Using Slashes in Alternate Encoding
Attack Pattern ID: 79 (Detailed Attack Pattern Completeness: Complete)Typical Severity: HighStatus: Draft
+ Description


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 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

+ 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.

A attacker can use a fuzzer in order to probe for a UTF-8 encoding vulnerability. The fuzzer should generate suspiscious 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 safelly 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 (

Be aware of the threat of alternative method of data encoding and obfuscation technique such as IP address endoding. (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
  • Information Leakage
  • Run Arbitrary Code
  • Privilege Escalation
+ 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.

+ Related Weaknesses
CWE-IDWeakness NameWeakness Relationship Type
173Failure to Handle Alternate EncodingTargeted
171Cleansing, Canonicalization, and Comparison ErrorsTargeted
180Incorrect Behavior Order: Validate Before CanonicalizeTargeted
181Incorrect Behavior Order: Validate Before FilterTargeted
20Improper Input ValidationTargeted
74Failure to Sanitize Data into a Different Plane ('Injection')Targeted
73External Control of File Name or PathTargeted
21Pathname Traversal and Equivalence ErrorsTargeted
22Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')Targeted
185Incorrect Regular ExpressionSecondary
200Information ExposureSecondary
697Insufficient ComparisonTargeted
707Improper Enforcement of Message or Data StructureTargeted
+ Related Attack Patterns
NatureTypeIDNameDescriptionView(s) this relationship pertains toView\(s\)
PeerOfAttack PatternAttack Pattern43Exploiting Multiple Input Interpretation Layers 
Mechanism of Attack1000
PeerOfAttack PatternAttack Pattern71Using Unicode Encoding to Bypass Validation Logic 
Mechanism of Attack1000
ChildOfCategoryCategory126Path Traversal 
Mechanism of Attack (primary)1000
ChildOfAttack PatternAttack Pattern267Leverage Alternate Encoding 
Mechanism of Attack (primary)1000
PeerOfAttack PatternAttack Pattern64Using Slashes and URL Encoding Combined to Bypass Validation Logic 
Mechanism of Attack1000
ParentOfAttack PatternAttack Pattern64Using Slashes and URL Encoding Combined to Bypass Validation Logic 
Mechanism of Attack1000
PeerOfAttack PatternAttack Pattern72URL Encoding 
Mechanism of Attack1000
ParentOfAttack PatternAttack Pattern78Using Escaped Slashes in Alternate Encoding 
Mechanism of Attack1000
+ Related Security Principles
  • Least privilege

  • Reluctance to trust

+ Purposes
  • Penetration
  • Exploitation
+ CIA Impact
Confidentiality Impact: HighIntegrity Impact: HighAvailability Impact: Medium
+ Technical Context
Architectural Paradigms
+ References
G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.

UTF-8 and Unicode FAQ for Unix/Linux, by Markus Kuhn -

URL encoded attacks, by Gunter Ollmann -

+ Content History
G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004.Cigital, Inc2007-03-01
Eric DalciCigital, Inc2007-02-13Fleshed out content to CAPEC schema from the original descriptions in "Exploiting Software"
Sean BarnumCigital, Inc2007-03-07Review and revise
Richard StruseVOXEM, Inc2007-03-26Review and feedback leading to changes in Related Attack Patterns
Sean BarnumCigital, Inc2007-04-16Modified pattern content according to review and feedback