Fix server fetches a URL the caller controls in Next.js
An outbound HTTP request is made to a URL that came from the caller. The server can reach hosts the caller cannot - cloud metadata endpoints, internal admin services, databases bound to localhost - so this turns the server into a proxy into its own network. Validate the destination against an allowlist before fetching it.
high likely Next.js CWE-918 / OWASP A10:2021
The vulnerable pattern in Next.js
This finding comes from the Next.js fixture in the owlwarden test
suite, in /api/proxy.
The destination of this request comes from the caller, so they choose which host the server connects to. That includes hosts they cannot reach themselves: the cloud metadata endpoint that hands out IAM credentials, internal services that skip authentication because they are 'not exposed', and anything bound to localhost.
The corrected handler
Validate the URL in the route handler before fetching, and disable redirect following.
const url = assertAllowedUrl(body.url)
const upstream = await fetch(url, { redirect: 'error' })On a different runtime
The fix above is written for the runtime Next.js usually runs on. These are the runtimes where it would not run at all - an import that does not exist, or an API the host does not have - and what to write instead.
Workers and other fetch-API runtimes
redirect: 'error' is not honoured on this runtime. Use redirect: 'manual' and refuse the response yourself, or an allowlisted host can redirect you to one that is not.
const url = assertAllowedUrl(input)
const upstream = await fetch(url, { redirect: 'manual' })
if (upstream.status >= 300 && upstream.status < 400) {
throw new Error('refusing to follow a redirect from an allowlisted host')
}If you are not using Next.js
Check the destination against an allowlist of hosts before fetching it. Blocklists do not work here: DNS rebinding, redirects, and IPv6-mapped addresses all defeat them.
Check your own repository
npx owlwarden scan
npx owlwarden explain ssrfRuns on your machine. No account, no telemetry, no network unless you ask. In CI, SARIF uploads to code scanning and the exit code is the gate.
Other Next.js checks
Rules with a tested Next.js example.
- cors-permissive medium Cross-origin policy accepts any origin
- hardcoded-secret high Credential hardcoded in source
- insecure-cookie medium Cookie set without its protective attributes
- install-lifecycle-script medium Package declares an install-time script
- open-redirect medium Redirect target comes from the caller
- sensitive-data-logged medium Sensitive data written to a log
- sql-injection high SQL query built by string interpolation
- stack-trace-leak high Stack trace leaked in error response
- weak-crypto high Broken cryptographic primitive protecting a secret