The Large-Scale Threat of Bad Data in DNS Since DNS was first created it has been commonly known that "bad data" in DNS is a potential security problem. DNSSEC is a new standard under development now that is designed to plug one of the security holes caused by "bad data" in DNS -- the fact that any DNS server can be filled with incorrect information by a malicious third party through a variety of techniques that don't require the third party to possess passwords that grant access to the computer that runs the DNS server. When an attacker fills a DNS server with malicious information, it gives the attacker full control over where on the network any client computer directs information sent to server computers within impacted DNS domains. This is commonly known as "DNS Hijacking" or "DNS Spoofing" or "DNS Poisoning". In and of itself DNS Hijacking is a significant threat, but one that is generally ignored because everyone knows that DNS is insecure (at least until DNSSEC gets deployed) and everyone lives with the risk. More to the point, everyone who needs security understands that DNS doesn't provide it, and they build additional security mechanisms on top of DNS. Secure Sockets Layer (SSL), for example. It is a commonly-held belief today, even amongst information security professionals, that it doesn't matter if DNS hijacking occurs, because it doesn't impact the security of SSL -- if a client is hijacked through "bad data" in DNS and redirected to a malicious attacker instead of the authentic SSL-secured server, the client will be informed of this fact because the SSL protocol authenticates the identity of the server reliably through the use of PKI certificates. There's no way for an attacker to spoof the SSL identity of the authentic server without stealing that server's secret key which corresponds to the public key contained within the server's PKI certificate as issued by a Certificate Authority (CA) such as Verisign. It is generally believed that secret keys are hard to steal, and that keeping secret keys secret is easy, so nobody should be especially worried about a DNS hijacker and "bad data" in DNS as long as they always use SSL or similar encryption and authentication software and protocols to protect sensitive communications on the Internet. Two recent software bugs discovered by researchers and reported on BugTraq, both of which impact Microsoft Internet Explorer as well as certain other software (a definitive list won't exist for some time, as there are many variations of the attack techniques and even the researchers themselves acknowledge that they haven't explored very many of the variations or looked into whether or not other software may also be vulnerable to these attacks) tear apart the belief that "bad data" in DNS is not a severe threat because of the existence of SSL and other standards that give everyone the security that DNS can not. Mike Benham [moxieat_private] and Slav Pidgorny [pidgornsat_private] reported separate but related discoveries about the fact that commonly-used SSL certificate creation software can be used to forge PKI certificates that are trusted by certain software such as Internet Explorer due to the fact that authentic certificates issued by Verisign, Thawte, and other CA's are improperly authorized (automatically) to function as CA certificates for the purpose of issuing and certifying malicious forged SSL certificates. Mike Benham demonstrated this week a forged SSL certificate for www.amazon.com that Internet Explorer trusts automatically as the authentic SSL certificate for Amazon's SSL-secured Web server. The technique and technical steps necessary to forge SSL certificates in this way is trivial and requires access only to an authentic certificate issued by a legitimate CA and the corresponding private key/public key pair that anyone must generate when applying to a legitimate CA to purchase an SSL certificate. (See references [1] and [2] below) Adam Megacz [adamat_private] of XWT Foundation reported a vulnerability in Internet Explorer's implementation of JavaScript that allows an attacker to hijack a user's browser and force it to attack Web servers located behind the user's firewall. The attack takes advantage of the fact that anyone who registers a DNS domain through any registrar can put "bad data" in their own DNS zone entries (called resource records) that contain addresses found only on internal (intranet) private networks of the type that exist commonly behind firewalls. RFC 1918 (see reference [3] below) documents the private address ranges, such as 10.x.x.x or 192.168.x.x, that have been reserved for private networks. This RFC makes it clear that addresses that reference private networks must not be exposed outside of the private network where they have ambiguous meaning, and this is supposed to have included restrictions on DNS servers to prevent them from serving, and perhaps restrictions on DNS resolvers to prevent them from accepting, such RFC 1918 "bad data" from DNS zone resource records outside of private networks. Because there are known buffer overflow vulnerabilities in Web servers that are installed behind firewalls, and because certain of these vulnerabilities (in particular Microsoft Internet Information Server) can be exploited by a simple URL with name/value pair parameters that trigger the buffer overflow and deliver malicious executable code to the compromised server, the idea of attacking intranet Web servers using RFC 1918 "bad data" addresses in DNS using both Adam Megacz's JavaScript exploit and buffer overflow URL exploits against servers that have not been properly patched due to the fact that they weren't supposed to be vulnerable to attacks originating from the Internet thanks to the protection allegedly provided by firewalls is now obvious to anyone who would undertake such attacks in the first place. (see reference [4] below) There ARE protections available to everyone impacted by "bad data" in DNS. The first protection is knowledge. 