Path Traversal in Buffalo with Firestore

Path Traversal occurs when user-controlled input is used to construct file or document paths without proper validation, allowing an attacker to access files or documents outside the intended directory or collection. In a Buffalo application that uses Google Cloud Firestore as a backend, this typically surfaces when a route parameter (e.g., :document_id or :collection) is directly interpolated into Firestore document paths. Because Firestore paths are hierarchical (e.g., users/{user_id}/data/{document_id}), unsanitized input can enable traversal across collections or documents via sequences like ../ or encoded equivalents.

Buffalo does not inherently validate route parameters; it passes them to handlers as strings. If a handler builds a Firestore path using string concatenation or formatting without normalizing or validating the input, an attacker can manipulate the path to read or target unintended documents. For example, a request to /documents/../../users/other_user/private could resolve to a different user’s document if the server does not enforce strict scoping. Firestore itself does not prevent path traversal at the API level; it trusts the client-supplied path. Therefore, the vulnerability arises from insecure server-side path construction rather than a flaw in Firestore.

Additionally, Firestore’s security rules can mitigate some risks, but they are not a substitute for server-side validation. Rules can restrict reads/writes to a user’s own subcollection, yet a traversal attack that stays within allowed collections but targets different document IDs may bypass rule-based protections if the server does not enforce ownership checks. The combination of Buffalo’s flexible routing, Firestore’s flexible path syntax, and missing input normalization creates an environment where Path Traversal can expose sensitive data or allow unauthorized operations.

middleBrick detects Path Traversal in Buffalo-Firestore integrations as part of its BOLA/IDOR and Input Validation checks. By submitting manipulated paths during an unauthenticated scan, the tool identifies whether the API reflects or returns data outside the intended scope. Findings include evidence of exposed document references or error messages that reveal path structure, along with remediation guidance aligned with OWASP API Top 10 and compliance frameworks such as SOC2.

To prevent Path Traversal in a Buffalo application using Firestore, validate and sanitize all user input before constructing Firestore paths. Use explicit allowlists for document IDs, avoid direct string interpolation of route parameters, and enforce ownership checks in both application logic and Firestore security rules. The following examples demonstrate secure patterns.

1. Validate document IDs with an allowlist

Restrict document IDs to alphanumeric characters and a limited set of safe symbols. Reject any input containing .., path separators, or unexpected characters before building a Firestore reference.

// handlers/documents.go
package handlers

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

    "cloud.google.com/go/firestore"
    "github.com/gobuffalo/buffalo"
    "google.golang.org/api/iterator"
)

var validDocID = regexp.MustCompile(`^[a-zA-Z0-9_-]{1,100}$`)

func GetDocument(c buffalo.Context) error {
    docID := strings.TrimSpace(c.Param("document_id"))
    if !validDocID.MatchString(docID) {
        return c.Render(400, r.JSON(map[string]string{"error": "invalid document_id"}))
    }

    ctx := c.Request().Context()
    client, err := firestore.NewClient(ctx, "your-project-id")
    if err != nil {
        return c.Render(500, r.JSON(map[string]string{"error": "internal error"}))
    }
    defer client.Close()

    docRef := client.Collection("users").Doc(c.Session().Get("user_id").(string)).Collection("data").Doc(docID)
    snapshot, err := docRef.Get(ctx)
    if err != nil {
        return c.Render(404, r.JSON(map[string]string{"error": "not found"}))
    }

    return c.Render(200, r.JSON(snapshot.Data()))
}

2. Use Firestore references with explicit scoping

Build document references using Firestore’s API rather than string concatenation. This ensures paths are resolved correctly and prevents accidental traversal.

// handlers/data.go
package handlers

import (
    "context"
    "net/http"

    "cloud.google.com/go/firestore"
    "github.com/gobuffalo/buffalo"
)

func GetScopedData(c buffalo.Context) error {
    userID := c.Session().Get("user_id").(string)
    if userID == "" {
        return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
    }

    ctx := c.Request().Context()
    client, err := firestore.NewClient(ctx, "your-project-id")
    if err != nil {
        return c.Render(500, r.JSON(map[string]string{"error": "internal error"}))
    }
    defer client.Close()

    // Explicitly scope to the authenticated user’s collection
    docRef := client.Collection("users").Doc(userID).Collection("data").Doc(c.Param("document_id"))
    snapshot, err := docRef.Get(ctx)
    if err != nil {
        return c.Render(404, r.JSON(map[string]string{"error": "not found"}))
    }

    return c.Render(200, r.JSON(snapshot.Data()))
}

3. Enforce ownership in Firestore security rules

Complement server-side checks with rules that restrict access to a user’s own data. While not a replacement for input validation, this adds a layer of defense.

// firestore.rules
rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{user_id}/data/{document=**} {
      allow read, write: if request.auth != null && request.auth.uid == user_id;
    }
  }
}

These practices reduce the risk of Path Traversal in Buffalo-Firestore integrations by ensuring that document paths are constructed from validated, scoped inputs and that access is controlled at multiple layers.