LOKI攻击原理

Volume Seven, Issue Forty-NineFile 06 of 16[ Project Loki ]whitepaper by daemon9 AKA routesourcecode by daemon9 && alhambrafor Phrack MagazineAugust 1996 Guild Productions, kidcomments to route@/alhambra@--[ Introduction ]--Ping traffic is ubiquitous to almost every TCP/IP based network and subnetwork. It has a standard packet format recognized by every IP-speakingrouter and is used universally for network management, testing, and measurement. As such, many firewalls and networks consider ping trafficto be benign and will allow it to pass through, unmolested. This project explores why that practice can be insecure. Ignoring the obvious threat ofthe done-to-death denial of service attack, use of ping traffic can open up covert channels through the networks in which it is allowed.Loki, Norse God of deceit and trickery, the 'Lord of Misrule' was well known for his subversive behavior. Inversion and reversal of all sortswas typical for him. Due to it's clandestine nature, we chose to name this project after him.The Loki Project consists of a whitepaper covering this covert channel in detail. The sourcecode is not for distribution at this time.--[ Overview ]--This whitepaper is intended as a complete description of the covert channel that exists in networks that allow ping traffic (hereon referred toin the more general sense of ICMP_ECHO traffic --see below) to pass. It is organized into sections:Section I. ICMP Background Info and the Ping ProgramSection II. Basic Firewall Theory and Covert ChannelsSection III. T he Loki PremiseSection IV. Discussion, Detection, and PreventionSection V. References(Note that readers unfamiliar with the TCP/IP protocol suite may wish to first read ftp:///pub/Philes/NetTech/TCP-IP/tcipIp.intro.txt.gz)Section I. ICMP Background Info and the Ping ProgramThe Internet Control Message Protocol is an adjunct to the IP layer. It is a connectionless protocol used to convey error messages and other information to unicast addresses. ICMP packets are encapsulated inside of IP datagrams. The first 4-bytes of the header are same for every ICMP message,with the remainder of the header differing for different ICMP message types.There are 15 different types of ICMP messages.The ICMP types we are concerned with are type 0x0 and type 0x8. ICMP type 0x0 specifies an ICMP_ECHOREPLY (the response) and type0x8 indicates an ICMP_ECHO (the query). The normal course of action isfor a type 0x8 to elicit a type 0x0 response from a listening server. (Normally, this server is actually the OS kernel of the target host. MostICMP traffic is, by default, handled by the kernel). This is what the ping program does.Ping sends one or more ICMP_ECHO packets to a host. The purposemay just be to determine if a host is in fact alive (reachable). ICMP_ECHO packets also have the option to include a data section. This data sectionis used when the record route option is specified, or, the more common case, (usually the default) to store timing information to determine round-triptimes. (See the ping(8) man page for more information on these topics).An excerpt from the ping man page:"...An IP header without options is 20 bytes. An ICMP ECHO_REQUEST packetcontains an additional 8 bytes worth of ICMP header followed by anarbitrary-amount of data. When a packetsize is given, this indicated thesize of this extra piece of data (the default is 56). Thus the amount ofdata received inside of an IP packet of type ICMP ECHO_REPLY will alwaysbe 8 bytes more than the requested data space (the ICMP header)..."Although the payload is often timing information, there is no check byany device as to the content of the data. So, as it turns out, this amount of data can also be arbitrary in content as well. Therein lies the covert channel.Section II. Basic Firewall Theory and Covert ChannelsThe basic tenet of firewall theory is simple: To shield one network from another. This can be clarified further into 3 provisional rules:1. All traffic passing between the two networks must pass through the firewall.2. Only traffic authorized by the firewall may pass through (as dictated bythe security policy of the site it protects).3. The firewall itself is immune to compromise.A covert channel is a vessel in which information can pass, but this vessel is not ordinarily used for information exchange. Therefore, as amatter of consequence, covert channels are impossible to detect and deterusing a system's normal (read: unmodified) security policy. In theory,almost any process or bit of