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CAPEC-79: Using Slashes in Alternate Encoding

Using Slashes in Alternate Encoding
Definition in a New Window Definition in a New Window
Attack Pattern ID: 79
Abstraction: Detailed
Status: Draft
Completeness: Complete
Presentation Filter:
+ Summary

This attack targets the encoding of the Slash characters. An attacker 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 attacker 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.

+ Attack Execution Flow
  1. The attacker has access to a resource path and required to use slashes as resource delimiter.

  2. The attacker tries variation and combination of the slashes characters in different encoding format.

  3. The attacker found an unfiltered combination which maps to a valid path and accesses unauthorized resources (directories, files, etc.)

+ Attack 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.

+ Typical Severity


+ Typical Likelihood of Exploit

Likelihood: High

+ Methods of Attack
  • Injection
  • Protocol Manipulation
  • API Abuse
+ Examples-Instances


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.

+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: Low

An attacker can try variation of the slashes characters.

Skill or Knowledge Level: Medium

An attacker can use more sophisticated tool or script to scan a website and find a path filtering problem.

+ Probing Techniques

An attacker can try different encoding formats for the slashes characters and see if they produce the same filtering results.

Automated tools such as fuzzer can be used to test the URL decoding and filtering. Custom scripts can also be used. For example, a good script for verifying the correct interpretation of UTF-8 encoded characters can be found at http://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-test.txt

+ Indicators-Warnings of Attack

If the first path decoding process has left some invalid or blacklisted 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 attacker can use a fuzzer in order to probe for a UTF-8 encoding vulnerability. The fuzzer should generate suspicious network activity.

+ Obfuscation Techniques

Typically the obfuscation here is the use of different alternate encoding format (UTF-8, Unicode, etc,)

+ Solutions and Mitigations

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 a white list that defines all valid input to the application based on the requirements specifications. Input that does not match against the white list should not be permitted to enter into the system.

+ Attack Motivation-Consequences
ScopeTechnical ImpactNote
Read application data
Execute unauthorized code or commands
Run Arbitrary Code
Gain privileges / assume identity
+ Injection Vector

The injection vector is a string path such as URL path.

+ Payload

The injection vector is a string path with malicious slashes characters. Alternate encoding format can also be used to code the slashes characters.

+ Payload Activation Impact

The impact of the payload is access to unauthorized resources.

+ Purposes
  • Penetration
  • Exploitation
+ CIA Impact
Confidentiality Impact: HighIntegrity Impact: HighAvailability Impact: Medium
+ Technical Context
Architectural Paradigms
+ References
[R.79.1] [REF-2] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.
[R.79.2] [REF-40] Markus Kuhn. "UTF-8 and Unicode FAQ for Unix/Linux". June 4, 1999. <http://www.cl.cam.ac.uk/~mgk25/unicode.html>.
[R.79.3] [REF-35] Gunter Ollmann. "URL Encoded Attacks - Attacks using the common web browser". CGISecurity.com. <http://www.cgisecurity.com/lib/URLEmbeddedAttacks.html>.
+ Content History
CAPEC Content TeamThe MITRE Corporation2014-06-23Internal_CAPEC_Team
CAPEC Content TeamThe MITRE Corporation2017-01-09Updated Related_Attack_PatternsInternal

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Page Last Updated or Reviewed: December 07, 2015