Path Traversal in Buffalo with Hmac Signatures

Path Traversal occurs when user-controlled input is used to construct file paths without proper validation, allowing an attacker to access files outside the intended directory. In the Buffalo web framework for Go, this risk can be compounded when Hmac Signatures are used to bind user-supplied parameters (such as an identifier in a URL or query string) to a server-generated signature for integrity verification. If the application uses the Hmac signature to authorize access to a file path derived from user input, an attacker who can manipulate the path component may leverage directory traversal sequences (e.g., ../../../) to resolve to sensitive files, even when the signature itself remains valid for the tampered input.

Consider a scenario where a request includes a file identifier and an Hmac signature intended to ensure the identifier has not been altered. If the server recomputes the Hmac over the received identifier and compares it to the provided signature, the comparison may succeed for a maliciously crafted identifier like ../../../etc/passwd if the server does not canonicalize or restrict the path before using it. The signature does not protect against path traversal because it only guarantees integrity of the identifier string, not safe resolution of that identifier to a file within a restricted directory. This mismatch between integrity checks and path resolution creates a vulnerability where an attacker can traverse the filesystem to read arbitrary files, potentially exposing configuration data, source code, or credentials stored outside the application’s document root.

In Buffalo, developers might use middleware or custom logic to verify Hmac Signatures for routing or parameter validation. If this logic feeds the user-supplied identifier directly into file operations (e.g., opening a file based on the identifier), the framework’s conventions do not automatically prevent path traversal. The Hmac layer provides no safeguards against path manipulation; it only ensures the identifier has not been modified in transit. Without additional controls such as path sanitization, strict base directory enforcement, or rejection of path traversal sequences, the combination of Hmac Signatures and dynamic file paths can inadvertently expose sensitive files through traversal attacks.

To mitigate Path Traversal risks when using Hmac Signatures in Buffalo, ensure that any file-path derivation from user-controlled identifiers is strictly confined to a designated base directory and that traversal sequences are rejected before path resolution. Hmac verification should be applied to a canonical, safe identifier (e.g., a filename without directory components or a sanitized key), and the resolved path must be constructed using path-cleaning functions that eliminate .. elements.

Example: verify the Hmac over a sanitized key and then safely join it with a base directory using Go’s path/filepath utilities.

// Example Hmac verification and safe path construction in Buffalo-style Go
package controllers

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
    "path/filepath"
    "strings"
)

// verifyHmac returns true if the provided key and signature match.
func verifyHmac(key, signature, secret string) bool {
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(key))
    expected := hex.EncodeToString(mac.Sum(nil))
    return hmac.Equal([]byte(expected), []byte(signature))
}

// FileHandler demonstrates safe handling of user input with Hmac Signatures.
func FileHandler(secret string) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // Assume key and signature are provided via query parameters.
        key := r.URL.Query().Get("key")
        sig := r.URL.Query().Get("sig")

        // Reject keys containing path traversal or directory components.
        if strings.Contains(key, "..") || strings.Contains(key, "/") || strings.Contains(key, "\\") {
            http.Error(w, "invalid key", http.StatusBadRequest)
            return
        }

        // Verify Hmac to ensure key integrity.
        if !verifyHmac(key, sig, secret) {
            http.Error(w, "invalid signature", http.StatusUnauthorized)
            return
        }

        // Canonicalize and safely join with a restricted base directory.
        baseDir := "/var/app/public/files"
        safeKey := filepath.Clean(key)
        filePath := filepath.Join(baseDir, safeKey)

        // Ensure the resolved path remains within the base directory.
        if !strings.HasPrefix(filePath, filepath.Clean(baseDir)+string(filepath.Separator)) && filePath != filepath.Clean(baseDir) {
            http.Error(w, "path traversal detected", http.StatusBadRequest)
            return
        }

        http.ServeFile(w, r, filePath)
    }
}

In this pattern, the Hmac signature is computed over a restricted key that disallows directory separators and traversal sequences. The server cleans the key with filepath.Clean and enforces that the final resolved path begins with the intended base directory. This ensures that even if an attacker supplies a crafted identifier, the Hmac verification and path resolution logic work together to prevent unauthorized file access. For Buffalo applications, implementing such validation around Hmac usage addresses the specific risk where integrity checks fail to constrain filesystem operations.