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COVERT Labs Security Advisory: Vulnerabilities in BIND 4 and 8

Jan 29, 2001, 21:29 (5 Talkback[s])

Date: Mon, 29 Jan 2001 06:31:55 -0800
From: COVERT Labs seclabs@NAI.COM
Subject: [COVERT-2001-01] Multiple Vulnerabilities in BIND

                     Network Associates, Inc.
                  COVERT Labs Security Advisory
                        January 29, 2001

                 Vulnerabilities in BIND 4 and 8


o Synopsis

BIND 8 contains a buffer overflow that allows a remote attacker to execute arbitrary code. The overflow is in the initial processing of a DNS request and therefore does not require an attacker to control an authoritative DNS server. In addition, the vulnerability is not dependent upon configuration options and affects both recursive and non-recursive servers. This vulnerability has been designated as CVE candidate CAN-2001-10.


BIND 4 contains a buffer overflow that can allow a remote attacker to execute arbitrary code. The overflow occurs when BIND reports an error while attempting to locate IP addresses for name servers. Exploitation of this vulnerability is restricted by the fact that the target name server be recursive and that the attacker has control of an authoritative DNS server. This vulnerability has been designated as CVE candidate CAN-2001-11.


BIND 4 contains a format string vulnerability that can allow a remote attacker to execute arbitrary code. This vulnerability also occurs when BIND reports an error while attempting to locate IP addresses for name servers, and thus has the same restrictions on exploitation as the buffer overflow. This vulnerability was fixed several versions prior to the current version of BIND 4, but is still present in certain Unix distributions. This vulnerability has been designated as CVE candidate CAN-2001-13.


o Vulnerable Systems

BIND 8 versions: 8.2, 8.2.1
                 8.2.2 through to 8.2.2-P7
                 8.2.3-T1A through to 8.2.3-T9B

BIND 4 versions: buffer overflow - 4.9.5 through to 4.9.7
                 format string   - 4.9.3 through to 4.9.5-P1

o Vulnerability Overview

BIND (Berkeley Internet Name Domain) is an implementation of the DNS (Domain Name System) protocol distributed by the Internet Software Consortium (www.isc.org). Two versions of BIND distributed by the ISC, BIND version 4 and BIND version 8, are vulnerable to the attacks described in this advisory. The most recent release of BIND, version 9, is not susceptible to these attacks.

BIND version 8 contains a buffer overflow in the implementation of Transaction Signatures (TSIG) for DNS security as defined in RFC 2845. Because the overflow occurs within the initial processing of a DNS request, both recursive and non-recursive DNS servers are vulnerable, independent of the DNS security configuration. The mechanisms employed by the DNS server make it susceptible to two potential methods of attack.

An attacker can perform a stack based buffer overflow, with two important qualifications: first, that the number of bytes past the end of the buffer that the attacker can overwrite is limited in length, and second, that the values of those bytes are mostly fixed. On the x86 architecture, the attacker can manipulate a sufficient number of bytes such that they can modify the saved frame pointer. Overwriting the least significant byte of the saved frame pointer can result in the execution of arbitrary code in certain predictable installations of the name server. The "infoleak" bug, discovered by Claudio Musmarra, and described in CERT advisory CA-2001-02, permits an attacker to remotely retrieve stack frames from named, which allows for direct calculation of the effect of the one byte overflow.

An attacker can also perform a heap overflow, overwriting malloc's internal variables. This method is very effective, though it requires that an operating system's implementation of malloc stores internal data structures in the allocated memory. For this attack to be successful, TCP port 53 must be accessible.

BIND version 4 contains a buffer overflow in a section of code that formulates a warning message for a call to syslog. There are several conditions that can lead to the triggering of this overflow, all of which involve the resolution of NS records into IP addresses. This vulnerability is a standard stack overflow, but the information an attacker is able to present is limited to printable characters. This limitation makes susceptibility to exploitation contingent upon the layout of the named process within memory, and possibly upon the amount of memory available to be allocated by the name server.

In older versions of BIND 4, the previously mentioned call to syslog utilizes a user controllable string as the second argument, which creates an exploitable condition. The same restriction applies, in that the format string is limited to printable characters. Despite this restriction, a remote attacker is still able to create a malicious format string to exploit the vulnerable syslog function call.

o Detailed Information

The BIND 8 vulnerability is the result of a DNS request utilizing a particular code path that invalidates the logic used to calculate the length of the request buffer.

