ByteOS Network

CAPEC-72:URL Encoding

Attack Pattern ID:72

Version:v3.9

Attack Pattern Name:URL Encoding

Abstraction:Detailed

Status:Draft

Likelihood of Attack:High

Typical Severity:High

Details Content History Related Weaknesses Reports

▼Description

This attack targets the encoding of the URL. An adversary can take advantage of the multiple way of encoding an URL and abuse the interpretation of the URL.

▼Extended Description

A URL may contain special character that need special syntax handling in order to be interpreted. Special characters are represented using a percentage character followed by two digits representing the octet code of the original character (%HEX-CODE).

For instance US-ASCII space character would be represented with %20. This is often referred as escaped ending or percent-encoding. Since the server decodes the URL from the requests, it may restrict the access to some URL paths by validating and filtering out the URL requests it received. An adversary will try to craft an URL with a sequence of special characters which once interpreted by the server will be equivalent to a forbidden URL.

It can be difficult to protect against this attack since the URL can contain other format of encoding such as UTF-8 encoding, Unicode-encoding, etc. The adversary could also subvert the meaning of the URL string request by encoding the data being sent to the server through a GET request. For instance an adversary may subvert the meaning of parameters used in a SQL request and sent through the URL string (See Example section).

▼Relationships

Nature	Type	ID	Name
ChildOf	S	267	Leverage Alternate Encoding

Nature: ChildOf

Type: Standard

ID: 267

Name: Leverage Alternate Encoding

▼Execution Flow

Explore

Survey web application for URLs with parameters

Using a browser, an automated tool or by inspecting the application, an adversary records all URLs that contain parameters.

Technique
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.

Experiment

Probe URLs to locate vulnerabilities

The adversary uses the URLs gathered in the "Explore" phase as a target list and tests parameters with different encodings of special characters to see how the web application will handle them.

Technique
Use URL encodings of special characters such as semi-colons, backslashes, or question marks that might be filtered out normally.
Combine the use of URL encodings with other encoding techniques such as the triple dot and escape slashes.

Exploit

Inject special characters into URL parameters

Using the information gathered in the "Experiment" phase, the adversary injects special characters into the URL using URL encoding. This can lead to path traversal, cross-site scripting, SQL injection, etc.

▼Prerequisites

The application should accepts and decodes URL input.

The application performs insufficient filtering/canonicalization on the URLs.

▼Skills Required

Low

An adversary can try special characters in the URL and bypass the URL validation.

Medium

The adversary may write a script to defeat the input filtering mechanism.

▼Indicators

If the first decoding process has left some invalid or denylisted characters, that may be a sign that the request is malicious.

Traffic filtering with IDS (or proxy) can detect requests with suspicious URLs. IDS may use signature based identification to reveal such URL based attacks.

▼Consequences

Scope	Likelihood	Impact	Note
Confidentiality	N/A	Read Data	N/A
Availability	N/A	Resource Consumption	Denial of Service
ConfidentialityIntegrityAvailability	N/A	Execute Unauthorized Commands	Run Arbitrary Code
ConfidentialityAccess ControlAuthorization	N/A	Gain Privileges	N/A

Scope: Confidentiality

Likelihood: N/A

Impact: Read Data

Note: N/A

Scope: Availability

Likelihood: N/A

Impact: Resource Consumption

Note: Denial of Service

Scope: Confidentiality, Integrity, Availability

Likelihood: N/A

Impact: Execute Unauthorized Commands

Note: Run Arbitrary Code

Scope: Confidentiality, Access Control, Authorization

Likelihood: N/A

Impact: Gain Privileges

Note: N/A

▼Mitigations

Refer to the RFCs to safely decode URL.

Regular expression can be used to match safe URL patterns. However, that may discard valid URL requests if the regular expression is too restrictive.

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).

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.

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. Test your decoding process against malicious input.

Be aware of the threat of alternative method of data encoding and obfuscation technique such as IP address encoding. (See related guideline section)

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.

▼Example Instances

▼Related Weaknesses

ID	Name
CWE-177	Improper Handling of URL Encoding (Hex Encoding)
CWE-172	Encoding Error
CWE-173	Improper Handling of Alternate Encoding
CWE-20	Improper Input Validation
CWE-73	External Control of File Name or Path
CWE-74	Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')