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CAPEC-72: URL Encoding

Attack Pattern ID: 72
Abstraction: Detailed
Status: Draft
Presentation Filter:
+ Description
This attack targets the encoding of the URL. An attacker can take advantage of the multiple way of encoding an URL and abuse the interpretation of the URL. An 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 attacker 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 attacker 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 attacker may subvert the meaning of parameters used in a SQL request and sent through the URL string (See Example section).
+ Likelihood Of Attack


+ Typical Severity


+ Relationships
Section HelpThis 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.
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
Section HelpThis table shows the views that this attack pattern belongs to and top level categories within that view.
+ Execution Flow
  1. The attacker accesses the server using a specific URL.
  1. The attacker tries to encode some special characters in the URL. The attacker finds out that some characters are not filtered properly.
  1. The attacker crafts a malicious URL string request and sends it to the server.
  2. The server decodes and interprets the URL string. Unfortunately since the input filtering is not done properly, the special characters may have harmful consequences.
+ Prerequisites
The application should accepts and decodes URL input.
The application performs insufficient filtering/canonicalization on the URLs.
+ Skills Required
[Level: Low]
An attacker can try special characters in the URL and bypass the URL validation.
[Level: Medium]
The attacker 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
Section HelpThis 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.
Read Data
Resource Consumption
Execute Unauthorized Commands
Access Control
Gain Privileges
+ 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 (
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

Attack Example: URL Encodings in IceCast MP3 Server.

The following type of encoded string has been known traverse directories against the IceCast MP3 server9:


or using

"/%25%25/" instead of "/../".

The control character ".." can be used by an attacker to escape the document root.

See also: CVE-2001-0784

Cross-Site Scripting

URL-Encoded attack:
HTML execution:
<script src=""></script>


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:
Executed database query:
SELECT preferences FROM logintable WHERE userid='bob'; update logintable set password='0wn3d';

From "URL encoded attacks", by Gunter Ollmann -

+ References
[REF-1] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. 2004-02.
[REF-495] Gunter Ollmann. "URL Encoded Attacks - Attacks using the common web browser". <>.
[REF-496] T. Berners-Lee, R. Fielding and L. Masinter. "RFC 3986 - Uniform Resource Identifier (URI): Generic Syntax". 2005-01. <>.
[REF-497] T. Berners-Lee, L. Masinter and M. McCahill. "RFC 1738 - Uniform Resource Locators (URL)". 1994-12. <>.
[REF-498] "HTML URL Encoding Reference". Refsnes Data. <>.
[REF-499] "The URLEncode and URLDecode Page". Albion Research Ltd. <>.
[REF-500] David Wheeler. "Secure Programming for Linux and Unix HOWTO". 5.11.4. Validating Hypertext Links (URIs/URLs). <>.
+ Content History
Submission DateSubmitterOrganization
2014-06-23CAPEC Content TeamThe MITRE Corporation
Modification DateModifierOrganization
2017-01-09CAPEC Content TeamThe MITRE Corporation
Updated Related_Attack_Patterns
2018-07-31CAPEC Content TeamThe MITRE Corporation
Updated References
2020-07-30CAPEC Content TeamThe MITRE Corporation
Updated Example_Instances, Indicators, Mitigations
2021-06-24CAPEC Content TeamThe MITRE Corporation
Updated Related_Weaknesses
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Page Last Updated or Reviewed: October 21, 2021