COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record

CAPEC-79

Using Slashes in Alternate Encoding

This attack targets the encoding of the Slash characters. An adversary would try to exploit common filtering problems related to the use of the slashes characters to gain access to resources on the target host. Directory-driven systems, such as file systems and databases, typically use the slash character to indicate traversal between directories or other container components. For murky historical reasons, PCs (and, as a result, Microsoft OSs) choose to use a backslash, whereas the UNIX world typically makes use of the forward slash. The schizophrenic result is that many MS-based systems are required to understand both forms of the slash. This gives the adversary many opportunities to discover and abuse a number of common filtering problems. The goal of this pattern is to discover server software that only applies filters to one version, but not the other.

Abstraction

Detailed

Typical severity

High

Likelihood of attack

High

Understand the attack

What is this attack pattern?

Execution flow

How does the attack proceed, step by step?

Explore

Survey the application for user-controllable inputs

Using a browser, an automated tool or by inspecting the application, an adversary records all entry points to the application.

Techniques used

Use a spidering tool to follow and record all links and analyze the web pages to find entry points. Make special note of any links that include parameters in the URL.
Use a proxy tool to record all user input entry points visited during a manual traversal of the web application.
Use a browser to manually explore the website and analyze how it is constructed. Many browsers' plugins are available to facilitate the analysis or automate the discovery.
Manually inspect the application to find entry points.

Experiment

Probe entry points to locate vulnerabilities

The adversary uses the entry points gathered in the "Explore" phase as a target list and looks for areas where user input is used to access resources on the target host. The adversary attempts different encodings of slash characters to bypass input filters.

Techniques used

Try both backslash and forward slash characters
Try different encodings for slash characters such as %5C

Exploit

Traverse application directories

Once the adversary determines how to bypass filters that filter out slash characters, they will manipulate the user input to include slashes in order to traverse directories and access resources that are not intended for the user.

Requirements

What does the attack require?

Prerequisites

The application server accepts paths to locate resources.
The application server does insufficient input data validation on the resource path requested by the user.
The access right to resources are not set properly.

Skills required

Low: An adversary can try variation of the slashes characters.
Medium: An adversary can use more sophisticated tool or script to scan a website and find a path filtering problem.

Resources required

Not stated in the MITRE record.

Detection

What are the indicators of the attack?

If the first path decoding process has left some invalid or denylisted characters, that may be a sign that the request is malicious.

Traffic filtering with IDS (or proxy) can detect request with suspicious URLs. IDS may use signature based identification to reveal such URL based attacks.

An adversary can use a fuzzer in order to probe for a UTF-8 encoding vulnerability. The fuzzer should generate suspicious network activity.

Impact

What does a successful attack lead to?

Read Data

Affected scopes: Confidentiality

Execute Unauthorized Commands

Affected scopes: Confidentiality, Integrity, Availability

Gain Privileges

Affected scopes: Confidentiality, Access Control, Authorization

Defence

How is it prevented and mitigated?

Any security checks should occur after the data has been decoded and validated as correct data format. Do not repeat decoding process, if bad character are left after decoding process, treat the data as suspicious, and fail the validation process. Refer to the RFCs to safely decode URL.

When client input is required from web-based forms, avoid using the "GET" method to submit data, as the method causes the form data to be appended to the URL and is easily manipulated. Instead, use the "POST method whenever possible.

There are tools to scan HTTP requests to the server for valid URL such as URLScan from Microsoft (http://www.microsoft.com/technet/security/tools/urlscan.mspx)

Be aware of the threat of alternative method of data encoding and obfuscation technique such as IP address encoding. (See related guideline section)

Test your path decoding process against malicious input.

In the case of path traversals, use the principle of least privilege when determining access rights to file systems. Do not allow users to access directories/files that they should not access.

Assume all input is malicious. Create an allowlist that defines all valid input to the application based on the requirements specifications. Input that does not match against the allowlist should not be permitted to enter into the system.

Real world

MITRE example instances

Example instance 1Detail

Attack Example: Slashes in Alternate Encodings The two following requests are equivalent on most Web servers: http://target server/some_directory\..\..\..\winnt is equivalent to http://target server/some_directory/../../../winnt Multiple encoding conversion problems can also be leveraged as various slashes are instantiated in URL-encoded, UTF-8, or Unicode. Consider the strings http://target server/some_directory\..%5C..%5C..\winnt where %5C is equivalent to the \ character.