cve.bz

CAPEC-199

XSS Using Alternate Syntax

High severityHigh likelihoodDetailed pattern

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.

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Show the promptExplain the attack pattern CAPEC-199: XSS Using Alternate Syntax: how it unfolds, what it needs and how to prevent and detect it in my systems. Use https://capec.mitre.org/data/definitions/199.html and https://cve.bz/cve/capec/199?via=ask-ai-copy, which lists recent CVEs with the weaknesses it exploits. Cite the sources.

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How the attack unfolds

  1. Explore

    The attacker looks for a way in.

    1. [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.

      3 techniques
      • 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.

  2. Experiment

    They probe what they found.

    1. [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.

      2 techniques
      • 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.

    2. [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.

      2 techniques
      • 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.

  3. Exploit

    They carry out the attack.

    1. [Get victim to click URL] In order for the attack to be successful, the victim needs to access the malicious URL.

      2 techniques
      • 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.

Prerequisites

  • Target client software must allow scripting such as JavaScript.

Skills required

  • Low skill

    To inject the malicious payload in a web page

  • High skill

    To bypass non trivial filters in the application

Resources required

  • Ability to send HTTP request to a web application.

Consequences

What a successful attack can lead to
ImpactAffectsNote
Modify DataIntegrity
Read DataConfidentiality
Execute Unauthorized CommandsAuthorizationRun Arbitrary Code
Gain PrivilegesAccountability, Authentication, Authorization, Non-Repudiation
Bypass Protection MechanismAccess Control, Authorization

Mitigations

  • 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.

Weaknesses it exploits

  • CWE-87 Improper Neutralization of Alternate XSS Syntax

Recent CVEs with these weaknesses

  1. 4.8CVE-2026-85293

    InvoicePlane is a self-hosted open source application for managing invoices, clients, and payments. In version 1.7.2-beta-1, InvoicePlane stores client_email values without enforcing email syntax and renders them unescaped inside double-qu…

  2. 5.3CVE-2026-79946

    Dell SCG 5.0 Appliance versions prior to 5.36.00.16 and Dell SCG 5.0 Application versions prior to 5.36.00.00, contains an Improper Neutralization of Alternate XSS Syntax vulnerability. An unauthenticated attacker with remote access could …

  3. 8.5CVE-2026-54002

    Kirby is an open-source content management system. Prior to 4.9.4 and 5.4.4, Kirby sites and plugins that use the writer or list fields or call Dom::sanitize(), Sane::sanitize(), Sane::Html::sanitize(), Sane::Svg::sanitize(), Sane::Xml::sa…

  4. 4.8CVE-2026-55661

    Tina is a headless content management system. In versions prior to @tinacms/mdx 2.1.7 and tinacms 3.9.3, rich-text parsing and the default link/image renderers did not sanitize the url field on Slate link/image nodes. Content containing …

  5. 8.8CVE-2026-55237

    AutoGPT is a workflow automation platform for creating, deploying, and managing continuous artificial intelligence agents. Versions prior to 0.6.62 have a DOM-based Cross-Site Scripting (XSS) vulnerability in AutoGPT's signup page. The app…

  6. 6.1CVE-2026-46492

    md-fileserver allows for local viewing of markdown files in a browser. Prior to version 1.10.3, a cross-site scripting (XSS) vulnerability exists in the application’s Markdown rendering logic. When user-supplied Markdown content is rendere…

  7. 6.1CVE-2026-25688

    Improper Neutralization of Alternate XSS Syntax vulnerability in Apache Answer. This issue affects Apache Answer: through 2.0.0. AI-generated response content was rendered in the browser without proper sanitization, allowing malicious sc…

  8. 7.4CVE-2026-45314

    Open WebUI is a self-hosted artificial intelligence platform designed to operate entirely offline. Prior to 0.9.3, the channel webhook create/update flow accepts arbitrary profile_image_url values, including data:image/svg+xml;base64,... p…

The newest records naming one of them. Each weakness's page lists them all.

Part of

Examples

In this example, the adversary tries to get <script>alert(1)</script> executed by the victim's browser. The t…

In this example, the adversary tries to get <script>alert(1)</script> executed by the victim's browser. The target application employs regular expressions to make sure no script is being passed through the application to the web page; such a regular expression could be ((?i)script), and the application would replace all matches by this regex by the empty string. An adversary will then create a special payload to bypass this filter:

<scriscriptpt>alert(1)</scscriptript>

when the applications gets this input string, it will replace all "script" (case insensitive) by the empty string and the resulting input will be the desired vector by the adversary:

<script>alert(1)</script>

In this example, we assume that the application needs to write a particular string in a client-side JavaScript context (e.g., <script>HERE</script>). For the adversary to execute the same payload as in the previous example, they would need to send alert(1) if there was no filtering. The application makes use of the following regular expression as filter

((\w+)\s*\(.*\)|alert|eval|function|document)

and replaces all matches by the empty string. For example each occurrence of alert(), eval(), foo() or even the string "alert" would be stripped. An adversary will then create a special payload to bypass this filter:

this['al' + 'ert'](1)

when the applications gets this input string, it won't replace anything and this piece of JavaScript has exactly the same runtime meaning as alert(1). The adversary could also have used non-alphanumeric XSS vectors to bypass the filter; for example,

($=[$=[]][(__=!$+$)[_=-~-~-~$]+({}+$)[_/_]+($$=($_=!''+$)[_/_]+$_[+$])])()[__[_/_]+__[_+~$]+$_[_]+$$](_/_)

would be executed by the JavaScript engine like alert(1) is.

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