Zip Slip in Buffalo with Firestore

Zip Slip is a path traversal vulnerability that occurs when an application constructs file paths using unsanitized user input, allowing an attacker to navigate outside the intended directory. When this pattern is applied in a Buffalo application that interacts with Google Cloud Firestore, the risk shifts from direct filesystem access to unintended data exposure or mutation. Buffalo typically serves as a Go web framework that manages HTTP requests and may use Firestore as a backend datastore. If user-supplied values—such as a document ID, collection name, or a filename used to generate a download or export path—are directly concatenated into Firestore operations or filesystem paths without validation, malicious inputs like ../../../firestore/export can traverse logical boundaries.

In a Buffalo + Firestore context, the vulnerability surfaces in two primary ways. First, if the application uses user input to determine which Firestore collection or document to read or write, a crafted path can lead to reading or modifying data outside the developer’s intended scope (e.g., accessing users/../../../admin/settings). Second, if the application generates local file paths for caching or export purposes based on Firestore metadata, a malicious path can escape the application’s working directory, potentially reaching sensitive system files. While Firestore itself enforces authentication and rules, the Buffalo layer may inadvertently expose internal logic or data mappings that an attacker can probe using path traversal techniques. The unauthenticated attack surface tested by middleBrick can detect such input validation weaknesses across the 12 security checks, including Input Validation and Property Authorization, highlighting how user-controlled data can traverse authorization boundaries.

For example, consider a handler that builds a Firestore document path from a URL parameter without normalization:

param := c.Param("collection")
ref := client.Collection(param).Doc("document")

An input like ../../../../etc/passwd does not directly affect Firestore due to its structured nature, but it can corrupt internal routing or be logged in ways that aid further attacks. MiddleBrick’s LLM/AI Security checks are particularly valuable here because they detect prompt injection and system prompt leakage patterns that could accompany API misuse in AI-integrated Buffalo services, ensuring that indirect exposures are also evaluated.

To mitigate Zip Slip in a Buffalo application using Firestore, you must validate and sanitize all user input that influences Firestore paths or local file operations. Use strict allow-listing for collection and document identifiers, and avoid direct string concatenation for path construction. The following examples demonstrate secure patterns for Buffalo handlers interacting with Firestore.

1. Validate collection and document identifiers

Ensure that identifiers conform to expected formats using regex or whitelisting. This prevents traversal sequences from being interpreted as valid paths.

import (
    "regexp"
    "github.com/gobuffalo/buffalo"
)

var validCollectionName = regexp.MustCompile(`^[a-z0-9_]+$`)

func GetDocument(c buffalo.Context) error {
    collection := c.Param("collection")
    if !validCollectionName.MatchString(collection) {
        return c.Error(400, fmt.Errorf("invalid collection name"))
    }
    ref := client.Collection(collection).Doc(c.Param("document"))
    // proceed with Firestore operations
    return nil
}

2. Use Firestore’s built-in APIs safely

Rely on Firestore’s native methods that accept structured data rather than raw path strings. Avoid using user input to dynamically construct collection or document references without validation.

func CreateRecord(c buffalo.Context) error {
    userID := c.Param("user_id")
    if !validUserID.MatchString(userID) {
        return c.Error(400, fmt.Errorf("invalid user identifier"))
    }
    _, err := client.Collection("users").Doc(userID).Set(c.Request().Context(), map[string]interface{}{
        "data": c.Param("value"),
    })
    if err != nil {
        return c.Error(500, err)
    }
    return nil
}

3. Sanitize file paths for local operations

If your Buffalo app writes files based on Firestore metadata, use filepath.Clean and restrict output directories to prevent directory traversal.

import (
    "path/filepath"
)

func ExportFile(c buffalo.Context) error {
    filename := filepath.Clean(c.Param("filename"))
    if filename == ".." || len(filename) == 0 {
        return c.Error(400, fmt.Errorf("invalid filename"))
    }
    target := filepath.Join("/safe/export/dir", filename)
    // proceed with file write using target
    return nil
}

4. Leverage middleBrick for continuous validation

Use the middleBrick CLI to scan your Buffalo endpoints regularly and detect input validation issues before they reach production. The CLI provides structured JSON output that can be integrated into CI/CD pipelines using the GitHub Action to enforce security gates.

$ middlebrick scan https://your-buffalo-api.example.com