ByteOS Network

CAPEC-64:Using Slashes and URL Encoding Combined to Bypass Validation Logic

Attack Pattern ID:64

Version:v3.9

Attack Pattern Name:Using Slashes and URL Encoding Combined to Bypass Validation Logic

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 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.

▼Relationships

Nature	Type	ID	Name
ChildOf	S	267	Leverage Alternate Encoding
CanFollow	D	80	Using UTF-8 Encoding to Bypass Validation Logic

Nature: ChildOf

Type: Standard

ID: 267

Name: Leverage Alternate Encoding

Nature: CanFollow

Type: Detailed

ID: 80

Name: Using UTF-8 Encoding to Bypass Validation Logic

▼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

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.

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

Likelihood: N/A

Impact: Read Data

Note: N/A

Scope: Confidentiality, Access Control, Authorization

Likelihood: N/A

Impact: Gain Privileges

Note: N/A

▼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

▼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-22	Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
CWE-697	Incorrect Comparison
CWE-707	Improper Neutralization
CWE-73	External Control of File Name or Path
CWE-74	Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')