使用bandit對目標(biāo)python代碼進(jìn)行安全函數(shù)掃描的案例分析

更新時(shí)間：2021年01月27日 11:08:20 作者：DechinPhy

這篇文章主要介紹了使用bandit對目標(biāo)python代碼進(jìn)行安全函數(shù)掃描,本文給大家介紹的非常詳細(xì)，對大家的學(xué)習(xí)或工作具有一定的參考借鑒價(jià)值，需要的朋友可以參考下

技術(shù)背景

在一些對python開源庫代碼的安全掃描中，我們有可能需要分析庫中所使用到的函數(shù)是否會對代碼的執(zhí)行環(huán)境造成一些非預(yù)期的影響。典型的例如python的沙箱逃逸問題，通過一些python的第三方庫可以執(zhí)行系統(tǒng)shell命令，而這就不在python的沙箱防護(hù)范圍之內(nèi)了。關(guān)于python的沙箱逃逸問題，這里不作展開，這也是困擾業(yè)界多年的一個(gè)問題，連python官方也提過python的沙箱是沒有完美的防護(hù)方案的，這里僅作為一個(gè)背景案例使用：

# subprocess_Popen.py

import subprocess
import uuid

subprocess.Popen('touch ' + str(uuid.uuid1()) +'.txt', shell = True)

這里演示的功能是使用subprocess函數(shù)庫開啟一個(gè)系統(tǒng)shell，并執(zhí)行一個(gè)touch的指令，可以生成一個(gè)指定文件名的文件，類似于mkdir產(chǎn)生一個(gè)文件夾。我們可以看到這個(gè)文件成功執(zhí)行后會在當(dāng)前的目錄下生成一個(gè)uuid隨機(jī)命名的txt文件：

[dechin@dechin-manjaro bandit_test]$ python3 subprocess_Popen.py 
[dechin@dechin-manjaro bandit_test]$ ll
總用量 4
-rw-r--r-- 1 dechin dechin 0 1月 26 23:03 b7aa0fc8-5fe7-11eb-b5d3-058313e110e4.txt
-rw-r--r-- 1 dechin dechin 123 1月 26 23:03 subprocess_Popen.py

然而，本次的關(guān)注點(diǎn)并不在與這個(gè)函數(shù)執(zhí)行了什么功能，而是這個(gè)函數(shù)中用到了subprocess這個(gè)函數(shù)庫。按照python的語言特點(diǎn)，當(dāng)你的系統(tǒng)中如果存在這樣的一個(gè)模塊引用了subprocess庫，那么任何可以調(diào)用該功能模塊的函數(shù)，都可以調(diào)用到subprocess這個(gè)函數(shù)，以下是另外一個(gè)惡意用戶的python代碼：

# bad.py

from subprocess_Popen import subprocess as subprocess

subprocess.Popen('touch bad.txt', shell = True)

該代碼的目的是在不直接import subprocess的條件下，通過前面創(chuàng)建好的subprocess_Popen.py來進(jìn)行搭橋調(diào)用subprocess的功能函數(shù)。這個(gè)腳本的執(zhí)行結(jié)果如下：

[dechin@dechin-manjaro bandit_test]$ python3 bad.py 
[dechin@dechin-manjaro bandit_test]$ ll
總用量 12
-rw-r--r-- 1 dechin dechin 0 1月 26 23:13 0fda7ede-5fe9-11eb-80a8-ad279ab4e0a6.txt
-rw-r--r-- 1 dechin dechin 0 1月 26 23:03 b7aa0fc8-5fe7-11eb-b5d3-058313e110e4.txt
-rw-r--r-- 1 dechin dechin 113 1月 26 23:13 bad.py
-rw-r--r-- 1 dechin dechin 0 1月 26 23:13 bad.txt
drwxr-xr-x 2 dechin dechin 4096 1月 26 23:13 __pycache__
-rw-r--r-- 1 dechin dechin 123 1月 26 23:03 subprocess_Popen.py

