Thursday, August 18, 2016

Backup-File Artifacts: The Underrated Web-Danger


Backup-File Artifacts: The Underrated Web-Danger Testing and Exploiting Backup-File Artifacts with BFAC



In August 13th, 2016, I have given a talk about Backup-File Artifacts, an attacking vector that is not commonly known, or being tested by penetration testers, yet, can be critical to the security of the web environment. At the end of the of the talk, I have released BFAC, an automated security tool that tests for Backup-File Artifacts that is missed on web-servers and can discloses the web-application's source-code.

This post will discuss different ideas on Backup-File Artifacts, will cover the types of Backup-File Artifacts (BFAs), and ways that it occurs, and how it can be related to web-servers. Also, it will cover the usage of BFAC  (Backup-File Artifacts Checker) in finding BFA issues.


What is Backup-File Artifacts that can affects web-applications?


Backup-File Artifacts are basically files that was left during a backup process. Offline backup artifacts is a something that I will not discuss, since it does not affect the security of the web, and it's not related to this research's point.


Most developers make a backup file before editing a script (or code editors or other programs), and then push it to the web-server along with the backup file. Also, one of the developers’ behaviours is to edit a script in the webroot of the web-server, and when finished the process, the backup-file is left on the webroot. This eventually  allows any unauthenticated party to retrieve the source-code by downloading the BFA. This is a general explanation of the BFA concept.


Types of Backup-File Artifacts

    1.   Backup-File Artifacts Caused by Code-Editors

Text-Editors and Code-Editors intentionally make a backup file for the file that is being edited. This is done so that developers can recover the files they are editing if they encountered any issues.
By default, the backup file is made in the same directory of the file that is being edited, also with a pattern based on the original file name.


For instance, when editing a file called index.php with nano, a backup file will be created with the name “index.php.save”. The same goes to other code editors, each code-editor has its own pattern that it follows when creating a backup file for the file that is being edited.

If the backup file was left on the web-server, an attacker can brute-forces the file, until he finds an existent backup file. Since index.php.save will not be executed as index.php, index.php.save will be the response of the request, showing the source-code of the script. Of course, the same goes to all different patterns.


    2.   Backup-File Artifacts Caused by Missed VCS Meta-Directories on The Webroot
Version Control Systems (VCS) are very popular systems in today’s world. All VCS creates a meta-directory on the same folder of the project. This folder includes tracks and logs for every change is done to the files of the project.


Developers tend to download/clone their project’s repositories on the web-server, without removing the VCS directory. This causes the project to be leaked, since the VCS is publicly accessible for unauthorized parties on the web-server.

    3.   Human-Based Missed Backup-File Artifacts

Developers who are not using Version Control Systems tend to do a manual backup file for scripts that needs to be edited. Also, developers uses similar patterns usually that can be easily guessed and enumerated.

For example, when a developer would like to edit index.php, he/she would make an initial copy in case of any errors or issues. The copy can be named with something that is really common.

index.php → index.php.bak
index.php → index.old
index.php → index.php.temp

Since human-based backup patterns can be easily guessed, we can perform the same damage that was mentioned above, which causes the disclosure of the web-application's source-code.

The Problem


Security Researchers and Penetration Testers who relies solely on web-application vulnerability scanners might not encountered this issues a lot in real-world. The reason is because Most web-application vulnerability scanners does not test against Backup-File Artifacts on web-servers. Also, there are no major tools till the release of this research that is dedicated to test Backup-File Artifacts.

Examples of  tested web-application Vulnerability Scanners that does not test perform Backup-File Artifacts testing

  • OWASP ZAP 2.4
  • Vega
  • W3AF – There are partial plugins that does basic testing, but it’s disabled by default.
  • Nikto 2.4.6
  • Burp Suite Professional 1.7.X

Note: these are the versions that I have tested of the products, future versions are mostly don’t support testing for Backup-File Artifacts too.


There are very few number of Web-Application Vulnerability Scanners that does test for Backup-File Artifacts. However, those scanners are using really basic testing of Backup-File Artifacts. This testing that are done by these rare portion of web-application vulnerability scanners misses most important checks, and do not satisfy the needs for professional security assessment done by security testers. For that reasons, I wrote BFAC.

Introducing BFAC

BFAC (Backup-File Artifacts Checker) is an automated tool that checks for backup artifacts that may discloses the web-application's source code. BFAC goal is to be an all-in-one tool for backup-file artifacts black-box testing.

Features


  • Testing all common types of backup-file artifacts patterns, including human-based BFAs, and BFA that can occur via code-editors.
  • Includes tests for common VCS artifacts, such as GIT, Subversion, Mercurial, and Bazaar.
  • Smart detection techniques; Capable of detecting "Not Found", and valid pages using different tests.
  • Stealthy Interaction with Web Servers.
  • Easy to edit and customize based on needs; easy to add custom BFA patterns.
  • Dynamic and generic; made to be not specified to test a specific environment or server.

How to Protect from Backup-File Artifacts on Web-Servers?


System Administrations
Actively scan web-servers for Backup-File Artifacts, and review if anything found should be placed publicly. If it’s not needed, then remove it to a temporary directory for review by developers responsible for it.

Developers
After doing any edits, check if a backup file exist, and remove it from the webroot if it exists.

Penetration Testers
Add testing for Backup-File Artifacts to your testing checklists. Also, you can use BFAC to test for Backup-File Artifacts.


This article is a summary of the presentation, you can find the slides with more information and details below:




2 comments:

  1. In proper environments, I doubt that web developers direclty SSH and VI into a webserver; as many would use source control. As mitigation, I'd add to ensure that (often already there by default) temp and swap files are not checked in.

    ReplyDelete