CAPEC-468
Generic Cross-Browser Cross-Domain Theft
An attacker makes use of Cascading Style Sheets (CSS) injection to steal data cross domain from the victim's browser. The attack works by abusing the standards relating to loading of CSS: 1. Send cookies on any load of CSS (including cross-domain) 2. When parsing returned CSS ignore all data that does not make sense before a valid CSS descriptor is found by the CSS parser.
On this page
By having control of some text in the victim's domain, the attacker is able to inject a seemingly valid CSS string. It does not matter if this CSS string is preceded by other data. The CSS parser will still locate the CSS string. If the attacker is able to control two injection points, one before the cross domain data that the attacker is interested in receiving and the other one after, the attacker can use this attack to steal all of the data in between these two CSS injection points when referencing the injected CSS while performing rendering on the site that the attacker controls. When rendering, the CSS parser will detect the valid CSS string to parse and ignore the data that "does not make sense". That data will simply be rendered. That data is in fact the data that the attacker just stole cross domain. The stolen data may contain sensitive information, such CSRF protection tokens.
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Explain the attack pattern CAPEC-468: Generic Cross-Browser Cross-Domain Theft: how it unfolds, what it needs and how to prevent and detect it in my systems. Use https://capec.mitre.org/data/definitions/468.html and https://cve.bz/cve/capec/468?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.
Prerequisites
No new lines can be present in the injected CSS stringProper HTML or URL escaping of the " and ' characters is not presentThe attacker has control of two injection points: pre-string and post-string
Skills required
- High skill
Ability to craft a CSS injection
Resources required
Attacker controlled site/page to render a page referencing the injected CSS string
Mitigations
Design: Prior to performing CSS parsing, require the CSS to start with well-formed CSS when it is a cross-domain load and the MIME type is broken. This is a browser level fix.
Implementation: Perform proper HTML encoding and URL escaping
Weaknesses it exploits
- CWE-149 Improper Neutralization of Quoting Syntax
- CWE-177 Improper Handling of URL Encoding (Hex Encoding)
- CWE-707 Improper Neutralization
- CWE-838 Inappropriate Encoding for Output Context
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The newest records naming one of them. Each weakness's page lists them all.
Related attack patterns
- Part of
- CAPEC-242 Code Injection
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