這個(gè)結(jié)果意味著，我們成功的使用bad.py調(diào)用了subprocess_Popen.py中所引用的subprocess，成功touch了一個(gè)bad.txt的文件。

到這里我們的背景案例演示結(jié)束，但我們需要重新梳理這些案例中所包含的邏輯：我們原本是希望在自己的系統(tǒng)中不引入python的沙箱逃逸問題，我們會對其他人傳遞過來的代碼進(jìn)行掃描，如使用下文中將要介紹的bandit工具來屏蔽subprocess等"危險(xiǎn)函數(shù)"。而如果我們在自己寫的python庫或者引入的第三方python庫中存在類似于subprocess的引用，這就會導(dǎo)致我們的屏蔽失效，用戶可以任意的通過這些引用的搭橋直接調(diào)用subprocess的函數(shù)功能。因此，在特殊的條件要求下，我們需要對自己的代碼進(jìn)行安全函數(shù)掃描，以免為其他人的系統(tǒng)帶來不可預(yù)期的安全風(fēng)險(xiǎn)。bandit只是其中的一種安全函數(shù)掃描的工具，接下來我們介紹一下其基本安裝和使用方法。

用pip安裝bandit

這里直接使用pip來安裝bandit，有需要的也可以從源碼直接安裝。關(guān)于在pip的使用中配置國內(nèi)鏡像源的方法，可以參考這篇博客中對python安裝第三方庫的介紹。

[dechin@dechin-manjaro bandit_test]$ python3 -m pip install bandit
Collecting bandit
 Downloading bandit-1.7.0-py3-none-any.whl (115 kB)
 |████████████████████████████████| 115 kB 101 kB/s 
Requirement already satisfied: PyYAML>=5.3.1 in /home/dechin/anaconda3/lib/python3.8/site-packages (from bandit) (5.3.1)
Collecting GitPython>=1.0.1
 Downloading GitPython-3.1.12-py3-none-any.whl (159 kB)
 |████████████████████████████████| 159 kB 28 kB/s 
Requirement already satisfied: six>=1.10.0 in /home/dechin/anaconda3/lib/python3.8/site-packages (from bandit) (1.15.0)
Collecting stevedore>=1.20.0
 Downloading stevedore-3.3.0-py3-none-any.whl (49 kB)
 |████████████████████████████████| 49 kB 25 kB/s 
Collecting gitdb<5,>=4.0.1
 Downloading gitdb-4.0.5-py3-none-any.whl (63 kB)
 |████████████████████████████████| 63 kB 28 kB/s 
Collecting pbr!=2.1.0,>=2.0.0
 Downloading pbr-5.5.1-py2.py3-none-any.whl (106 kB)
 |████████████████████████████████| 106 kB 26 kB/s 
Collecting smmap<4,>=3.0.1
 Downloading smmap-3.0.5-py2.py3-none-any.whl (25 kB)
Installing collected packages: smmap, gitdb, GitPython, pbr, stevedore, bandit
Successfully installed GitPython-3.1.12 bandit-1.7.0 gitdb-4.0.5 pbr-5.5.1 smmap-3.0.5 stevedore-3.3.0

安裝結(jié)束之后，可以通過以下指令驗(yàn)證是否安裝成功：

[dechin@dechin-manjaro bandit_test]$ bandit -h
usage: bandit [-h] [-r] [-a {file,vuln}] [-n CONTEXT_LINES] [-c CONFIG_FILE] [-p PROFILE] [-t TESTS] [-s SKIPS] [-l] [-i] [-f {csv,custom,html,json,screen,txt,xml,yaml}] [--msg-template MSG_TEMPLATE] [-o [OUTPUT_FILE]] [-v] [-d] [-q]
  [--ignore-nosec] [-x EXCLUDED_PATHS] [-b BASELINE] [--ini INI_PATH] [--exit-zero] [--version]
  [targets [targets ...]]

