Executive Summary

CVE-2026-56104 is a session hijacking vulnerability in Chainlit versions before 2.10.1. It allows unauthenticated attackers to restore and take over authenticated user sessions by presenting a valid sessionId during WebSocket session restoration without verifying if the attacker actually owns that session.

This happens because the system does not check ownership in the restore_existing_session path, enabling attackers to assume the victim's permissions and roles.

As a result, attackers can invoke tools and access data that should be restricted to the authenticated victim.

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Impact Analysis

This vulnerability can have serious impacts as it allows attackers to hijack user sessions without authentication.

• Attackers can gain unauthorized access to tools and sensitive data that are normally restricted to the legitimate user.
• They can assume the victim's permissions and roles, potentially leading to unauthorized actions within the system.
• This can result in data breaches, unauthorized data manipulation, and compromise of user privacy.

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Detection Guidance

This vulnerability involves unauthorized restoration of WebSocket sessions by presenting a valid sessionId without ownership verification. Detection would involve monitoring WebSocket session restoration attempts, especially those using the restore_existing_session path.

To detect potential exploitation, you can look for unusual or repeated WebSocket session restoration requests with sessionIds that do not match the authenticated user.

Since the vulnerability is specific to Chainlit versions before 2.10.1, verifying the version of Chainlit running on your system is a primary step.

• Check Chainlit version: run `chainlit --version` or check the installed package version.
• Monitor WebSocket traffic for restore_existing_session requests with sessionIds.
• Use network packet capture tools like `tcpdump` or `Wireshark` to filter WebSocket traffic and analyze session restoration messages.
• Look for authorization errors or unusual session restoration failures in Chainlit logs, which may indicate attempts to hijack sessions.

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Mitigation Strategies

The primary mitigation step is to upgrade Chainlit to version 2.10.1 or later, where the vulnerability has been fixed by adding proper session ownership validation during WebSocket session restoration.

Until the upgrade can be applied, consider restricting access to the Chainlit WebSocket endpoints to trusted networks or users to reduce the risk of exploitation.

Monitor logs for suspicious session restoration attempts and revoke any suspicious sessionIds if possible.

• Upgrade Chainlit to version 2.10.1 or later.
• Restrict network access to Chainlit WebSocket endpoints.
• Monitor and audit session restoration logs for unauthorized attempts.

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Compliance Impact

The vulnerability allows unauthenticated attackers to hijack authenticated user sessions and assume the victim's permissions and roles, enabling unauthorized access to restricted tools and data.

Such unauthorized access to sensitive user data and functionality could lead to violations of data protection regulations and standards like GDPR and HIPAA, which require strict controls on access to personal and protected health information.

By enabling session hijacking without ownership verification, the vulnerability undermines the confidentiality and integrity of user sessions, potentially resulting in non-compliance with these regulations.

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