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IE, Firefox, Opera: bypassing DNS pinning
Synthesis of the vulnerability
An attacker can create a HTML page bypassing the DNS pinning protection included in web browsers.
Vulnerable software: IE, Firefox, SeaMonkey, Opera.
Severity of this announce: 1/4.
Consequences of an intrusion: data reading, data flow.
Attacker's origin: internet server.
Number of vulnerabilities in this bulletin: 4.
Creation date: 11/10/2007.
Références of this computer vulnerability: CVE-2007-5276, CVE-2007-5277, VIGILANCE-VUL-7237.
Description of the vulnerability
A "DNS rebinding" attack has the objective to force the web browser to connect to a server different than the one which provided the HTML document. This vulnerability for example permits to scan ports or to obtain information without going through the firewall.
This attack uses the following method:
- Attacker setups a DNS server for his "attacker.dom" domain. This server answers that IP address of www.attacker.dom is 126.96.36.199, with a TTL of 10 seconds.
- Attacker setups a web server to host a HTML page containing a script creating a connection to the originating server.
- Attacker invites victim to connect to his web server.
- When the HTML page is displayed, the script tries to access to the server: as the TTL expired, the web browser sends a new DNS query. However, this time, attacker's DNS server indicates the IP address of www.attacker.dom is 192.168.1.1.
- Script thus connects to the 192.168.1.1 address which is an internal address.
To protect against this attack, web browsers implement "DNS pinning" which consists in storing IP addresses in the cache whatever the duration indicated by the TTL. However, several vulnerabilities permit to bypass this protection.
Internet Explorer 7 keeps addresses in its cache during 30 minutes. However, if attacker's DNS server indicates several addresses, and if the first one becomes unreachable, the cache is cleared. [severity:1/4]
Internet Explorer 6 keeps addresses in its cache during 30 minutes. However, if the HTML page indicates to connect to an unreachable port, the cache is cleared. [severity:1/4; CVE-2007-5277]
Firefox keeps addresses in its cache during at most 2 minutes, which is too short: a script can wait. [severity:1/4]
Internet Explorer 6 keeps addresses in its cache during 12 minutes. However, if the HTML page indicates to connect to an unreachable port, the cache is cleared. [severity:1/4; CVE-2007-5276]
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