Xpath Injection in Chi with Firestore

Chi is a common HTTP router for Go applications, and when it passes user-controlled values into XPath expressions that are later used to query Firestore (typically via a translation layer or custom backend logic), Xpath Injection can occur. Firestore does not use XPath natively, but if your application builds XPath strings to select or filter data before mapping results to Firestore documents, the risk arises at the intersection of runtime routing, input handling, and backend data access.

An attacker can supply crafted input such as /ChiTest/user[1]/id or inject union-like syntax to probe data structures if XPath evaluation is performed client-side or in a service layer. For example, if Chi passes a URL parameter directly into an XPath expression without sanitization, an attacker could use ' or 1=1 or ' to alter predicate logic and retrieve unintended nodes. Even though Firestore stores data in collections and documents, any intermediate service that converts Firestore reads into XPath—such as an XML/JSON transformation layer—becomes vulnerable.

The 12 security checks in middleBrick run in parallel and include Input Validation and Property Authorization, which help detect whether user input reaches XPath construction paths. When combined with the OWASP API Top 10 category 'Broken Object Level Authorization (BOLA/IDOR)', this scenario may expose paths that should be restricted. middleBrick also checks for Unsafe Consumption patterns, which can surface places where untrusted input is used to influence query logic, including XPath-like selectors that ultimately map to Firestore document paths.

In a scan using the middleBrick CLI—run with middlebrick scan <url>—the tool tests unauthenticated attack surfaces and returns a security risk score with per-category breakdowns. If XPath construction appears in your runtime behavior, findings will include severity levels and remediation guidance, helping you understand how an attacker might manipulate input to reach sensitive Firestore-mapped data.

Because Firestore relies on structured queries rather than native XPath, the vulnerability is not in Firestore itself but in how your application composes queries. The risk is that XPath manipulation leads to unauthorized data access or inference, which aligns with BOLA/IDOR and Input Validation failures. middleBrick’s LLM/AI Security checks do not apply here, but its standard authentication and authorization checks can highlight whether access controls are bypassed through malformed input paths.

To illustrate, consider a handler in Chi that receives an id parameter and uses it to build an XPath expression for a backend service that maps to Firestore documents. Without proper validation, an input like user/*/profile could return more data than intended. By using the middleBrick Dashboard to track scans over time, you can detect when new endpoints introduce such patterns and apply fixes before deploying changes, especially with the Pro plan’s continuous monitoring and configurable alerts.

Remediation focuses on strict input validation, avoiding string concatenation for query construction, and using Firestore’s native mechanisms. In Chi, define explicit parameter parsing and reject any user input that attempts to influence path or predicate logic. Never build XPath-like strings from URL parameters; instead, map validated identifiers directly to Firestore document IDs or collection paths.

Use Chi’s middleware to sanitize and constrain inputs. For example, enforce that IDs match a known pattern (alphanumeric and length limits) before using them in any backend call. This prevents injection attempts that rely on unexpected characters or structure.

Below are concrete Firestore code examples for Chi handlers that avoid XPath-style construction entirely. Instead of interpreting user input as query syntax, treat it strictly as a document identifier and enforce allow-lists where possible.

// Chi handler using firego or another Firestore client in Go
package handlers

import (
    "context"
    "net/http"
    "regexp"

    "cloud.google.com/go/firestore"
    "github.com/go-chi/chi/v5"
)

func GetUserProfile(db *firestore.Client) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        userID := chi.URLParam(r, "userID")
        // Strict validation: only alphanumeric IDs, 1-64 chars
        matched, _ := regexp.MatchString(`^[a-zA-Z0-9_-]{1,64}$`, userID)
        if !matched {
            http.Error(w, "invalid user identifier", http.StatusBadRequest)
            return
        }
        ctx := r.Context()
        docRef := db.Collection("users").Doc(userID)
        snap, err := docRef.Get(ctx)
        if err != nil {
            http.Error(w, "unable to fetch profile", http.StatusInternalServerError)
            return
        }
        // Process snap.Data() as needed
        _ = snap.Data()
    }
}

If you must support dynamic paths, map them to predefined structures rather than evaluating them as code. For instance, use a map of allowed document kinds to collections, and validate against that map before accessing Firestore. This approach eliminates XPath-style traversal and ensures that Firestore queries remain predictable and authorized.

In the middleBrick Pro plan, you can enable continuous monitoring to ensure that new routes or changes to parameter handling do not reintroduce unsafe patterns. The GitHub Action can fail builds if a security score drops below your set threshold, providing CI/CD enforcement. Combined with the Web Dashboard’s per-category breakdowns, this helps maintain secure handling of identifiers across your API surface.

Finally, always apply the principle of least privilege to Firestore access. Even with validated inputs, ensure service accounts used by Chi handlers have read access only to the collections and documents required. This reduces the impact of any potential logic flaws and aligns with secure API design practices.