CVE-2026-9186: Langflow is vulnerable to stored cross-site scripting and IP spoofing due to unsanitized Markdown rendering and untrusted proxy header trust
IBM Langflow OSS 1.0.0 through 1.11.2 allows remote authenticated attackers to bypass localhost-only MCP configuration installation by spoofing X-Forwarded-For: 127.0.0.1 header, enabling arbitrary writes to IDE config files (~/.cursor/mcp.json, etc.).
Other sources
Langflow OSS allows remote authenticated attackers to bypass localhost-only MCP configuration installation by spoofing X-Forwarded-For: 127.0.0.1 header, enabling arbitrary writes to IDE config files (~/.cursor/mcp.json, etc.).
— IBM
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Langflow OSSto a version that resolves this vulnerability.Fixed in 1.11.3 - Configuration
Configure Langflow OSS (and any reverse proxy) to not allow clients to spoof the X-Forwarded-For header (e.g., ensure the application only trusts X-Forwarded-For from approved/proxy IPs) so localhost-only MCP configuration installation cannot be bypassed.
Langflow OSS X-Forwarded-For trust = Do not trust X-Forwarded-For: 127.0.0.1 for localhost-only MCP configuration installation
Event History
Frequently Asked Questions
Who can exploit this issue?
A remote attacker must be authenticated to Langflow OSS. The affected versions identified are 1.0.0 through 1.11.2.
What does an attacker need to do to bypass the localhost-only restriction?
The attacker sends an X-Forwarded-For header with the value 127.0.0.1. This causes Langflow to treat the request as local when installing MCP configuration.
What is the impact of a successful bypass?
An attacker can perform arbitrary writes to IDE MCP configuration files, including paths such as ~/.cursor/mcp.json. This can allow installation of attacker-controlled MCP configuration.