Bandit - a Python source code security analyzer

positional arguments:
 targets  source file(s) or directory(s) to be tested

optional arguments:
 -h, --help  show this help message and exit
 -r, --recursive find and process files in subdirectories
 -a {file,vuln}, --aggregate {file,vuln}
   aggregate output by vulnerability (default) or by filename
 -n CONTEXT_LINES, --number CONTEXT_LINES
   maximum number of code lines to output for each issue
 -c CONFIG_FILE, --configfile CONFIG_FILE
   optional config file to use for selecting plugins and overriding defaults
 -p PROFILE, --profile PROFILE
   profile to use (defaults to executing all tests)
 -t TESTS, --tests TESTS
   comma-separated list of test IDs to run
 -s SKIPS, --skip SKIPS
   comma-separated list of test IDs to skip
 -l, --level  report only issues of a given severity level or higher (-l for LOW, -ll for MEDIUM, -lll for HIGH)
 -i, --confidence report only issues of a given confidence level or higher (-i for LOW, -ii for MEDIUM, -iii for HIGH)
 -f {csv,custom,html,json,screen,txt,xml,yaml}, --format {csv,custom,html,json,screen,txt,xml,yaml}
   specify output format
 --msg-template MSG_TEMPLATE
   specify output message template (only usable with --format custom), see CUSTOM FORMAT section for list of available values
 -o [OUTPUT_FILE], --output [OUTPUT_FILE]
   write report to filename
 -v, --verbose  output extra information like excluded and included files
 -d, --debug  turn on debug mode
 -q, --quiet, --silent
   only show output in the case of an error
 --ignore-nosec do not skip lines with # nosec comments
 -x EXCLUDED_PATHS, --exclude EXCLUDED_PATHS
   comma-separated list of paths (glob patterns supported) to exclude from scan (note that these are in addition to the excluded paths provided in the config file) (default:
   .svn,CVS,.bzr,.hg,.git,__pycache__,.tox,.eggs,*.egg)
 -b BASELINE, --baseline BASELINE
   path of a baseline report to compare against (only JSON-formatted files are accepted)
 --ini INI_PATH path to a .bandit file that supplies command line arguments
 --exit-zero  exit with 0, even with results found
 --version  show program's version number and exit

CUSTOM FORMATTING
-----------------

Available tags:

 {abspath}, {relpath}, {line}, {test_id},
 {severity}, {msg}, {confidence}, {range}

Example usage:

 Default template:
 bandit -r examples/ --format custom --msg-template \
 "{abspath}:{line}: {test_id}[bandit]: {severity}: {msg}"

 Provides same output as:
 bandit -r examples/ --format custom

 Tags can also be formatted in python string.format() style:
 bandit -r examples/ --format custom --msg-template \
 "{relpath:20.20s}: {line:03}: {test_id:^8}: DEFECT: {msg:>20}"

 See python documentation for more information about formatting style:
 https://docs.python.org/3/library/string.html

The following tests were discovered and loaded:
-----------------------------------------------
 B101 assert_used
 B102 exec_used
 B103 set_bad_file_permissions
 B104 hardcoded_bind_all_interfaces
 B105 hardcoded_password_string
 B106 hardcoded_password_funcarg
 B107 hardcoded_password_default
 B108 hardcoded_tmp_directory
 B110 try_except_pass
 B112 try_except_continue
 B201 flask_debug_true
 B301 pickle
 B302 marshal
 B303 md5
 B304 ciphers
 B305 cipher_modes
 B306 mktemp_q
 B307 eval
 B308 mark_safe
 B309 httpsconnection
 B310 urllib_urlopen
 B311 random
 B312 telnetlib
 B313 xml_bad_cElementTree
 B314 xml_bad_ElementTree
 B315 xml_bad_expatreader
 B316 xml_bad_expatbuilder
 B317 xml_bad_sax
 B318 xml_bad_minidom
 B319 xml_bad_pulldom
 B320 xml_bad_etree
 B321 ftplib
 B323 unverified_context
 B324 hashlib_new_insecure_functions
 B325 tempnam
 B401 import_telnetlib
 B402 import_ftplib
 B403 import_pickle
 B404 import_subprocess
 B405 import_xml_etree
 B406 import_xml_sax
 B407 import_xml_expat
 B408 import_xml_minidom
 B409 import_xml_pulldom
 B410 import_lxml
 B411 import_xmlrpclib
 B412 import_httpoxy
 B413 import_pycrypto
 B501 request_with_no_cert_validation
 B502 ssl_with_bad_version
 B503 ssl_with_bad_defaults
 B504 ssl_with_no_version
 B505 weak_cryptographic_key
 B506 yaml_load
 B507 ssh_no_host_key_verification
 B601 paramiko_calls
 B602 subprocess_popen_with_shell_equals_true
 B603 subprocess_without_shell_equals_true
 B604 any_other_function_with_shell_equals_true
 B605 start_process_with_a_shell
 B606 start_process_with_no_shell
 B607 start_process_with_partial_path
 B608 hardcoded_sql_expressions
 B609 linux_commands_wildcard_injection
 B610 django_extra_used
 B611 django_rawsql_used
 B701 jinja2_autoescape_false
 B702 use_of_mako_templates
 B703 django_mark_safe

