Hallucination Attacks in Gorilla Mux with Firestore

Hallucination attacks in a Gorilla Mux routing layer interacting with Firestore occur when an attacker supplies path or query parameters that cause the handler to reference non-existent Firestore documents or collections, then fabricates or misrepresents the returned data. Because Gorilla Mux uses route variables (e.g., /users/{userID}) that map directly to Firestore lookups, an attacker can inject unexpected IDs or malformed queries that do not map to any real document. If the handler does not properly validate the existence of the document and instead constructs a synthetic response, it may hallucinate user data, permissions, or metadata.

In this combination, the risk is amplified by two factors: dynamic route parameters and Firestore’s flexible document model. Firestore does not enforce strict schema, so a missing document does not inherently cause an error if the code performs a type assertion or constructs a default response. An attacker can probe endpoints like /orgs/{orgID}/settings with random UUIDs; when Firestore returns nil, poor error handling may cause the handler to generate plausible but false settings, effectively hallucinating data. This becomes an information leakage or privilege escalation vector when the hallucinated data includes roles, permissions, or PII that should remain hidden.

Additionally, Firestore security rules are evaluated at read/write time, but application-layer logic in Gorilla Mux must enforce authorization before constructing queries. If the handler uses unchecked route variables to build Firestore queries without validating that the requesting subject has access to the target document, hallucination can bypass intended access controls. For example, an attacker might supply an orgID that belongs to another tenant; if the handler hallucinates a default org object rather than returning a 404, it may expose cross-tenant data. The attack chain typically involves enumeration, injection, and fabrication, leveraging Firestore’s index-based lookups and the router’s pattern matching to produce convincing false outputs.

Real-world indicators include inconsistent responses for valid versus invalid IDs, verbose errors revealing document paths, and missing existence checks before data assembly. Because Firestore can return partial results or merge fields from multiple documents, hallucination can also arise from incorrect aggregation logic in handlers that combine Firestore streams with in-memory transformations. These vulnerabilities map closely to OWASP API Top 10 API1:2023 Broken Object Level Authorization, where missing or misapplied authorization checks allow attackers to manipulate references to Firestore documents.

Remediation focuses on strict input validation, existence checks, and canonical error handling when querying Firestore from Gorilla Mux handlers. Always validate route variables against expected formats before using them in Firestore queries, and treat missing documents as errors rather than synthetic data sources. Use context timeouts and consistent error types to avoid leaking internal paths.

Example: a user profile endpoint that safely checks document existence and returns 404 for missing resources:

// Firestore client setup (outside handler)
client, err := firestore.NewClient(ctx, "my-project")
if err != nil {
    log.Fatalf("firestore.NewClient: %v", err)
}
defer client.Close()

// Gorilla Mux handler with validation and existence check
func getUserProfile(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    userID := vars["userID"]
    // Validate format to mitigate injection and malformed IDs
    if !isValidUserID(userID) {
        http.Error(w, `{"error":"invalid userID"}`, http.StatusBadRequest)
        return
    }
    docRef := client.Collection("users").Doc(userID)
    doc, err := docRef.Get(r.Context())
    if err != nil {
        // Log the error internally; do not expose Firestore internals
        http.Error(w, `{"error":"unable to retrieve profile"}`, http.StatusInternalServerError)
        return
    }
    if doc == nil || !doc.Exists() {
        http.Error(w, `{"error":"profile not found"}`, http.StatusNotFound)
        return
    }
    var data map[string]interface{}
    if err := doc.DataTo(&data); err != nil {
        http.Error(w, `{"error":"data parse error"}`, http.StatusInternalServerError)
        return
    }
    w.Header().Set("Content-Type", "application/json")
    json.NewEncoder(w).Encode(data)
}

func isValidUserID(id string) bool {
    // Allow only safe patterns (e.g., UUID or numeric)
    return regexp.MustCompile(`^[a-zA-Z0-9_-]{1,128}$`).MatchString(id)
}

For org-scoped settings, enforce tenant ownership before querying Firestore. Use Firestore security rules as a baseline but enforce checks in application code to prevent hallucination across tenants:

func getOrgSettings(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    orgID := vars["orgID"]
    userID := r.Context().Value("userID").(string)
    if !isValidOrgID(orgID) || !isValidUserID(userID) {
        http.Error(w, `{"error":"invalid parameters"}`, http.StatusBadRequest)
        return
    }
    // Ensure user belongs to org before fetching settings
    if !userHasOrgAccess(r.Context(), userID, orgID) {
        http.Error(w, `{"error":"forbidden"}`, http.StatusForbidden)
        return
    }
    settingsRef := client.Collection("orgs").Doc(orgID).Collection("settings").Doc("current")
    doc, err := settingsRef.Get(r.Context())
    if err != nil || !doc.Exists() {
        http.Error(w, `{"error":"settings not found"}`, http.StatusNotFound)
        return
    }
    var settings map[string]interface{}
    if err := doc.DataTo(&settings); err != nil {
        http.Error(w, `{"error":"data parse error"}`, http.StatusInternalServerError)
        return
    }
    w.Header().Set("Content-Type", "application/json")
    json.NewEncoder(w).Encode(settings)
}

func userHasOrgAccess(ctx context.Context, userID, orgID string) bool {
    // Check membership in Firestore or a local cache; keep logic explicit
    membershipRef := client.Collection("org_memberships").Doc(orgID).Collection("members").Doc(userID)
    snap, err := membershipRef.Get(ctx)
    return err == nil && snap.Exists()
}

Additional practices: avoid merging Firestore fields from multiple documents into a single hallucinated object; prefer explicit joins with existence checks. Monitor logs for repeated 404s on similar patterns to detect probing behavior. Combine these measures with Firestore rules that align with your authorization model to reduce the attack surface.