep: grep -i 'Client:' captured_traffic.txt
• Alternatively, use tshark to extract HTTP headers: tshark -r capture.pcap -Y 'http.request.method == "POST" && http.host contains "jellyfin"' -T fields -e http.host -e http.request.full_uri -e http.header.Client

Note that detection requires analyzing the Client header for injected HTML or JavaScript, which may require custom scripts or manual inspection.

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Executive Summary

This vulnerability exists in Jellyfin, an open source self-hosted media server, prior to version 10.11.9. It is a potential Cross-Site Scripting (XSS) attack that allows a non-privileged user to execute arbitrary JavaScript code in the context of a logged-in Administrative user.

The issue arises because the Client header during an AuthenticateByName request can contain arbitrary HTML and JavaScript. When an Administrative user visits the Access tab of the user in question within the dashboard, this malicious code is executed.

This vulnerability was fixed in version 10.11.9 of Jellyfin.

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Impact Analysis

The vulnerability can allow a non-privileged user to execute arbitrary JavaScript code in the context of an Administrative user. This can lead to various security issues, such as unauthorized actions performed with administrative privileges.

Since the attack targets the Administrative user's session, it can result in compromise of administrative controls, potentially allowing attackers to manipulate the media server or access sensitive information.

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Mitigation Strategies

To mitigate this vulnerability, you should upgrade Jellyfin to version 10.11.9 or later, where the issue has been fixed.

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Compliance Impact

The vulnerability allows a non-privileged user to execute arbitrary JavaScript in the context of an administrative user, leading to a high confidentiality impact. This could potentially expose sensitive information managed by the Jellyfin server.

Such unauthorized access or exposure of confidential data may affect compliance with data protection regulations like GDPR and HIPAA, which require safeguarding personal and sensitive information against unauthorized access.

However, the provided information does not explicitly state the direct impact on compliance with these standards.

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