flawopen.com/path-traversal/Python

● CWE-918 · 高危
安全研究 · FlawOpen

漏洞深度剖析：Path Traversal in Python

How os.path.join silently discards the root directory when input starts with a slash, and how to use pathlib.Path.resolve() and is_relative_to().

💡 通俗易懂的原理解析 (ELI5)

设想一家酒店的房卡扫描锁原本只允许开启二楼的客房。如果客人在门禁键盘上输入 '../../master-safe'，有缺陷的门锁就会跳出当前楼道，直接打开酒店经理办公室的主保险箱。

核心概念与专有名词

Web Application Security
技术概念 (Web Application Security)：Core architecture component affected by CWE-918.
CWE-918
技术概念 (CWE-918)：Standard Common Weakness Enumeration classification for path-traversal-python.
Defense-in-Depth
技术概念 (Defense-in-Depth)：Multi-layered engineering verification and runtime boundary isolation.

攻击执行流程分解

Step 1

客户端提交文件路径参数

应用程序接口通过 HTTP 参数接收用户指定的文件名、报告路径或静态资源标识。

Step 2

目录遍历序列注入

攻击者在文件名参数中注入相对路径遍历符号（如 '../', '..%2f'）或绝对路径覆盖。

Step 3

文件系统根目录边界逃逸

后端直接将不可信输入与基础目录拼接，未进行规范化绝对路径解析（Canonicalization）与边界校验。

Step 4

敏感文件未授权读取或覆盖

系统运行时打开并回传敏感配置文件（如 /etc/passwd、源码凭证、环境变量密钥），造成数据外泄。

源代码对比：漏洞与安全实现

✕ 存在漏洞的实现
# VULNERABLE: os.path.join discards BASE_DIR if filename starts with '/'
import os
from flask import Flask, request, abort, send_file

app = Flask(__name__)
BASE_DIR = '/var/app/public/user_uploads'

@app.route('/download')
def download_file():
    filename = request.args.get('file')
    # If filename is "/etc/passwd", os.path.join returns "/etc/passwd"!
    # If filename is "../../etc/passwd", it traverses outside BASE_DIR
    target_path = os.path.join(BASE_DIR, filename)
    
    if not os.path.exists(target_path):
        abort(404)
    return send_file(target_path)
✓ 加固后的安全修复
# HARDENED: Resolve canonical path and enforce strict boundary containment
from pathlib import Path
from flask import Flask, request, abort, send_file

app = Flask(__name__)
BASE_DIR = Path('/var/app/public/user_uploads').resolve()

@app.route('/download')
def download_file():
    filename = request.args.get('file', '')
    # 1. Strip leading slashes and resolve relative components
    clean_name = filename.lstrip('/\\')
    target_path = (BASE_DIR / clean_name).resolve()
    
    # 2. Strict containment check: target MUST be inside BASE_DIR
    if not target_path.is_relative_to(BASE_DIR) or not target_path.is_file():
        abort(403)
        
    return send_file(target_path)

工程与系统安全加固清单

References