Zip Slip in Gin with Firestore

Zip Slip is a path traversal vulnerability that occurs when user-supplied filenames or paths are used to construct extraction or file operations without proper validation. In a Gin-based Go service that interacts with Google Cloud Firestore, the risk emerges not from Firestore itself, which is a managed NoSQL database, but from how application code handles file downloads, temporary storage, and data imports that involve file paths.

Consider a scenario where an endpoint accepts a filename or archive upload, extracts it, and uses the contained file paths to write or reference data in Firestore. If the application uses filepath.Join() without cleaning or validating user input, an attacker can provide a filename such as ../../../etc/passwd. During extraction or file operations, this path escapes the intended directory, potentially overwriting system or application files. Even though Firestore stores structured data rather than files, a compromised file operation can lead to unauthorized read/write access to service account credentials or configuration files that the application uses to initialize Firestore clients. This can result in privilege escalation or data leakage.

Additionally, if the Gin application processes archive files (e.g., ZIP) and uses unsanitized member paths to determine Firestore document IDs or batch writes, an attacker can inject path sequences that traverse directories. While Firestore enforces its own access controls, the surrounding infrastructure—service accounts, local disk, and environment configuration—becomes exposed through these file system interactions. An attacker who manipulates file paths can overwrite critical initialization scripts or certificates used by the Firestore client library, leading to authentication bypass or data tampering downstream.

The combination of Gin’s flexible routing and middleware with Firestore’s client library can inadvertently amplify risks if input validation is limited to HTTP layer checks alone. Security practices must extend to file handling, archive extraction, and any interaction with the local filesystem that could influence Firestore client behavior or configuration integrity.

To mitigate Zip Slip in a Gin application that uses Firestore, enforce strict path validation and avoid direct concatenation of user input into file paths. Use Go’s standard library functions to clean and restrict paths, and ensure that any file operations remain confined to a designated directory.

Secure File Extraction Example

When extracting archives, validate each member path to ensure it resolves within the target directory:

import (
    "archive/zip"
    "io"
    "os"
    "path/filepath"
    "strings"
)

func safeExtract(zipPath, destDir string) error {
    r, err := zip.OpenReader(zipPath)
    if err != nil {
        return err
    }
    defer r.Close()

    for _, f := range r.File {
        // Clean and validate the path
        target := filepath.Join(destDir, f.Name)
        if !strings.HasPrefix(filepath.Clean(target), filepath.Clean(destDir)+string(os.PathSeparator)) {
            return os.ErrInvalid
        }

        if f.FileInfo().IsDir() {
            os.MkdirAll(target, f.Mode())
            continue
        }

        if err := os.MkdirAll(filepath.Dir(target), f.Mode()); err != nil {
            return err
        }

        src, err := f.Open()
        if err != nil {
            return err
        }
        defer src.Close()

        dst, err := os.OpenFile(target, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, f.Mode())
        if err != nil {
            return err
        }
        defer dst.Close()

        if _, err := io.Copy(dst, src); err != nil {
            return err
        }
    }
    return nil
}

Firestore Document Write with Validated Input

Ensure that any data derived from file operations is sanitized before being used in Firestore writes. Use explicit field mapping instead of dynamic paths:

import (
    "context"
    "cloud.google.com/go/firestore"
)

func writeFirestoreFromValidatedData(ctx context.Context, client *firestore.Client, docID string, data map[string]interface{}) error {
    // Validate docID to prevent path-like traversal
    if docID == "" || strings.ContainsAny(docID, " /\\") {
        return os.ErrInvalid
    }

    _, err := client.Collection("records").Doc(docID).Set(ctx, data)
    return err
}

Middleware Path Sanitization

Add a Gin middleware to reject requests with suspicious path patterns before they reach business logic:

func PathValidationMiddleware() gin.HandlerFunc {
    return func(c *gin.Context) {
        path := c.Param("path")
        if strings.Contains(path, "..") || strings.Contains(path, "%2e%2e") {
            c.AbortWithStatusJSON(400, gin.H{"error": "invalid path"})
            return
        }
        c.Next()
    }
}

These measures ensure that file handling logic does not expose the application or its Firestore integration to traversal attacks, maintaining separation between user input and system paths.