Home > CAPEC List > CAPEC-71: Using Unicode Encoding to Bypass Validation Logic (Version 3.0)  

CAPEC-71: Using Unicode Encoding to Bypass Validation Logic

Attack Pattern ID: 71
Abstraction: Detailed
Status: Draft
Presentation Filter:
+ Description
An attacker may provide a Unicode string to a system component that is not Unicode aware and use that to circumvent the filter or cause the classifying mechanism to fail to properly understanding the request. That may allow the attacker to slip malicious data past the content filter and/or possibly cause the application to route the request incorrectly.
+ Likelihood Of Attack

Medium

+ Typical Severity

High

+ Relationships

The table(s) below shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.

+ Relevant to the view "Mechanisms of Attack" (CAPEC-1000)
NatureTypeIDName
ChildOfStandard Attack PatternStandard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.267Leverage Alternate Encoding
PeerOfDetailed Attack PatternDetailed Attack Pattern - A detailed level attack pattern in CAPEC provides a low level of detail, typically leveraging a specific technique and targeting a specific technology, and expresses a complete execution flow. Detailed attack patterns are more specific than meta attack patterns and standard attack patterns and often require a specific protection mechanism to mitigate actual attacks. A detailed level attack pattern often will leverage a number of different standard level attack patterns chained together to accomplish a goal.80Using UTF-8 Encoding to Bypass Validation Logic
+ Execution Flow
Explore
  1. Survey the application for user-controllable inputs: Using a browser or an automated tool, an attacker follows all public links and actions on a web site. He records all the links, the forms, the resources accessed and all other potential entry-points for the web application. 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.

    Techniques
    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.
Experiment
  1. Probe entry points to locate vulnerabilities: The attacker uses the entry points gathered in the "Explore" phase as a target list and injects various Unicode encoded payloads to determine if an entry point actually represents a vulnerability with insufficient validation logic and to characterize the extent to which the vulnerability can be exploited. Try to use Unicode encoding of content in Scripts in order to bypass validation routines. Try to use Unicode encoding of content in HTML in order to bypass validation routines. Try to use Unicode encoding of content in CSS in order to bypass validation routines.

    Techniques
    Try to use Unicode encoding of content in Scripts in order to bypass validation routines.
    Try to use Unicode encoding of content in HTML in order to bypass validation routines.
    Try to use Unicode encoding of content in CSS in order to bypass validation routines.
+ Prerequisites
Filtering is performed on data that has not be properly canonicalized.
+ Skills Required
[Level: Medium]
An attacker needs to understand Unicode encodings and have an idea (or be able to find out) what system components may not be Unicode aware.
+ Indicators
Unicode encoded data is passed to APIs where it is not expected
+ Consequences

The table below specifies different individual consequences associated with the attack pattern. The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Confidentiality
Access Control
Authorization
Bypass Protection Mechanism
Confidentiality
Integrity
Availability
Execute Unauthorized Commands
Integrity
Modify Data
Availability
Unreliable Execution
+ Mitigations
Ensure that the system is Unicode aware and can properly process Unicode data. Do not make an assumption that data will be in ASCII.
Ensure that filtering or input validation is applied to canonical data.
Assume all input is malicious. Create a white list that defines all valid input to the software system based on the requirements specifications. Input that does not match against the white list should not be permitted to enter into the system.
+ Example Instances
Attack Example: Unicode Encodings in the IIS Server

A very common technique for a Unicode attack involves traversing directories looking for interesting files. An example of this idea applied to the Web is

http://target.server/some_directory/../../../winnt

In this case, the attacker is attempting to traverse to a directory that is not supposed to be part of standard Web services. The trick is fairly obvious, so many Web servers and scripts prevent it. However, using alternate encoding tricks, an attacker may be able to get around badly implemented request filters.

In October 2000, an adversary publicly revealed that Microsoft's IIS server suffered from a variation of this problem. In the case of IIS, all the attacker had to do was provide alternate encodings for the dots and/or slashes found in a classic attack. The Unicode translations are

. yields C0 AE / yields C0 AF \ yields C1 9C

Using this conversion, the previously displayed URL can be encoded as

http://target.server/some_directory/%C0AE/%C0AE/%C0AE%C0AE/%C0AE%C0AE/winnt
See also: CVE-2000-0884
+ References
[REF-1] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. 2004-02.
+ Content History
Submissions
Submission DateSubmitterOrganization
2014-06-23CAPEC Content TeamThe MITRE Corporation
Modifications
Modification DateModifierOrganization
2017-01-09CAPEC Content TeamThe MITRE Corporation
Updated Related_Attack_Patterns
2018-07-31CAPEC Content TeamThe MITRE Corporation
Updated References

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Page Last Updated or Reviewed: July 31, 2018