Budibase: Unanchored Regex in matchers.ts Allows CSRF Bypass via Query String Injection in Budibase Worker
Summary
The buildMatcherRegex() / matches() functions in packages/backend-core/src/middleware/matchers.ts share the same structural root cause as the recently patched CVE-2026-31816: route patterns are compiled into unanchored regular expressions and tested against ctx.request.url, which includes the full query string. The CSRF middleware in the Budibase Worker uses this matching system to decide whether to skip CSRF token validation. An unauthenticated attacker can forge state-changing cross-origin requests against any Worker API endpoint by injecting a public route pattern into the query string, causing the CSRF middleware to skip token validation entirely. This allows actions such as sending admin invites, modifying global configuration, and managing users without a valid CSRF token.
CVE-2026-31816 fixed the same unanchored-regex-on-full-URL bug in server/middleware/utils.ts but left backend-core/middleware/matchers.ts untouched.
Details
Root cause — packages/backend-core/src/middleware/matchers.ts:
```typescript
export const buildMatcherRegex = (patterns: EndpointMatcher[]): RegexMatcher[] => {
return patterns.map(pattern => {
let route = pattern.route
// replaces :param segments with /.*
const matches = route.match(PARAM_REGEX)
if (matches) {
for (let match of matches) {
const suffix = match.endsWith("/") ? "/" : ""
route = route.replace(match, "/.*" + suffix)
}
}
return { regex: new RegExp(route), method, route }
// ^ no ^ anchor, no $ anchor — matches anywhere in string
})
}
export const matches = (ctx: Ctx, options: RegexMatcher[]) => {
return options.find(({ regex, method }) => {
const urlMatch = regex.test(ctx.request.url) // full URL including query string
const methodMatch = method === "ALL" ? true
: ctx.request.method.toLowerCase() === method.toLowerCase()
return urlMatch && methodMatch
})
}
```
Two compounding bugs identical to the patched CVE:
1. new RegExp(route) — no ^ start anchor, no $ end anchor.
2. ctx.request.url — full URL string including ?query=value, not just the path.
CSRF middleware — packages/backend-core/src/middleware/csrf.ts:
```typescript
export function csrf(
opts: { noCsrfPatterns: EndpointMatcher[] } = { noCsrfPatterns: [] }
) {
const noCsrfOptions = buildMatcherRegex(opts.noCsrfPatterns)
return (async (ctx: Ctx, next: Next) => {
const found = matches(ctx, noCsrfOptions)
if (found) {
return next() // <-- CSRF check entirely skipped when pattern matches
}
// ... CSRF token validation ...
}) as Middleware
}
```
Worker registration — packages/worker/src/api/index.ts:
```typescript
const NO_CSRF_ENDPOINTS = [...PUBLIC_ENDPOINTS]
// PUBLIC_ENDPOINTS includes (among others):
// { route: "/api/global/auth/:tenantId", method: "POST" }
// { route: "/api/global/users/init", method: "POST" }
// { route: "/api/system/restored", method: "POST" }
router
.use(auth.buildCsrfMiddleware({ noCsrfPatterns: NO_CSRF_ENDPOINTS }))
```
buildMatcherRegex compiles "/api/global/auth/:tenantId" into the regex /api/global/auth/.* (via PARAM_REGEX replacing /:tenantId → /.*). Since the regex is unanchored, it matches the substring "/api/global/auth/" anywhere in ctx.request.url — including inside a query string parameter on a completely different endpoint.
Triggering condition:
```
POST /api/global/users/invite?x=/api/global/auth/evil
```
ctx.request.url = "/api/global/users/invite?x=/api/global/auth/evil"new RegExp("/api/global/auth/.*").test(ctx.request.url) → true (substring found in query string)ctx.request.method === "POST" → truematches() returns the pattern entry → CSRF skipped- The protected user-invite POST proceeds without any CSRF token
Additional affected middleware (same matches() call):
| Middleware | Pattern list | Security effect when bypassed |
|---|---|---|
| csrf() | NO_CSRF_ENDPOINTS | CSRF token validation skipped |
| tenancy() | NO_TENANCY_ENDPOINTS | allowNoTenant = true, bypasses tenant ID requirement |
| authenticated() | PUBLIC_ENDPOINTS | Marks endpoint as publicEndpoint = true |
The NO_TENANCY_ENDPOINTS entry { route: "/api/system", method: "ALL" } compiles to /api/system (no param replacement). Since method: "ALL" matches every HTTP verb and the pattern is unanchored, any request with ?x=/api/system/x in its URL matches, potentially bypassing tenant isolation in multi-tenant deployments.
PoC
Prerequisites: Victim user is logged into a Budibase Worker instance (e.g., https://budibase.target.com) in their browser. Attacker hosts a page at https://evil.com.
Step 1 — Verify CSRF is normally enforced:
```bash
# Without the bypass — CSRF token missing → rejected
cur
