COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record

CAPEC-78

Using Escaped Slashes in Alternate Encoding

This attack targets the use of the backslash in alternate encoding. An adversary can provide a backslash as a leading character and causes a parser to believe that the next character is special. This is called an escape. By using that trick, the adversary tries to exploit alternate ways to encode the same character which leads to filter problems and opens avenues to attack.

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 attempts to escape multiple different special characters using a backslash.

Techniques used

Escape a special character with a backslash to bypass input validation.
Try different encodings of both the backslash and the special character to see if this bypasses input validation

Exploit

Manipulate input

Once the adversary determines how to bypass filters that filter out special characters using an escaped slash, they will manipulate the user input in a way that is not intended by the application.

Requirements

What does the attack require?

Prerequisites

The application accepts the backlash character as escape character.
The application server does incomplete input data decoding, filtering and validation.

Skills required

Low: The adversary can naively try backslash character and discover that the target host uses it as escape character.
Medium: The adversary may need deep understanding of the host target in order to exploit the vulnerability. The adversary may also use automated tools to probe for this vulnerability.

Resources required

Not stated in the MITRE record.

Detection

What are the indicators of the attack?

An adversary can use a fuzzer in order to probe for this vulnerability. The fuzzer should generate suspicious network activity noticeable by an intrusion detection system.

Impact

What does a successful attack lead to?

Read Data

Affected scopes: Confidentiality

Resource Consumption

Affected scopes: Availability

Execute Unauthorized Commands

Affected scopes: Confidentiality, Integrity, Availability

Bypass Protection Mechanism

Affected scopes: Confidentiality, Access Control, Authorization

Defence

How is it prevented and mitigated?

Verify that the user-supplied data does not use backslash character to escape malicious characters.

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

Be aware of the threat of alternative method of data encoding.

Regular expressions can be used to filter out backslash. Make sure you decode before filtering and validating the untrusted input data.

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.

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.

Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.

Real world

MITRE example instances

Example instance 1Detail

For example, the byte pair \0 might result in a single zero byte (a NULL) being sent. Another example is \t, which is sometimes converted into a tab character. There is often an equivalent encoding between the back slash and the escaped back slash. This means that \/ results in a single forward slash. A single forward slash also results in a single forward slash. The encoding looks like this: / yields / \/ yields /

Example instance 2Detail

An attack leveraging escaped slashes in slternate encodings is very simple. If you believe the target may be filtering the slash, attempt to supply \/ and see what happens. Example command strings to try out include CWD ..\/..\/..\/..\/winnt which converts in many cases to CWD ../../../../winnt To probe for this kind of problem, a small C program that uses string output routines can be very useful. File system calls make excellent testing fodder. The simple snippet int main(int argc, char* argv[]) { puts("\/ \\ \? \. \| "); return 0; } produces the output / \ ? . | Clearly, the back slash is ignored, and thus we have hit on a number of alternative encodings to experiment with. Given our previous example, we can extend the attack to include other possibilities: CWD ..\?\?\?\?\/..\/..\/..\/winnt CWD \.\.\/\.\.\/\.\.\/\.\.\/winnt CWD ..\|\|\|\|\/..\/..\/..\/winnt