GHSA-wxq7-x3qp-vcr8 Budibase: Unanchored Regex in `matchers.ts` Allows CSRF Bypass via Query String Injection in Budibase Worker
SummaryThe `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`:**Two compounding bugs identical to the patched CVE:`new RegExp(route)` — no `^` start anchor, no `$` end anchor.`ctx.request.url` — full URL string including `?query=value`, not just the path.**CSRF middleware — `packages/backend-core/src/middleware/csrf.ts`:****Worker registration — `packages/worker/src/api/index.ts`:**`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:**`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"` → `true``matches()` returns the pattern entry → CSRF skippedThe 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:****Step 2 — CSRF bypass via query string injection:****Step 3 — Cross-site exploitation (victim visits attacker page):**When the authenticated victim visits `https://evil.com/csrf.html`, the browser sends the cross-origin POST including the victim's session cookie. The Worker's CSRF middleware matches `/api/global/auth/.*` in the query string, skips validation, and processes the admin invite.ImpactAn unauthenticated attacker can forge state-changing requests on behalf of any authenticated Budibase admin by injecting a public endpoint pattern into the query string of a Worker API call. Operations exposed by the Worker that become exploitable via CSRF include:**User management**: send admin/operator invites, delete users, modify roles**Global configuration**: update authentication settings (SSO, OIDC, SMTP), change branding**Tenant administration**: in multi-tenant instances, tenant isolation bypass via `NO_TENANCY_ENDPOINTS` (`/api/system` match) combined with the CSRF bypass allows cross-tenant administrative actionsThe vulnerability affects all Budibase self-hosted deployments with an internet-accessible Worker service up to and including version **3.32.3**, which is the latest released version. It is a direct sibling of CVE-2026-31816 and stems from the same unanchored-regex-on-full-URL pattern in `packages/backend-core/src/middleware/matchers.ts`, which was not addressed by the patch for that CVE.
CVSS v3.1 score: 6.5