從這個(gè)列表中的屏蔽函數(shù)我們可以看出所謂的"危險(xiǎn)函數(shù)"到底都有哪些，比如常用的subprocess和random都被包含在內(nèi)。subprocess是因?yàn)槠鋵hell的調(diào)用而被列為"危險(xiǎn)函數(shù)"，而random則是因?yàn)槠鋫坞S機(jī)數(shù)的性質(zhì)(這里簡單說明一下，現(xiàn)在一般推薦使用secrets中的所謂安全隨機(jī)數(shù)，但是實(shí)際上只有量子疊加測量才能夠真正實(shí)現(xiàn)真隨機(jī)數(shù))。

bandit常用使用方法

直接對py文件進(jìn)行掃描：

[dechin@dechin-manjaro bandit_test]$ bandit subprocess_Popen.py 
[main] INFO profile include tests: None
[main] INFO profile exclude tests: None
[main] INFO cli include tests: None
[main] INFO cli exclude tests: None
[main] INFO running on Python 3.8.5
[node_visitor] INFO Unable to find qualified name for module: subprocess_Popen.py
Run started:2021-01-26 15:31:00.425603

Test results:
>> Issue: [B404:blacklist] Consider possible security implications associated with subprocess module.
 Severity: Low Confidence: High
 Location: subprocess_Popen.py:3
 More Info: https://bandit.readthedocs.io/en/latest/blacklists/blacklist_imports.html#b404-import-subprocess
2
3 import subprocess
4 import uuid

--------------------------------------------------
>> Issue: [B602:subprocess_popen_with_shell_equals_true] subprocess call with shell=True identified, security issue.
 Severity: High Confidence: High
 Location: subprocess_Popen.py:6
 More Info: https://bandit.readthedocs.io/en/latest/plugins/b602_subprocess_popen_with_shell_equals_true.html
5
6 subprocess.Popen('touch ' + str(uuid.uuid1()) +'.txt', shell = True)

--------------------------------------------------

Code scanned:
 Total lines of code: 3
 Total lines skipped (#nosec): 0

Run metrics:
 Total issues (by severity):
  Undefined: 0.0
  Low: 1.0
  Medium: 0.0
  High: 1.0
 Total issues (by confidence):
  Undefined: 0.0
  Low: 0.0
  Medium: 0.0
  High: 2.0
Files skipped (0):

通過對剛才所創(chuàng)建的調(diào)用了危險(xiǎn)函數(shù)subprocess的py文件subprocess_Popen.py的掃描，我們識別出了其中的"危險(xiǎn)函數(shù)"，注意這里的Issue編號是602，定級是Severity: Low Confidence: High。但是如果我們用bandit去掃描利用了其他函數(shù)對危險(xiǎn)函數(shù)的調(diào)用搭橋來二次調(diào)用的bad.py文件，我們發(fā)現(xiàn)是另外一種結(jié)果：