1) SSL can no longer be presumed to provide automatic server identity authentication; especially not when Internet Explorer is used. 2) Any domain name can resolve to an address that points at computers behind your firewall if your firewall doesn't have the ability to filter out this type of RFC 1918 "bad data" in DNS lookups; as a result, any URL can cause any Web browser to attack computers behind the firewall -- INCLUDING your own computer: 127.0.0.1 is the loopback adapter address present in every TCP/IP-enabled computer which causes the computer to refer to itself without knowledge of its own name or address, and this address can ALSO be configured as "bad data" in any DNS zone resource record. 3) Sending code, including script, over the network is a very risky practice that should be discouraged; even when digital signatures are used to authenticate the trustworthiness of such code, these signatures do not provide a guarantee of safety and software bugs (or key theft) can render them useless -- automatic trust should never be granted to code that travels over the network and better tools for human users to use for making trust determinations should be deployed wherever possible. The second protection is software patches. 1) Web browsers and other software vulnerable right now to the various "bad data" DNS threats need to be fixed or uninstalled. 2) DNS resolvers should properly implement RFC 1918 and reject private network addresses received from any DNS server not located on the same private network. 3) Firewalls need to be upgraded with the ability to properly implement RFC 1918 AND filter out 127.0.0.1 RR's (not mentioned explicitly in RFC 1918) 4) ISP's need to update the resolvers they have deployed in the DNS servers they operate on behalf of customers so that they will never relay "bad data", which would ideally include 5) DEPLOY DNSSEC! (see reference [5] below) The third protection is enhanced security policy for Internet infrastructure operators. 1) Enforce a new common-sense policy for DNS and DNSSEC (which is not currently designed to prevent "bad data" if that "bad data" is entered on purpose by an attacker who legitimately, or through key or credential theft, has control of a DNS zone) which says that IP address can't be misappropriated without permission by anyone who registers a domain name; instead, everyone who wishes to map a domain name to an IP address should be required to obtain permission first from the authority who has been assigned control (and responsibility) for managing the IP address block that contains the IP address. 2) Require, as a condition of participating in the DNS (or DNSSEC) that legitimate, verifiable contact information be registered with the appropriate authority (IANA, ARIN, RIPE, etc.) for the legal entity that has been assigned, and has accepted, responsibility (and possibly legal liability) for each address block. 3) Exclude from participation in DNS (or DNSSEC) any IP address that belongs to an address block that is under the control of anonymous or unverifiable entities. Comments or suggestions on these issues are welcome. We seek to facilitate a properly-secured Internet that takes into consideration the reality of software bugs and implements minimal security policy to provide common-sense safeguards for all users. With respect to DNS, and its eventual successor DNSSEC, it can no longer be tolerated by Internet users that the DNS makes no attempt to restrict the spread of "bad data". Existing RFC's need to be implemented properly, new operational procedures need to be developed, and bad software that contains DNS-based security bugs needs to be patched promptly by all vendors. On a related subject, everyone involved in the process of computer security vulnerability discovery, disclosure, and software bug fixes should take a moment to familiarize themselves with the internet draft of the Responsible Vulnerability Disclosure Process, and in particular note the important role of a third-party "coordinator" in cases where any party involved in the process needs help communicating with any other party to ensure proper handling and comprehensive understanding of complex technical materials: http://www.ietf.org/internet-drafts/draft-christey-wysopal-vuln-disclosure-0 0.txt Most vulnerability disclosures occur today without comprehensive cross-vendor research facilitated by a coordinator. Our group of forensic experts makes its members available to function as Security Coordinators to any party who needs this type of technical assistance. Thank you for your time and attention to these important matters. Jason Coombs [jasoncat_private] FORENSICS.ORG Security Coordinator secalertat_private REFERENCES [1] http://online.securityfocus.com/archive/1/286290 [2] http://online.securityfocus.com/archive/1/273101 [3] http://www.ietf.org/rfc/rfc1918.txt [4] http://www.xwt.org/sop.txt [5] http://www.ietf.org/html.charters/dnsext-charter.html
This archive was generated by hypermail 2b30 : Tue Aug 13 2002 - 07:33:32 PDT