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Vulnerability of Chrome, Firefox: site spoofing via homographs

Synthesis of the vulnerability

An attacker can use a url with Unicode characters looking like ASCII characters, in order to deceive the victim.
Severity of this bulletin: 2/4.
Creation date: 18/04/2017.
Références of this threat: 1332714, CVE-2017-5090, VIGILANCE-VUL-22467.

Description of the vulnerability

Several Unicode characters (such as U+0430) look like the ASCII 'a' character. Some attackers use domain names containing these variants, in order to invite the victim to click on a link.

This type of attack, based on homographs, was already described in several bulletins (VIGILANCE-VUL-4729 and VIGILANCE-VUL-8497). Fixes are applied in most software.

However, when the name is only composed of Unicode characters, the Chrome and Firefox protections are bypassed. For example, is displayed as Moreover, as a valid certificate for this domain can be obtained via Let's Encrypt, an attacker can easily spoof a TLS site.

An attacker can therefore use a url with Unicode characters looking like ASCII characters, in order to deceive the victim.
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This computer threat bulletin impacts software or systems such as Chrome, Firefox, SeaMonkey, Opera.

Our Vigil@nce team determined that the severity of this security threat is medium.

The trust level is of type confirmed by the editor, with an origin of document.

A proof of concept or an attack tool is available, so your teams have to process this alert. An attacker with a technician ability can exploit this computer vulnerability alert.

Solutions for this threat

Chrome: version 59.
The version 59 is probably fixed:
The version 59 was released but its changelog does not explicitely mention this issue.

Firefox: workaround for homographs.
A workaround is to set "network.IDN_show_punycode" to "true" in about:config.
Unicode domain names will then be displayed as https://www.xn--[...].com/

Opera: version 44.0.2510.1449.
The version 44.0.2510.1449 is fixed:
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