CVE-2026-18904 Overview
CVE-2026-18904 is an authorization vulnerability in IBM Langflow OSS versions 1.0.0 through 1.11.1. The flaw stems from a namespace collision between user identifiers, which allows a remote attacker to access data belonging to other users and inject unauthorized messages. The issue is categorized under [CWE-639] (Authorization Bypass Through User-Controlled Key). Langflow is an open-source framework used to build applications on top of large language models, so exploitation exposes conversational context, prompts, and workflow data across tenant boundaries.
Critical Impact
A remote, unauthenticated attacker can obtain sensitive information belonging to other users and inject unauthorized messages into their sessions by exploiting the collision between user identifiers.
Affected Products
- IBM Langflow OSS 1.0.0 through 1.11.1
- Langflow deployments exposing the affected API surface
- Any downstream service embedding vulnerable Langflow releases
Discovery Timeline
- 2026-08-28 - CVE-2026-18904 published to the National Vulnerability Database (NVD)
- 2026-08-31 - Entry last modified in NVD
Technical Details for CVE-2026-18904
Vulnerability Analysis
The vulnerability resides in how IBM Langflow OSS builds and resolves user identifiers across its multi-user API surface. Two distinct users can resolve to the same internal namespace, allowing one account to reach objects, flows, or messages owned by another. The confidentiality impact is rated High, and the integrity impact is Low because the attacker can inject messages but cannot arbitrarily rewrite records. Availability is not affected. The attack requires network access with no privileges and no user interaction, which broadens the exposure of internet-facing Langflow instances.
Root Cause
The root cause is an insecure design pattern that treats user-controlled identifier fragments as unique authorization keys. When two user identifiers collide within the same namespace, Langflow does not differentiate the underlying account context. This is a textbook [CWE-639] Insecure Direct Object Reference (IDOR), where authorization decisions rely on values that are neither globally unique nor cryptographically bound to the requesting principal.
Attack Vector
An attacker interacts with the Langflow API over the network without prior authentication. By registering or supplying a user identifier that collides with a target's namespace, the attacker gains read access to that user's data and can post messages that appear to originate from the shared context. Refer to the IBM Support advisory for vendor guidance on the affected code paths. No public proof-of-concept or exploit code is currently available.
Detection Methods for CVE-2026-18904
Indicators of Compromise
- Unexpected read operations against flows, sessions, or message stores owned by other users
- Creation of user accounts with identifiers that closely mirror existing usernames or IDs
- Message records with author metadata inconsistent with the originating authenticated session
- API requests to Langflow endpoints from IP addresses that have never interacted with the target tenant
Detection Strategies
- Correlate authenticated session identifiers with the user_id field on every message and flow retrieval to detect mismatches
- Alert on user registration events where new identifiers are substrings, permutations, or case variants of existing identifiers
- Baseline normal per-user query volumes to Langflow APIs and surface deviations that could indicate cross-tenant scraping
Monitoring Recommendations
- Enable verbose audit logging on Langflow authentication, user creation, and message APIs
- Forward Langflow application logs to a centralized analytics platform for retention and correlation
- Track outbound requests from Langflow to LLM providers for prompt content that references unfamiliar users or projects
How to Mitigate CVE-2026-18904
Immediate Actions Required
- Upgrade IBM Langflow OSS to a fixed release above 1.11.1 as directed by the IBM Support advisory
- Restrict network exposure of Langflow instances to trusted networks or authenticated reverse proxies
- Audit existing user accounts for identifier collisions and remove or rename duplicates
- Review message and flow histories for unauthorized reads or injections during the exposure window
Patch Information
IBM has published remediation guidance on the IBM Support Page. Administrators should apply the fixed version referenced in that advisory and validate the upgrade against test environments before promoting to production.
Workarounds
- Place Langflow behind an authenticating gateway that enforces strong, globally unique user identifiers
- Disable public registration and require administrator provisioning of new users until patched
- Rotate API keys and session tokens after upgrading to invalidate credentials that may have been observed
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
