CAPEC-71: Using Unicode Encoding to Bypass Validation Logic
Attack Pattern ID: 71
Abstraction: Detailed
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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
This table shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, 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.
Nature
Type
ID
Name
ChildOf
Standard 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.
Detailed 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.
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. They record all the links, the forms, the resources accessed and all other potential entry-points for the web application.
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
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.
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
This table 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.
Scope
Impact
Likelihood
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 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.
Example Instances
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
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
A Related Weakness relationship associates a weakness with this attack pattern. Each association implies a weakness that must exist for a given attack to be successful. If multiple weaknesses are associated with the attack pattern, then any of the weaknesses (but not necessarily all) may be present for the attack to be successful. Each related weakness is identified by a CWE identifier.
CAPEC mappings to ATT&CK techniques leverage an inheritance model to streamline and minimize direct CAPEC/ATT&CK mappings. Inheritance of a mapping is indicated by text stating that the parent CAPEC has relevant ATT&CK mappings. Note that the ATT&CK Enterprise Framework does not use an inheritance model as part of the mapping to CAPEC.
Relevant to the ATT&CK taxonomy mapping (see parent)
Description
This table shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, 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.
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