COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONMITRE canlı kaydı

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.

Soyutlama

Detailed

Tipik ciddiyet

High

Saldırı ihtimali

High

Saldırıyı anla

Bu saldırı kalıbı nedir?

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.

Uygulama akışı

Saldırı adım adım nasıl ilerler?

1

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.

Kullanılan teknikler

  • 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.
2

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.

Kullanılan teknikler

  • 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
3

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.

Gereksinimler

Saldırının gerçekleşmesi için ne gerekir?

Ön koşullar

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

Gerekli beceri

  • 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.

Gerekli kaynak

MITRE kaydında belirtilmemiş.

Tespit

Saldırının göstergeleri nelerdir?

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.

Etki

Başarılı saldırı neye yol açar?

Read Data

Etkilenen alanlar: Confidentiality

Resource Consumption

Etkilenen alanlar: Availability

Execute Unauthorized Commands

Etkilenen alanlar: Confidentiality, Integrity, Availability

Bypass Protection Mechanism

Etkilenen alanlar: Confidentiality, Access Control, Authorization

Savunma

Nasıl önlenir ve etkisi azaltılır?

1
Verify that the user-supplied data does not use backslash character to escape malicious characters.
2
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.
3
Be aware of the threat of alternative method of data encoding.
4
Regular expressions can be used to filter out backslash. Make sure you decode before filtering and validating the untrusted input data.
5
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.
6
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.
7
Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.

Gerçek dünya

MITRE örnek olayları

Örnek olay 1Detay
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 /
Örnek olay 2Detay
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