[dechin@dechin-manjaro bandit_test]$ bandit bad.py 
[main] INFO profile include tests: None
[main] INFO profile exclude tests: None
[main] INFO cli include tests: None
[main] INFO cli exclude tests: None
[main] INFO running on Python 3.8.5
[node_visitor] INFO Unable to find qualified name for module: bad.py
Run started:2021-01-26 15:30:47.370468

Test results:                            
>> Issue: [B404:blacklist] Consider possible security implications associated with subprocess module.
 Severity: Low Confidence: High                          
 Location: bad.py:3                           
 More Info: https://bandit.readthedocs.io/en/latest/blacklists/blacklist_imports.html#b404-import-subprocess                
2
3 from subprocess_Popen import subprocess as subprocess
4
5 subprocess.Popen('touch bad.txt', shell = True)

--------------------------------------------------
>> Issue: [B604:any_other_function_with_shell_equals_true] Function call with shell=True parameter identified, possible security issue.
 Severity: Medium Confidence: Low                         
 Location: bad.py:5                           
 More Info: https://bandit.readthedocs.io/en/latest/plugins/b604_any_other_function_with_shell_equals_true.html                
4
5 subprocess.Popen('touch bad.txt', shell = True)

--------------------------------------------------

Code scanned:                            
 Total lines of code: 2
 Total lines skipped (#nosec): 0

Run metrics:                         
 Total issues (by severity):
  Undefined: 0.0
  Low: 1.0
  Medium: 1.0
  High: 0.0
 Total issues (by confidence):
  Undefined: 0.0
  Low: 1.0
  Medium: 0.0
  High: 1.0
Files skipped (0):

注意這里雖然實(shí)現(xiàn)的功能跟上面那個(gè)例子是一樣的，但是這里的Issue編號為604，定級也變成了Severity: Medium Confidence: Low。這里的關(guān)鍵并不是定級變成了什么，而是定級被改變了，這是因?yàn)?code>bandit是通過對字符串的處理來識別危險(xiǎn)函數(shù)的，因此對于這種二次調(diào)用的特殊場景，bandit不一定都能夠準(zhǔn)確的識別出來對危險(xiǎn)函數(shù)的調(diào)用，甚至可能出現(xiàn)二次調(diào)用后，完全無法識別風(fēng)險(xiǎn)函數(shù)的使用的可能性。

2.掃描一個(gè)目錄下的所有py文件，并將結(jié)果寫入txt文件

[dechin@dechin-manjaro bandit_test]$ bandit *.py -o test_bandit.txt -f txt
[main] INFO profile include tests: None
[main] INFO profile exclude tests: None
[main] INFO cli include tests: None
[main] INFO cli exclude tests: None
[main] INFO running on Python 3.8.5
[node_visitor] INFO Unable to find qualified name for module: bad.py
[node_visitor] INFO Unable to find qualified name for module: subprocess_Popen.py
[text] INFO Text output written to file: test_bandit.txt

該案例就掃描了當(dāng)前目錄下的所有py文件，其實(shí)就是bad.py和subprocess_Popen.py這兩個(gè)，并且將最終的掃描結(jié)果保存至test_bandit.txt文件中，這里我們就不展示txt文件的具體內(nèi)容，大概就是將上一章節(jié)的兩個(gè)執(zhí)行結(jié)果進(jìn)行了整合。

3.掃描一個(gè)目錄下的多層文件夾中的py文件，并將結(jié)果寫入html文件

假如我們有如下所示的一個(gè)目錄結(jié)構(gòu)需要進(jìn)行掃描測試：

[dechin@dechin-manjaro bandit_test]$ tree
.
├── bad.py
├── bad.txt
├── level2
│ └── test_random.py
├── subprocess_Popen.py
├── test_bandit.html
└── test_bandit.txt

1 directory, 6 files
[dechin@dechin-manjaro bandit_test]$ cat level2/test_random.py 
# test_bandit.py

import random

a = random.random()

