Zip Slip in Buffalo with Hmac Signatures

Zip Slip is a path traversal vulnerability that occurs when an archive’s entries are extracted without proper validation, allowing malicious paths such as ../../../etc/passwd to escape the intended extraction directory. When Hmac Signatures are used to verify integrity or authenticity of the archive, a mismatch between the expected signature and the runtime behavior can expose or compound the risk. In Buffalo, a Go web framework that encourages rapid development, handling file uploads and archive extraction is common in features like import/export or plugin loading. If an endpoint accepts an uploaded archive, verifies its Hmac Signatures to confirm authenticity, but then extracts files without sanitizing paths, an attacker can supply a validly signed archive containing malicious traversal paths.

The vulnerability chain involves three dimensions:

• Zip Slip: unsanitized member paths in zip archives enable directory traversal during extraction.
• Buffalo: server-side handlers written in Buffalo may use standard Go archive/zip utilities and custom middleware, increasing the chance of overlooked path checks.
• Hmac Signatures: while Hmac Signatures ensure data integrity and origin authenticity, they do not guarantee safe extraction logic; trusting a verified signature can lead to skipping additional validation steps.

For example, an attacker could craft a zip file with a member named ../../../malicious.sh, sign it with a shared secret known to the server, and have the Buffalo handler verify the Hmac Signatures before extraction. Because the signature matches, the handler might proceed to extract the file directly into a web-accessible directory, resulting in arbitrary file write and potential remote code execution. This shows how relying solely on Hmac Signatures in Buffalo without strict path canonicalization and directory traversal prevention turns a trusted signature into an enabler of exploitation.

Remediation focuses on validating archive member paths before extraction and ensuring Hmac Signatures are part of a broader safe handling strategy. Always canonicalize and inspect each file path, rejecting any that traverse outside the target directory. Below are concrete, syntactically correct examples demonstrating secure handling in a Buffalo application using Hmac Signatures for verification.

package actions

import (
    "archive/zip"
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "io"
    "net/http"
    "os"
    "path/filepath"
    "strings"
)

const sharedSecret = "your-secure-shared-secret"

func VerifyHmac(data, receivedMac string) bool {
    key := []byte(sharedSecret)
    mac := hmac.New(sha256.New, key)
    mac.Write([]byte(data))
    expected := hex.EncodeToString(mac.Sum(nil))
    return hmac.Equal([]byte(expected), []byte(receivedMac))
}

func ExtractSafely(srcPath, destDir, receivedMac string) error {
    // Example data that would be signed (e.g., file content or a manifest)
    fileData, err := os.ReadFile(srcPath)
    if err != nil {
        return err
    }
    if !VerifyHmac(string(fileData), receivedMac) {
        return http.ErrAbortHandler
    }

    r, err := zip.OpenReader(srcPath)
    if err != nil {
        return err
    }
    defer r.Close()

    for _, f := range r.File {
        // Securely construct the destination path
        destPath := filepath.Join(destDir, f.Name)
        if !strings.HasPrefix(destPath, filepath.Clean(destDir)+string(os.PathSeparator)) {
            return http.ErrAbortHandler // path traversal attempt
        }
        if f.FileInfo().IsDir() {
            os.MkdirAll(destPath, os.ModePerm)
            continue
        }
        if err := os.MkdirAll(filepath.DestDir, os.ModePerm); err != nil {
            return err
        }
        rc, err := f.Open()
        if err != nil {
            return err
        }
        defer rc.Close()
        dstFile, err := os.OpenFile(destPath, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, f.Mode())
        if err != nil {
            return err
        }
        defer dstFile.Close()
        if _, err := io.Copy(dstFile, rc); err != nil {
            return err
        }
    }
    return nil
}

In this example, VerifyHmac checks the Hmac Signatures using SHA-256 and hmac.Equal to prevent timing attacks. The ExtractSafely function opens the zip archive, iterates over each file, and ensures the resolved path remains within the intended destination directory by checking the prefix after canonicalization with filepath.Clean and filepath.Join. This approach mitigates Zip Slip even when Hmac Signatures confirm the archive’s authenticity.

For teams using the Buffalo CLI, integrate this logic into your action methods and leverage the Pro plan’s continuous monitoring to detect risky file handling patterns across your APIs. If you automate scans in CI/CD, the GitHub Action can fail builds when unsafe extraction patterns are detected in your codebase, while the MCP Server allows you to run checks directly from your AI coding assistant within the IDE.