CAPEC-64: Using Slashes and URL Encoding Combined to Bypass Validation Logic |
 Description This attack targets the encoding of the URL combined with the encoding of the slash characters. An attacker can take advantage of the multiple ways of encoding a 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 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. Likelihood Of Attack Typical Severity Execution Flow Explore The attacker accesses the server using a specific URL.
Experiment The attacker tries to encode some special characters in the URL. The attacker find out that some characters are not filtered properly.
Exploit The attacker crafts a malicious URL string request and sends it to the server. The server decodes and interprets the URL string. Unfortunately since the input filtering is not done properly, the special characters have harmful consequences.
Prerequisites
| The application accepts and decodes URL string request. |
| 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  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 |
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Availability | Resource Consumption | | Confidentiality Integrity Availability | Execute Unauthorized Commands | | Confidentiality | Read Data | | Confidentiality Access Control Authorization | Gain Privileges | |
Mitigations
| 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. |
| 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 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). |
Example Instances
Attack Example: 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 attacker 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 |
Taxonomy Mappings 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
) References
[REF-1] G. Hoglund and
G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. 2004-02.
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 Content History | Submissions |
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| Submission Date | Submitter | Organization |
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| 2014-06-23 (Version 2.6) | CAPEC Content Team | The MITRE Corporation | | | Modifications |
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| Modification Date | Modifier | Organization |
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| 2017-01-09 (Version 2.9) | CAPEC Content Team | The MITRE Corporation | | Updated Related_Attack_Patterns | | 2017-08-04 (Version 2.11) | CAPEC Content Team | The MITRE Corporation | | Updated Description Summary | | 2018-07-31 (Version 2.12) | CAPEC Content Team | The MITRE Corporation | | Updated Obfuscation_Techniques, References | | 2020-07-30 (Version 3.3) | CAPEC Content Team | The MITRE Corporation | | Updated Indicators, Mitigations | | 2021-06-24 (Version 3.5) | CAPEC Content Team | The MITRE Corporation | | Updated Related_Weaknesses | | 2022-09-29 (Version 3.8) | CAPEC Content Team | The MITRE Corporation | | Updated Example_Instances |
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