CVE-2026-56278: Flowise - Session Hijacking via Weak Default Express Session Secret

Synopsis

Session hijacking via weak hardcoded default secret in Flowise, the open-source LLM workflow builder. Flowise versions 3.0.13 and earlier fall back to the publicly known string 'flowise' as the express-session signing secret when the EXPRESS_SESSION_SECRET environment variable is not configured. An unauthenticated remote attacker can use this known secret to forge valid signed session cookies and impersonate any user, bypassing authentication entirely. A patched-image rebuild at version 3.1.0 is available on HarborGuard for affected environments.

HarborGuard Coverage

Exploit Conditions

• Network reachabilityRequired

  The attacker must be able to reach the Flowise service over the network; the vulnerability is exposed on any internet- or intranet-accessible deployment.

• AuthenticationNot required

  No account or credentials are needed; the attacker forges a valid session cookie using the publicly known default secret without prior authentication.

• Victim interactionNot required

  No user action is needed; the attacker crafts and submits the forged cookie directly to the server without involving any victim.

• Attack complexityDetail

  Exploit complexity is low: the default secret is static and publicly visible in source code, so cookie forgery is straightforward and requires no special conditions or timing.

Blast Radius

• Attacker forges a session cookie signed with the known default secret and gains authenticated access as any target user, including administrators.
• With administrative session access, the attacker reads all workflow definitions, credentials stored in Flowise, and any data accessible through the impersonated account.
• The attacker modifies or deletes LLM workflows, alters stored API keys or tool configurations, and injects malicious nodes into production pipelines.
• Because the session secret is a deployment-wide default, every active user session in an unpatched deployment with the default secret is vulnerable to impersonation simultaneously.