我們可以在當(dāng)前目錄下執(zhí)行如下指令：

[dechin@dechin-manjaro bandit_test]$ bandit -r . -f html -o test_bandit.html
[main] INFO profile include tests: None
[main] INFO profile exclude tests: None
[main] INFO cli include tests: None
[main] INFO cli exclude tests: None
[main] INFO running on Python 3.8.5
[html] INFO HTML output written to file: test_bandit.html

這里我們得到的結(jié)果是一個(gè)test_bandit.html文件，文件內(nèi)容如下圖所示：

4.使用配置文件禁用部分Issue
在執(zhí)行目錄下創(chuàng)建一個(gè).bandit文件，作如下配置就可以避免對B404的審查：

[bandit]
skips: B404

執(zhí)行的掃描結(jié)果如下圖所示，我們可以看到B404相關(guān)的Issue已經(jīng)不在列表中了：

5.在py文件中直接逃避bandit審計(jì)
在待掃描的py文件的對應(yīng)風(fēng)險(xiǎn)函數(shù)后加上如下注釋，即可在bandit審計(jì)過程中自動忽略：

# bad.py

from subprocess_Popen import subprocess as sb

sb.Popen('touch bad.txt', shell = 1) # nosec

這里我們可以看到最終的審計(jì)結(jié)果中，B604也隨之而不見了，如下圖所示。從這個(gè)案例中我們也可以知悉，bandit并不是一個(gè)用來作安全防護(hù)的工具，僅僅是用來做比較初步的python代碼安全函數(shù)使用規(guī)范的審查工作，而掃描出來的問題是否處理，其實(shí)最終還是取決于開發(fā)者自己。

bandit簡單性能測試

眾所周知python語言的性能是極其受限的，因此bandit的性能也有可能十分的低下，這里讓我們來定量的測試一下bandit的性能到底在什么水準(zhǔn)。首先我們創(chuàng)建一個(gè)10000行的py文件，內(nèi)容全部為危險(xiǎn)函數(shù)的使用：

# gen.py

import os

with open('test_bandit_power.py', 'w') as py_file:
 py_file.write('import subprocess as sb\n')
 for i in range(10000):
 py_file.write('sb.Popen(\'whoami\', shell = 1)\n')

通過執(zhí)行python3 gen.py就可以生成一個(gè)10000行的危險(xiǎn)函數(shù)文件test_bandit_power.py，大約300KB的大小。此時(shí)我們針對這單個(gè)的文件進(jìn)行bandit掃描測試，我們發(fā)現(xiàn)這個(gè)過程極為漫長，并且生成了大量的錯(cuò)誤日志：

[dechin@dechin-manjaro bandit_test]$ time bandit test_bandit_power.py -f html -o test_power.html
[main] INFO profile include tests: None
[main] INFO profile exclude tests: None
[main] INFO cli include tests: None
[main] INFO cli exclude tests: None
[main] INFO running on Python 3.8.5
[node_visitor] INFO Unable to find qualified name for module: test_bandit_power.py
[html] INFO HTML output written to file: test_power.html

real 0m6.239s
user 0m6.082s
sys 0m0.150s

我們可以簡單估算，如果10000行的代碼都需要6s的時(shí)間來進(jìn)行掃描，那么對于比較大的項(xiàng)目的1000000+的代碼的掃描時(shí)間，則有可能達(dá)到10min往上，這個(gè)時(shí)間雖然也不是特別長，但是對于大型的項(xiàng)目而言這絕對不是一個(gè)非常高效的選擇。

總結(jié)概要

在一些對安全性要求較高的開發(fā)項(xiàng)目中，有可能會禁止使用危險(xiǎn)函數(shù)，如subprocess等。而bandit的作用旨在通過對代碼的掃描自動化的給出安全危險(xiǎn)函數(shù)分析意見，至于是否采納，還是以不同項(xiàng)目的管理者需求為準(zhǔn)。同時(shí)經(jīng)過我們的測試發(fā)現(xiàn)，bandit在實(shí)際使用場景下性能表現(xiàn)并不如意，因此在大型項(xiàng)目中我們并不推薦使用，如果一定要使用也可以考慮進(jìn)行針對性的配置。

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作者ID：DechinPhy
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