CAPEC-199: XSS Using Alternate Syntax
CAPEC版本: 3.9
更新日期: 2023-01-24
攻击模式描述
An adversary uses alternate forms of keywords or commands that result in the same action as the primary form but which may not be caught by filters. For example, many keywords are processed in a case insensitive manner. If the site's web filtering algorithm does not convert all tags into a consistent case before the comparison with forbidden keywords it is possible to bypass filters (e.g., incomplete black lists) by using an alternate case structure. For example, the "script" tag using the alternate forms of "Script" or "ScRiPt" may bypass filters where "script" is the only form tested. Other variants using different syntax representations are also possible as well as using pollution meta-characters or entities that are eventually ignored by the rendering engine. The attack can result in the execution of otherwise prohibited functionality.
执行流程
步骤 1 Explore
[Survey the application for user-controllable inputs] Using a browser or an automated tool, an adversary follows all public links and actions on a web site. They record all the links, the forms, the resources accessed and all other potential entry-points for the web application.
- Use a spidering tool to follow and record all links. Make special note of any links that include parameters in the URL.
- Use a proxy tool to record all links visited during a manual traversal of the web application. Make special note of any links that include parameters in the URL. Manual traversal of this type is frequently necessary to identify forms that are GET method forms rather than POST forms.
- Use a browser to manually explore the website and analyze how it is constructed. Many browser's plugins are available to facilitate the analysis or automate the URL discovery.
步骤 2 Experiment
[Probe identified potential entry points for XSS vulnerability] Possibly using an automated tool, an adversary requests variations on the inputs they surveyed before using alternate syntax. These inputs are designed to bypass incomplete filtering (e.g., incomplete HTML encoding etc.) and try many variations of characters injection that would enable the XSS payload. They record all the responses from the server that include unmodified versions of their script.
- Use a list of XSS probe strings to inject in parameters of known URLs. If possible, the probe strings contain a unique identifier. Attempt numerous variations based on form, format, syntax & encoding.
- Use a proxy tool to record results of manual input of XSS probes in known URLs.
步骤 3 Experiment
[Craft malicious XSS URL] Once the adversary has determined which parameters are vulnerable to XSS, they will craft a malicious URL containing the XSS exploit. The adversary can have many goals, from stealing session IDs, cookies, credentials, and page content from the victim.
- Change a URL parameter to include a malicious script tag created using alternate syntax to bypass filters.
- Send information gathered from the malicious script to a remote endpoint.
步骤 4 Exploit
[Get victim to click URL] In order for the attack to be successful, the victim needs to access the malicious URL.
- Send a phishing email to the victim containing the malicious URL. This can be hidden in a hyperlink as to not show the full URL, which might draw suspicion.
- Put the malicious URL on a public forum, where many victims might accidentally click the link.
前提条件
- Target client software must allow scripting such as JavaScript.
所需技能
所需资源
- Ability to send HTTP request to a web application.
后果影响
影响范围: Integrity
技术影响: Modify Data
影响范围: Confidentiality
技术影响: Read Data
影响范围: Authorization
技术影响: Execute Unauthorized Commands
说明: Run Arbitrary Code
影响范围: Accountability Authentication Authorization Non-Repudiation
技术影响: Gain Privileges
影响范围: Access Control Authorization
技术影响: Bypass Protection Mechanism
缓解措施
Design: Use browser technologies that do not allow client side scripting.
Design: Utilize strict type, character, and encoding enforcement
Implementation: Ensure all content that is delivered to client is sanitized against an acceptable content specification.
Implementation: Ensure all content coming from the client is using the same encoding; if not, the server-side application must canonicalize the data before applying any filtering.
Implementation: Perform input validation for all remote content, including remote and user-generated content
Implementation: Perform output validation for all remote content.
Implementation: Disable scripting languages such as JavaScript in browser
Implementation: Patching software. There are many attack vectors for XSS on the client side and the server side. Many vulnerabilities are fixed in service packs for browser, web servers, and plug in technologies, staying current on patch release that deal with XSS countermeasures mitigates this.