What Is XPath Injection?
XPath Injection is a type of attack targeting applications that use XML data for queries and operations. This form of injection exploits the way web applications handle user input for XPath queries, which are used to search and navigate XML documents.
Attackers manipulate XPath queries by injecting malicious code, aiming to alter query execution or access unauthorized data. The vulnerability arises from the application’s failure to adequately sanitize user inputs, allowing attackers to inject XPath expressions that can compromise data integrity and security.
XPath attacks can have severe consequences for web applications that interact with XML data:
- Data exposure: When attackers successfully exploit XPATH injection vulnerabilities, they can gain access to sensitive information stored in XML data. This can include passwords, user IDs, and other sensitive information.
- Data integrity compromise: Data integrity breaches occur when an attacker uses XPATH injection to modify the information stored within an XML file. This manipulation can alter system output, corrupt data, or change application behavior.
- Denial of service: Through specially crafted input, an attacker can create an XPath query that significantly slows down the server, leading to a Denial of Service (DoS). In severe cases, this might crash the system or make the application unresponsive.
This is part of a series of articles about cyber attack
How Does XPath Work?
XPath, or XML Path Language, is a query language designed for selecting nodes from an XML document. It allows users to navigate through elements and attributes in an XML document, enabling precise selection of data based on its hierarchical structure.
XPath uses path expressions to identify and select nodes or node-sets in an XML document. For example, an XPath expression can select all nodes with a particular attribute value, or it can be used to find data within nested elements.
Here are a few examples of path expressions:
- Selecting all nodes with a specific name: /bookstore/book selects all book elements that are children of the bookstore element.
- Selecting nodes by attribute: /bookstore/book[@category=’fiction’] selects all book elements within bookstore that have an attribute category with the value fiction.
- Selecting specific node: /bookstore/book[1] selects the first book element that is a child of the bookstore element.
Here are examples of predicates, which enable selecting nodes based on some condition:
- Filtering by position: /bookstore/book[position()<3] selects the first two book elements under bookstore.
- Conditional selection: /bookstore/book[price>35.00] selects all book elements under bookstore with a price element value greater than 35.00.
Finally, here are examples of the use of wildcards to select unknown XML nodes:
- Wildcards for elements: /bookstore/* selects all child elements of bookstore.
- Wildcard for any node: //book/*/price selects all price elements grandchild to any book element, regardless of the intermediate node.
XPath Injection Example
Imagine a scenario where XML data is being used by a web page’s user authentication system. To verify a user’s credentials, the system might use XPath queries, like this:
“//Customer[UserName/text()='” + Request(“Username”) + “‘ And
Password/text()='” + Request(“Password”) + “‘]”;
Normally, this method effectively locates the correct user node using the provided username and password. However, an attacker could exploit this system by injecting a username and password combination that bypasses the authentication check, such as:
Username: lol’ or 1=1 or ‘a’=’a
Password: lol
This effectively transforms the XPath query into:
//Customer[UserName/text()=’lol’ or 1=1 or ‘a’=’a’ And Password/text()=’lol’]
In this scenario, the XPath query is manipulated in such a way that the password check becomes irrelevant. The portion including “1=1” is always true, making the entire query succeed for any customer in the XML document. This exposes sensitive information without the need for valid credentials.
4 Ways to Prevent XPath Injection
Here are some important measures for preventing and mitigating XPath vulnerabilities.
1. Use an Allowlist
Implementing an allowlist involves explicitly defining which user inputs are acceptable for XPath queries. By doing so, you filter out potentially harmful inputs that could exploit XPath vulnerabilities. An allowlist ensures that only pre-approved patterns or strings are processed, significantly reducing the risk of injection attacks.
2. Avoid Linking User Input and XPath Queries
Directly linking user input to XPath queries exposes applications to injection risks. Attackers can craft malicious inputs to manipulate query outcomes or access unauthorized data. To mitigate this, developers should separate user inputs from the query execution process. This means user inputs should undergo validation and sanitization before being considered for any XPath expression.
Implementing a layer of abstraction between user inputs and XPath execution can significantly reduce vulnerability. This approach involves verifying user inputs against expected formats or values before incorporating them into any query. By doing so, it creates a buffer that helps prevent malicious data from influencing the query logic.
3. Use Parameterized XPath Queries
This technique involves predefining query templates with placeholders for user inputs rather than concatenating strings to form queries. By circumventing direct input concatenation, it renders injected malicious inputs ineffective, as they’re treated as data, not executable code.
Adoption of parameterized queries necessitates a shift in how developers construct XPath expressions. They must identify where user inputs intersect with queries and replace direct insertion with placeholders. This enhances security and makes code maintenance easier by centralizing input handling.
4. Implement Proper Error Handling
When errors are not appropriately managed, they can reveal information about the backend structure or database schema. This information can be invaluable for attackers. Implementing generic error messages that do not disclose sensitive information is a fundamental step in safeguarding against XPath injection.
Besides obscuring potential entry points for attackers, effective error handling involves logging errors for internal review. These logs can provide insights into attempted attacks or areas where the application might be vulnerable.
XPath Injection Protection with Imperva
Imperva’s Web Application Firewall can prevent XPath injection attacks with world-class analysis of web traffic to your applications.
Beyond the WAF, Imperva provides comprehensive protection for applications, APIs, and microservices:
Runtime Application Self-Protection (RASP) – Real-time attack detection and prevention from your application runtime environment goes wherever your applications go. Stop external attacks and injections and reduce your vulnerability backlog.
API Security – Automated API protection ensures your API endpoints are protected as they are published, shielding your applications from exploitation.
Advanced Bot Protection – Prevent business logic attacks from all access points – websites, mobile apps and APIs. Gain seamless visibility and control over bot traffic to stop online fraud through account takeover or competitive price scraping.
DDoS Protection – Block attack traffic at the edge to ensure business continuity with guaranteed uptime and no performance impact. Secure your on premises or cloud-based assets – whether you’re hosted in AWS, Microsoft Azure, or Google Public Cloud.
Attack Analytics – Ensures complete visibility with machine learning and domain expertise across the application security stack to reveal patterns in the noise and detect application attacks, enabling you to isolate and prevent attack campaigns.
Client-Side Protection – Gain visibility and control over third-party JavaScript code to reduce the risk of supply chain fraud, prevent data breaches, and client-side attacks.
