Path Traversal in Echo Go with Hmac Signatures

Path Traversal occurs when user-controlled input is used to construct file system paths without proper validation, allowing an attacker to access files outside the intended directory. In Echo Go, this often arises when a route parameter or query value is directly concatenated into a file path. When Hmac Signatures are used to authenticate or integrity-check requests, developers may assume that signed parameters are safe and do not require additional validation, which can lead to a false sense of security.

Consider an endpoint that serves documents by ID and uses an HMAC to verify the request originates from a trusted source. If the handler uses the signed document ID directly in a filesystem operation like os.Open(filepath.Join(baseDir, docID)) without checking for .. or path separators, an authenticated attacker can still traverse directories. The Hmac Signature ensures the request is authorized, but it does not enforce that the ID maps to a safe location within baseDir. This combination creates a Path Traversal vulnerability because the integrity check passes while the underlying path resolution remains unchecked.

Another scenario involves query parameters used to select files, where the developer signs the query to prevent tampering. If the signed value is used in a path join without canonicalization, an attacker can provide inputs such as ../../../etc/passwd. The Hmac Signature may validate successfully, but the resolved path escapes the intended directory. This is especially problematic when the signature covers only a subset of parameters, leaving the file path parameter unchecked despite being derived from authenticated data.

Echo Go handlers may also use middleware that verifies Hmac Signatures and then sets a context value used later for file operations. If the middleware does not enforce strict path constraints, downstream handlers remain vulnerable even though the request is authenticated. Attackers can leverage this by sending specially crafted paths that traverse directories, potentially reading sensitive configuration files or application sources. The vulnerability is not in the Hmac verification itself, but in the lack of output encoding and path sanitization after the signature check.

Real-world attack patterns mirror standard Path Traversal techniques, such as using encoded characters or repeated traversal sequences to bypass naive filters. Because the signature validates the request, rate limiting and anomaly detection may be less effective, increasing the risk of exploitation. This highlights the importance of treating authenticated input as untrusted when it influences filesystem paths, even when protected by cryptographic integrity checks.

To remediate Path Traversal in Echo Go when using Hmac Signatures, always validate and sanitize file paths independently of authentication or integrity checks. Use a strict allowlist approach for file identifiers and resolve paths against a controlled base directory before any filesystem operation.

Below is a secure example demonstrating how to handle signed requests safely in Echo Go. The handler verifies the Hmac signature, then ensures the document ID is safe before constructing the filesystem path.

// Verify Hmac signature and safely resolve file path
func secureFileHandler(c echo.Context) error {
    // Assume signature is validated by middleware; if not, verify here
    docID := c.Param("id")

    // Validate document ID: only allow alphanumeric and hyphens
    if !regexp.MustCompile(`^[a-zA-Z0-9-]+$`).MatchString(docID) {
        return echo.NewHTTPError(http.StatusBadRequest, "invalid document ID")
    }

    baseDir := "/safe/documents"
    // Use filepath.Clean to eliminate any leftover path traversal attempts
    cleanID := filepath.Clean(docID)
    filePath := filepath.Join(baseDir, cleanID)

    // Ensure the resolved path remains within baseDir
    absBase, _ := filepath.Abs(baseDir)
    absPath, _ := filepath.Abs(filePath)
    if !strings.HasPrefix(absPath, filepath.Clean(absBase)+string(os.PathSeparator)) && absPath != absBase {
        return echo.NewHTTPError(http.StatusForbidden, "path traversal detected")
    }

    file, err := os.Open(filePath)
    if err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "unable to open file")
    }
    defer file.Close()

    return c.File(filePath)
}

In this example, the Hmac signature verification is assumed to be handled by middleware; if you implement verification directly, ensure it runs before path validation. The key remediation steps are input validation with a strict regex, canonicalization with filepath.Clean, and a prefix check to guarantee the final path resides within the intended directory. This approach neutralizes Path Traversal regardless of the strength of the Hmac Signatures.

For API designs that reference files by name, prefer storing files in a database or object storage and serving them through a controlled endpoint rather than direct filesystem access. If filesystem access is necessary, maintain a mapping between signed identifiers and filesystem locations server-side, ensuring that identifiers never directly influence paths.