CVE-2025-30157 - Inside Envoy’s ext_proc Crash—Crash and Exploit Details Demystified

Envoy is a powerhouse in the world of cloud-native networking. Its name is synonymous with high-performance service proxies. If you run APIs, manage traffic between microservices, or operate a service mesh, chances are you’ve considered or deployed Envoy.

But like all software, updates and vigilance are crucial. In early 2025, a security flaw was discovered: CVE-2025-30157. This vulnerability is particularly troublesome for those using the ext_proc HTTP filter, especially with websockets. And if you’re running a version earlier than 1.33.1, 1.32.4, 1.31.6, or 1.30.10, your system could be at risk.

What is CVE-2025-30157?

The ext_proc HTTP filter in Envoy allows for out-of-process HTTP request/response modification. That means it can send requests to an external server for inspection or changes, which is great for advanced API management.

The catch: if a “local reply” (an immediate error response) is triggered—say, if a websocket handshake fails—Envoy crashes. This crash is rooted in a lifetime (use-after-free) issue in the filter. Effectively, after Envoy sends a reply to the external processor, that reply can outlive the filter object, leading to a crash.

Why does this matter?
If someone can trigger this edge case on purpose, they could repeatedly crash an Envoy proxy—potentially causing denial-of-service (DoS) and disrupting all downstream services relying on Envoy.

Exploit Walkthrough

To see this in action, let’s walk through how an attacker—or even an innocent user—might unwittingly crash Envoy:

Envoy’s configuration may include something like

http_filters:
  - name: envoy.filters.http.ext_proc
    typed_config:
      "@type": type.googleapis.com/envoy.extensions.filters.http.ext_proc.v3.ExternalProcessor
      grpc_service:
        envoy_grpc:
          cluster_name: ext_proc_backend

2. Websocket Handshake Failure

Suppose your service allows websocket connections, but—for some reason—a handshake fails (e.g., authentication error).

The failure leads Envoy to send a local reply rather than proxying to backend services.

Triggering Code Snippet (Python, using requests-websocket)

import websocket

try:
    ws = websocket.create_connection("ws://your-envoy-proxy.example.com/ws")
    ws.send("Test message")
    ws.close()
except websocket.WebSocketBadStatusException as err:
    print("Handshake failed, response:", err)

If the handshake isn’t successful (for any reason), the error triggers a local reply inside Envoy.

The local reply is sent while the ext_proc filter is still engaged

- Due to a race condition/free-after-use bug, Envoy tries to access a filter object that was already freed

Real-World Impact

- Both public and private services are affected if they use the ext_proc HTTP filter and accept websocket traffic
- Attackers do not need privileged access—just the ability to attempt websockets (which is common in many apps)

References & More Reading

- Official Envoy Security Advisory - GHSA-84hp-f7qq-cv43
- CVE-2025-30157 on NVD
- Envoy Ext_Proc Filter Documentation

Final Take: Stay Ahead of Threats

This bug highlights how subtle lifetime issues can have massive ripple effects in cloud-native stacks. Always keep up-to-date with upstream releases—check if you have websocket endpoints and ext_proc, and patch as soon as possible.

Questions or want a deeper dive? Drop a comment below or check out the references for more technical details.

Timeline

Published on: 03/21/2025 15:15:43 UTC
Last modified on: 04/01/2025 20:22:34 UTC