cve.bz

CAPEC-247

XSS Using Invalid Characters

Medium severityDetailed pattern

An adversary inserts invalid characters in identifiers to bypass application filtering of input. Filters may not scan beyond invalid characters but during later stages of processing content that follows these invalid characters may still be processed. This allows the adversary to sneak prohibited commands past filters and perform normally prohibited operations. Invalid characters may include null, carriage return, line feed or tab in an identifier. Successful bypassing of the filter can result in a XSS attack, resulting in the disclosure of web cookies or possibly other results.

On this page

Ask an AI about this attack pattern

ChatGPTClaudeGoogle AI Mode

Show the promptExplain the attack pattern CAPEC-247: XSS Using Invalid Characters: how it unfolds, what it needs and how to prevent and detect it in my systems. Use https://capec.mitre.org/data/definitions/247.html and https://cve.bz/cve/capec/247?via=ask-ai-copy, which lists recent CVEs with the weaknesses it exploits. Cite the sources.

Opens a third-party assistant; its answer is its own, not cve.bz's. The prompt holds only this attack pattern's id and public links.

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 and analyze the web pages to find entry points. 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.
      • Use a browser to manually explore the website and analyze how it is constructed. Many browsers' plugins are available to facilitate the analysis or automate the discovery.

  2. Experiment

    They probe what they found.

    1. [Probe identified potential entry points for XSS vulnerabilities using invalid characters] The adversary uses the entry points gathered in the "Explore" phase as a target list and injects various common script payloads and special characters preceded by an invalid character(s) to determine if an entry point actually represents a vulnerability and to characterize the extent to which the vulnerability can be exploited. The adversary is looking for cases where an invalid character causes an input filter to stop processing, allowing the malicious input that follows to bypass the filter

      3 techniques
      • Use a list of XSS probe strings preceded by an invalid character(s) such as null, carriage return, line feed, or tab to inject script in parameters of known URLs. If possible, the probe strings contain a unique identifier.
      • Use a proxy tool to record results of manual input of XSS probes in known URLs.
      • Use a list of HTML special characters preceded by an invalid character(s) to inject into parameters of known URLs and check if they were properly encoded, replaced, or filtered out.

    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 preceded by invalid character(s).
      • 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

  • The target must fail to remove invalid characters from input and fail to adequately scan beyond these characters.

Resources required

  • None: No specialized resources are required to execute this type of attack.

Mitigations

  • Design: Use libraries and templates that minimize unfiltered input.

  • Implementation: Normalize, filter and use an allowlist for any input that will be included in any subsequent web pages or back end operations.

  • Implementation: The victim should configure the browser to minimize active content from untrusted sources.

Weaknesses it exploits

  • CWE-86 Improper Neutralization of Invalid Characters in Identifiers in Web Pages

Recent CVEs with these weaknesses

  1. 6.1CVE-2026-71478

    league/commonmark is a PHP library for parsing and rendering CommonMark Markdown. From 1.5.0 until 2.9.0, the AttributesExtension's href and src unsafe-link filter can be bypassed by embedding control bytes, such as a tab, carriage return,…

  2. 7.8CVE-2026-28417

    Vim is an open source, command line text editor. Prior to version 9.2.0073, an OS command injection vulnerability exists in the `netrw` standard plugin bundled with Vim. By inducing a user to open a crafted URL (e.g., using the `scp://` pr…

  3. 2.1CVE-2025-66606

    A vulnerability has been found in FAST/TOOLS provided by Yokogawa Electric Corporation. This product does not properly encode URLs. An attacker could tamper with web pages or execute malicious scripts. The affected products and versi…

  4. 5.4CVE-2025-20168

    A vulnerability in the web-based management interface of Cisco Common Services Platform Collector (CSPC) could allow an authenticated, remote attacker to conduct cross-site scripting (XSS) attacks against a user of the interface. This v…

  5. 5.4CVE-2025-20167

    A vulnerability in the web-based management interface of Cisco Common Services Platform Collector (CSPC) could allow an authenticated, remote attacker to conduct cross-site scripting (XSS) attacks against a user of the interface. This v…

  6. 5.4CVE-2025-20166

    A vulnerability in the web-based management interface of Cisco Common Services Platform Collector (CSPC) could allow an authenticated, remote attacker to conduct cross-site scripting (XSS) attacks against a user of the interface. This v…

  7. 6.5CVE-2024-10941

    A malicious website could have included an iframe with an malformed URI resulting in a non-exploitable browser crash. This vulnerability affects Firefox < 126.

  8. 7.8CVE-2024-21864

    Improper neutralization in some Intel(R) Arc(TM) & Iris(R) Xe Graphics software before version 31.0.101.5081 may allow an unauthenticated user to potentially enable escalation of privilege via adjacent network access.

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

Part of

MITRE's words, unchanged: CAPEC-247 at MITRE. CAPEC is Copyright © 2007–2026, The MITRE Corporation, used under the CAPEC terms of use: “The MITRE Corporation (MITRE) hereby grants you a non-exclusive, royalty-free license to use Common Attack Pattern Enumeration and Classification (CAPEC™) for research, development, and commercial purposes. Any copy you make for such purposes is authorized provided that you reproduce MITRE’s copyright designation and this license in any such copy.” CAPEC and the CAPEC logo are trademarks of The MITRE Corporation. Credits. All attack patterns.