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| Home > CAPEC List > Individual CAPEC Dictionary Definition (Release 1.1) | View the CAPEC List |
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Individual CAPEC Dictionary Definition (Release 1.1)
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Using Slashes and URL Encoding Combined to Bypass Validation Logic | |||||||||||||||||||||||||||||||||||||
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| Attack Pattern ID | Pattern Abstraction: Detailed 64 | ||||||||||||||||||||||||||||||||||||
| Typical Severity | High | ||||||||||||||||||||||||||||||||||||
| Description | Summary This attack targets the encoding of the URL combined with the encoding of the slash characters. 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. Attack Execution Flow
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| Attack Prerequisites | The application accepts and decodes URL string request. The application performs insufficient filtering/canonicalization on the URLs. | ||||||||||||||||||||||||||||||||||||
| Typical Likelihood of Exploit | High | ||||||||||||||||||||||||||||||||||||
| Methods of Attack |
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| Examples-Instances | Description Attack Example: Combined Encodings CesarFTP Related Vulnerability CVE-2001-1335 | ||||||||||||||||||||||||||||||||||||
| Attacker Skill or Knowledge Required | Low - An attacker can try special characters in the URL and bypass the URL validation. Medium - The attacker may write a script to defeat the input filtering mechanism. | ||||||||||||||||||||||||||||||||||||
| Probing Techniques | An attacker can manually inject special characters in the URL string request and observe the results of the request. Custom scripts can also be used. For example, a good script for verifying the correct interpretation of UTF-8 encoded characters can be found at http://www.cl.cam.ac.uk/~mgk25/ucs/examples/UTF-8-test.txt Automated tools such as fuzzer can be used to test the URL decoding and filtering. | ||||||||||||||||||||||||||||||||||||
| Indicators-Warnings of Attack | If the first decoding process has left some invalid or blacklisted 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. | ||||||||||||||||||||||||||||||||||||
| Obfuscation Techniques | Sometime the percent escaping can be used to obfuscate the attack itself. Alternative method of data encoding can be used. Obfuscation technique such as IP address encoding can also be used (See reference section : "URL encoded attacks", by Gunter Ollmann). | ||||||||||||||||||||||||||||||||||||
| Solutions and Mitigations | 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. 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. 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. 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. Refer to the RFCs to safelly 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). | ||||||||||||||||||||||||||||||||||||
| Attack Motivation- |
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| Context Description | Most web programming languages have built in API to perform URL Encoding and URL Decoding. For instance PHP uses "urlencode(string)" and "urldecode(string)". Java uses "java.net.URLEncode.encode(String)". | ||||||||||||||||||||||||||||||||||||
| Related Weaknesses |
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| Related Attack Patterns |
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| Related Security Principles |
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| Purpose | Penetration | ||||||||||||||||||||||||||||||||||||
| CIA Impact |
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| Technical Context |
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| References | G. Hoglund and G. McGraw. Exploiting Software: How to Break Code. Addison-Wesley, February 2004. CWE – Input Validation URL encoded attacks, by Gunter Ollmann - http://www.cgisecurity.com/ Uniform Resource Identifier (URI): Generic Syntax, RFC 3886 - http://www.ietf.org/rfc/rfc URL Uniform Resource Locators (URL) RFC - http://rfc.net/rfc1738.html URL encoding reference - http://www.w3schools.com/ta The URLEncode and URLDecode Page - http://www.albionresearch.c David Wheeler - Validating URIs - http://www.dwheeler.com/sec | ||||||||||||||||||||||||||||||||||||
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