CVE-2026-56104: Chainlit Session Hijacking Vulnerability (CVSS 8.2)

Technical summary

CVE-2026-56104 is a session restoration bypass in Chainlit's WebSocket implementation. The restore_existing_session endpoint accepts a sessionId parameter from unauthenticated or low-privileged clients without cryptographic ownership verification, violating the principle of secure session management. An attacker who obtains a valid sessionId—through network interception, logs, or side-channel disclosure—can initiate a WebSocket session and inherit all permissions and role assignments bound to that session. The vulnerability exists because the session restoration logic trusts the client-supplied sessionId without validating that the requester is the legitimate session owner. This enables privilege escalation and lateral movement within the application context.

Business impact

Session hijacking in Chainlit exposes organizations to data exfiltration, unauthorized tool execution, and potential lateral movement within connected workflows. If Chainlit is used for conversational access to backend systems, databases, or APIs, a hijacked session can bypass access controls entirely. Regulatory compliance risks include HIPAA, PCI-DSS, and SOC 2 violations if user sessions contain sensitive health, financial, or audit data. For enterprises using Chainlit as a customer-facing or internal interface layer, a widespread session hijacking attack could compromise multiple users simultaneously, leading to reputational damage and incident response costs.

Affected systems

Chainlit instances running versions prior to 2.10.1 are affected. The vulnerability is present in the WebSocket session restoration mechanism, so any Chainlit deployment exposing WebSocket endpoints to network access is at risk. Organizations should inventory Chainlit versions in production and pre-production environments. Verify your deployment version by checking the application code, package manifest, or running-instance metadata (e.g., via Docker image tags or pip freeze output).

Exploitability

The CVSS 3.1 score of 8.2 (HIGH) reflects the moderate complexity of exploitation: an attacker must possess at least user-level access to the system initially (PR:L), but the attack does not require user interaction and can compromise confidentiality and integrity across service boundaries (S:C, C:H, I:H). Exploitation is practical: an attacker with low privileges can enumerate or obtain valid session IDs from logs, error messages, or memory inspection, then replay them to escalate. The attack surface is bounded by WebSocket availability, but in typical deployments WebSocket endpoints are as accessible as the main application. No public exploit is tracked in CISA's Known Exploited Vulnerabilities catalog as of the latest data.

Remediation

Immediately upgrade Chainlit to version 2.10.1 or later. This release includes cryptographic ownership verification in the session restoration logic, ensuring that only the legitimate session owner can restore a session. Prior to upgrading, apply network-level mitigations such as IP whitelisting, WebSocket endpoint rate limiting, and mandatory VPN access if Chainlit is not strictly internal. Rotate or invalidate all active sessions post-patch to clear any hijacked tokens from circulation. Review access logs for suspicious session restoration activity.

Patch guidance

Upgrade Chainlit to 2.10.1 or later via pip (pip install --upgrade chainlit>=2.10.1) or your container registry. Test the upgrade in a non-production environment first to validate compatibility with custom extensions and connected tools. After upgrading, restart all Chainlit processes and workers. Verify the upgrade by checking the application version string in logs or the /info endpoint if exposed. No database migration or configuration change is required. Consider scheduling the patch during a maintenance window to minimize user disruption if Chainlit is customer-facing.

Detection guidance

Monitor WebSocket logs for repeated restore_existing_session requests using the same or different sessionIds from a single source IP or within an abnormal time window. Alert on session restoration attempts from IPs that do not match the original session creation location (if geo-location data is available). Implement session integrity checks: log all session restoration events with requester IP, user agent, and timestamp, then correlate with actual user login records. Use SIEM rules to detect deviations from baseline session lifecycle patterns. If Chainlit exports logs to a centralized system, search for sessionId values appearing in restoration requests that differ from the session creator's identity.

Why prioritize this

This vulnerability merits immediate attention due to its HIGH severity (8.2 CVSS), low attack barrier once initial access exists, and direct impact on user data and system integrity. While not yet in active exploitation (KEV status false), the flaw is straightforward to weaponize and affects all users of Chainlit. Organizations relying on Chainlit for sensitive workflows—such as AI-assisted data analysis, customer service automation, or backend system orchestration—should prioritize patching within 24–48 hours. The requirement for low-level initial access means internal threats, compromised accounts, or supply-chain scenarios are realistic threat models.

Risk score, explained

The 8.2 HIGH risk score reflects a high-confidence vulnerability with significant impact. Confidentiality and integrity are both rated as HIGH because a hijacked session grants full access to user data and the ability to modify or execute restricted operations. The requirement for prior low-level access (PR:L) prevents this from being a CRITICAL 10.0, but the cross-service scope impact (S:C) elevates it above typical user-level exploits. The lack of active exploitation (KEV false) and the absence of public tooling somewhat mitigate the score, but the technical simplicity of exploitation once access is obtained keeps it firmly in the HIGH band.

Frequently asked questions