Path Traversal in Buffalo with Bearer Tokens

Path Traversal occurs when user-controlled input is used to construct file system paths without proper validation or sanitization, allowing an attacker to access files outside the intended directory. In a Buffalo application, this risk can be inadvertently exposed even when requests include Bearer Tokens for authentication. Bearer Tokens are typically passed in the Authorization header and are often treated as a strong authentication mechanism, which may lead developers to assume that requests are trusted. However, authentication and access control are separate concerns; a valid token confirms identity but does not enforce authorization or input safety.

Consider a Buffalo handler that serves user-provided filenames from a local directory, such as logs or reports, while requiring a Bearer Token for access:

// Example: unsafe file serving in a Buffalo handler
func ServeFile(c buffalo.Context) error {
    // Assume authentication middleware has validated the Bearer Token
    filename := c.Param("filename")
    filePath := filepath.Join("/var/app/reports/", filename)
    http.ServeFile(c.Response(), c.Request(), filePath)
    return nil
}

An attacker authenticated with a valid Bearer Token could supply a filename like ../../../etc/passwd. The filepath.Join function does not sanitize prior path segments, and the resulting path escapes the intended directory. Because the handler trusts the token, the request proceeds, exposing sensitive system files. This scenario illustrates how Bearer Tokens do not mitigate Path Traversal; they only verify who is making the request, not whether the request is safe.

Moreover, API specifications such as OpenAPI may document endpoints with Bearer Token requirements while overlooking path parameter validation. Security scans that correlate runtime behavior with spec definitions can detect such mismatches. middleBrick’s OpenAPI/Swagger spec analysis resolves $ref definitions and cross-references them with runtime findings, identifying cases where authentication is present but input validation is weak. This is particularly important because Path Traversal in Buffalo with Bearer Tokens may appear fully authorized yet remain critically vulnerable.

Remediation focuses on strict input validation and avoiding direct filesystem path construction from user input. Even when Bearer Tokens are used for authentication, handlers must treat all user-controlled data as untrusted. The following patterns demonstrate secure approaches in Buffalo.

1. Validate and sanitize filenames

Use a whitelist of allowed filenames or a strict allowlist pattern. Avoid relying on path traversal prevention alone.

func SafeServeFile(c buffalo.Context) error {
    // Restrict filenames to alphanumeric and limited safe characters
    filename := c.Param("filename")
    matched, err := regexp.MatchString(`^[a-zA-Z0-9_.-]+$`, filename)
    if err != nil || !matched {
        return c.Error(400, errors.New("invalid filename"))
    }
    baseDir := "/var/app/reports/"
    filePath := filepath.Join(baseDir, filename)
    // Ensure the resolved path remains within baseDir
    if !strings.HasPrefix(filePath, filepath.Clean(baseDir)) {
        return c.Error(403, errors.New("access denied"))
    }
    http.ServeFile(c.Response(), c.Request(), filePath)
    return nil
}

2. Use http.Dir with OpenFile to prevent escapes

The standard library’s http.Dir implements Open with Clean and checks that the resolved path remains inside the root. This is a robust approach for file serving.

func DirServeFile(c buffalo.Context) error {
    filename := c.Param("filename")
    root := http.Dir("/var/app/reports/")
    file, err := root.Open(filename)
    if err != nil {
        return c.Error(404, errors.New("file not found"))
    }
    defer file.Close()
    // Optionally inspect file info to ensure it’s a regular file
    info, err := file.Stat()
    if err != nil || info.IsDir() {
        return c.Error(403, errors.New("forbidden"))
    }
    http.ServeContent(c.Response(), c.Request(), info.Name(), info.ModTime(), file)
    return nil
}

3. Avoid user input in filesystem operations entirely when possible

If files are accessed by an identifier, map identifiers to filesystem paths server-side rather than exposing paths directly. For example, use a lookup table or database mapping tokenized references to file paths.

type FileRecord struct {
    ID   string
    Path string
}

func LookupServeFile(c buffalo.Context) error {
    id := c.Param("id")
    var record FileRecord
    // Assume DB lookup by ID
    if db.Where("id = ?", id).First(&record).RecordNotFound() {
        return c.Error(404, errors.New("not found"))
    }
    http.ServeFile(c.Response(), c.Request(), record.Path)
    return nil
}

These patterns ensure that even with Bearer Token authentication, Path Traversal risks are mitigated through rigorous input validation and secure path resolution. middleBrick’s CLI and GitHub Action integrations can help detect such issues during development by scanning endpoints and flagging insecure path handling patterns.