data can be a covert channel. In practice, itis usually quite difficult to elicit meaningful data from most covertchannels in a timely fashion. In the case of Loki, however, it is quitesimple to exploit.A firewall, in it's most basic sense, seeks to preserve the security policy of the site it protects. It does so by enforcing the 3 rules above. Covert channels, however, by very definition, are not subject to a site'snormal security policy.Section III. The Loki PremiseThe concept of the Loki Project is simple: arbitrary information tunneling in the data portion of ICMP_ECHO and ICMP_ECHOREPLY packets. Loki exploits the covert channel that exists inside of ICMP_ECHO traffic. This channel exists because network devices do not filter the contents of ICMP_ECHO traffic. They simply pass them, drop them, or return them. The trojan packets themselves are masqueraded as common ICMP_ECHO traffic. We can encapsulate (tunnel) any information we want. From here on out, Loki traffic will referto ICMP_ECHO traffic that tunnels information. (Astute readers will note that Loki is simply a form of steganography).Loki is not a compromise tool. It has many uses, none of which arebreaking into a machine. It can be used as a backdoor into a system byproviding a covert method of getting commands executed on a target machine.It can be used as a way of clandestinely leeching information off of amachine. It can be used as a covert method of user-machine or user-user communication. In essence the channel is simply a way to secretly shuffledata (confidentiality and authenticity can be added by way of cryptography).Loki is touted as a firewall subversion technique, but in reality it is simple a vessel to covertly move data. *Through* exactly what we move this data is not so much an issue, as long as it passes ICMP_ECHO traffic. It doesnot matter: routers, firewalls, packet-filters, dual-homed hosts, etc... allcan serve as conduits for Loki.Section IV. Discussion, Detection and PreventionIf ICMP_ECHO traffic is allowed, then this channel exists. If this channel exists, then it is unbeatable for a backdoor (once the system is compromised). Even with extensive firewalling and packet-filteringmechanisms in place, this channel continues to exist (provided, of course,they do not deny the passing of ICMP_ECHO traffic). With a proper implementation, the channel can go completely undetected for the duration ofits existence.Detection can be difficult. If you know what to look for, you mayfind that the channel is being used on your system. However, knowing whento look, where to look, and the mere fact that you *should* be looking allhave to be in place. A surplus of ICMP_ECHOREPLY packets with a garbledpayload can be ready indication the channel is in use. The standalone Loki server program can also be a dead give-away. However, if the attacker cankeep traffic on the channel down to a minimum, and was to hide the Lokiserver *inside* the kernel, detection suddenly becomes much more difficult.Disruption of this channel is simply preventative. Disallow ICMP_ECHO traffic entirely. ICMP_ECHO traffic, when weighed against the securityliabilities it imposes, is simply not *that* necessary. Restricting ICMP_ECHO traffic to be accepted from trusted hosts only is ludicrous with aconnectionless protocol such as ICMP. Forged traffic can still reach thetarget host. The LOKI packet with a forged source IP address will arrive atthe target (and will elicit a legitimate ICMP_ECHOREPLY, which willtravel to the spoofed host, and will be subsequently dropped silently) andcan contain the 4-byte IP address of the desired target of the Loki response packets, as well as 51-bytes of malevolent data... While the possibilityexists for a smart packet filter to check the payload field and ensure thatit *only* contains legal information, such a filter for ICMP is not in wideusage, and could still be open to fooling. The only sure way to destroy thischannel is to deny ALL ICMP_ECHO traffic into your network.NOTE: This channel exists in many other protocols. Loki Simply coversICMP, but in theory (and practice) any protocol is vulnerable to covertdata tunneling. All that is required is the ingenuity...Section V. ReferencesBooks: TCP Illustrated vols. I, II, IIIRFCs: rfc 792Source: Loki v1.0Ppl: We did not pioneer this concept To our knowledge,it was discovered independently of our efforts, prior to ourresearch. This party wishes to remain aloof.This project made possible by a grant from the Guild Corporation.EOF。