Insufficient Visual Distinction of Homoglyphs Presented to User

Opis

The product displays information or identifiers to a user, but the display mechanism does not make it easy for the user to distinguish between visually similar or identical glyphs (homoglyphs), which may cause the user to misinterpret a glyph and perform an unintended, insecure action.

Some glyphs, pictures, or icons can be semantically distinct to a program, while appearing very similar or identical to a human user. These are referred to as homoglyphs. For example, the lowercase "l" (ell) and uppercase "I" (eye) have different character codes, but these characters can be displayed in exactly the same way to a user, depending on the font. This can also occur between different character sets. For example, the Latin capital letter "A" and the Greek capital letter "Α" (Alpha) are treated as distinct by programs, but may be displayed in exactly the same way to a user. Accent marks may also cause letters to appear very similar, such as the Latin capital letter grave mark "À" and its equivalent "Á" with the acute accent.

Adversaries can exploit this visual similarity for attacks such as phishing, e.g. by providing a link to an attacker-controlled hostname that looks like a hostname that the victim trusts. In a different use of homoglyphs, an adversary may create a back door username that is visually similar to the username of a regular user, which then makes it more difficult for a system administrator to detect the malicious username while reviewing logs.

Inne nazwy

Homograph Attack

"Homograph" is often used as a synonym of "homoglyph" by researchers, but according to Wikipedia, a homograph is a word that has multiple, distinct meanings.

Skutki

• Integrity, Confidentiality

  Other

  An attacker may ultimately redirect a user to a malicious website, by deceiving the user into believing the URL they are accessing is a trusted domain. However, the attack can also be used to forge log entries by using homoglyphs in usernames. Homoglyph manipulations are often the first step towards executing advanced attacks such as stealing a user's credentials, Cross-Site Scripting (XSS), or log forgery. If an attacker redirects a user to a malicious site, the attacker can mimic a trusted domain to steal account credentials and perform actions on behalf of the user, without the user's knowledge. Similarly, an attacker could create a username for a website that contains homoglyph characters, making it difficult for an admin to review logs and determine which users performed which actions.

Zapobieganie

• Implementation

  Use a browser that displays Punycode for IDNs in the URL and status bars, or which color code various scripts in URLs.

  Due to the prominence of homoglyph attacks, several browsers now help safeguard against this attack via the use of Punycode. For example, Mozilla Firefox and Google Chrome will display IDNs as Punycode if top-level domains do not restrict which characters can be used in domain names or if labels mix scripts for different languages.

• Implementation

  Use an email client that has strict filters and prevents messages that mix character sets to end up in a user's inbox.

  Certain email clients such as Google's GMail prevent the use of non-Latin characters in email addresses or in links contained within emails. This helps prevent homoglyph attacks by flagging these emails and redirecting them to a user's spam folder.

Przykłady podatności

• CVE-2013-7236 web forum allows impersonation of users with homoglyphs in account names
• CVE-2012-0584 Improper character restriction in URLs in web browser
• CVE-2009-0652 Incomplete denylist does not include homoglyphs of "/" and "?" characters in URLs
• CVE-2017-5015 web browser does not convert hyphens to punycode, allowing IDN spoofing in URLs
• CVE-2005-0233 homoglyph spoofing using punycode in URLs and certificates
• CVE-2005-0234 homoglyph spoofing using punycode in URLs and certificates
• CVE-2005-0235 homoglyph spoofing using punycode in URLs and certificates

Słabość nadrzędna

• CWE-451 User Interface (UI) Misrepresentation of Critical Information

Kiedy powstaje

• Architecture and Design — This weakness may occur when characters from various character sets are allowed to be interchanged within a URL, username, email address, etc. without any notification to the user or underlying system being used.
• Implementation

Dotyczy

Not Language-Specific, Not Technology-Specific, Web Based

Źródła

1. Writing Secure Code — Michael Howard, David LeBlanc, Chapter 11, "Canonical Representation Issues", Page 382, Microsoft Press, 2002
2. The 2011 IDN Homograph Attack Mitigation Survey — Gregory Baatard, Peter Hannay, ECU Publications, 2012