Authentication Bypass in Letta through 0.16.8

Executive Summary

CVE-2026-77937 is an authentication bypass vulnerability in Letta through version 0.16.8. It allows attackers to impersonate other users by setting a self-asserted user_id HTTP header to another tenant's ID. This works even with LETTA_SERVER_SECURE enabled, enabling unauthorized access to read and modify agents, memory, and configuration of other users.

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Detection Guidance

Check HTTP headers in Letta API requests for the user_id field being set to arbitrary values. Monitor for unauthorized access to other users' agents, memory, or configuration. Review logs for requests with mismatched user_id and actor_id values.

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Impact Analysis

Attackers could gain unauthorized access to other users' data, modify configurations, or interact with agents as if they were the legitimate user. This could lead to data breaches, loss of sensitive information, or disruption of services relying on Letta.

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Compliance Impact

This vulnerability could lead to unauthorized access to personal or sensitive data, violating GDPR's data protection principles and HIPAA's security requirements. Organizations using Letta may face compliance violations, legal penalties, and reputational damage due to potential data exposure.

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Mitigation Strategies

Upgrade Letta to a version beyond 0.16.8. Disable or restrict the user_id header in API requests. Implement strict validation to ensure user_id matches authenticated actor_id. Review and audit all user permissions and access logs.

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