When a request is received, it is either stored in the heap or on the stack, depending on the transport mechanism. Upon receipt of a UDP request, it is read into a 513 byte buffer on the stack called "u.buf" by the function datagram_read(). When a TCP request is received, the message is read by stream_getlen() into a 64k buffer called "sp->s_buf", which is allocated from the heap for every socket. An interesting feature of BIND is that it uses the incoming buffer of both transport mechanisms to read the request from the network and then modifies it to create an appropriate response. Two key variables are maintained to track the usage of the buffer: one containing the actual length of the data in the buffer, called "msglen", and a second variable "buflen" that tracks the remaining length free in the buffer.

When a DNS message is received, msglen is initialized to the length of data received from the network. With a UDP message, this is the amount of data returned by a recvfrom() call, whereas with a TCP message, it is the value provided as the length by the client. buflen is set to the size of the buffer used to read the message (512 for UDP, 64k for TCP).

Under normal circumstances, as BIND processes a request, it appends the answer, authoritative, and additional records to the query. It then modifies the DNS header to reflect these changes and delivers the response. During this processing, msglen will reflect the length of the response as it is being formed, and buflen will be used to track the remaining space available in the buffer. Throughout the processing, BIND assumes that msglen plus buflen will equal the original length of the buffer.

Upon receipt of a DNS message, it is processed as either a request or response based upon the query response flag set in the message header. If a request is received, BIND then determines whether it is a query, iquery, update or notification. Beginning with BIND 8.2, prior to request processing, the additional section of the DNS message is examined for a TSIG resource record. The function
ns_find_tsig() is called to perform this functionality as well as to enforce a basic level of validity on the TSIG resource record. If a valid TSIG is identified but an appropriate security key can not be found, an error is signaled and BIND bypasses the normal request processing. As a result, msglen and buflen remain close to their initial values, instead of being set to their "working" values.

BIND processes the request as an error since a TSIG was identified but an appropriate security key was not found. As part of the error generation, BIND reuses the request buffer and appends a TSIG after the question section. At this point, BIND assumes that the size of the request is msglen plus buflen which, under normal circumstances, would be correct. However, in this special case, the request was never processed and "msglen + buflen" is in fact almost twice the size of the original buffer. BIND is then willing to append a TSIG via ns_sign() beyond the limits of the buffer.

Since a valid security key was not found, ns_sign() will only append a small number of bytes with limited values. As mentioned above, this makes the vulnerable BIND installation susceptible to two types of attack.

Combining this oversight with the way a compiler positions the stack variables in datagram_read(), it is possible for an attacker to overwrite portions of the saved stack activation records in
datagram_read() with certain fixed values. In this case, executing arbitrary code is possible under the x86 architecture by overwriting the saved frame pointer's least significant byte with zero resulting in the saved frame pointer pointing into the original DNS request in the majority of cases.

Predicting the effect of this one byte overflow can be difficult as it varies depending upon how BIND was started. However, the "infoleak" bug allows an attacker to retrieve the stack activation record of datagram_read(). This information can then be used to calculate the exact number of bytes that will displace the frame pointer when the least significant byte of the saved ebp is overwritten with 0.

The second method of attack utilizes certain implementations of dynamic memory allocation. It is possible to overwrite malloc's boundary tags with predictable values, changing the standard libraries' notion of the length of the buffer following the buffer processed in the DNS request. Thus, the next set of boundary information is read from within a buffer that an attacker can control, allowing for a malicious pointer overwrite upon compaction.

This technique is applicable to malloc implementations that store linkage information in the actual allocated memory. The following implementations are known by COVERT to be exploitable: IRIX libc, Linux glibc, and Solaris libc.

The BIND 4 vulnerability is a sprintf into a 999 byte stack buffer that occurs when BIND formulates a message warning the administrator of an inconsistency or error resolving a Name Server record to an IP address. The vulnerability occurs within nslookupComplain(), which is a static utility function used by nslookup().

When BIND encounters a query that it can not answer from its cache or zone files, it attempts to forward the query to a name server that is capable of resolving it or referring BIND to a more appropriate server. When BIND forwards a query, it creates a qinfo structure to keep track of the request. It also creates this structure in order to track requests initiated by itself in order to find various linkage information. BIND can determine potential name servers for which to forward to by walking through each label in the query in its database, looking for stored NS records.

The purpose of the nslookup() function is to take a list of NS records and populate a qinfo structure with their corresponding IP addresses. BIND can then use those IP addresses as a list of name servers for which to attempt forwarding or sending a query.

nslookup() performs certain sanity checks on the information that it retrieves. For example, if it finds that a particular name server has an IP address of,, or a multicast address, then it will flag this condition as an error, warn the administrator via syslog, and move on to the next NS record. The function nslookupComplain() is called to warn the administrator and, as mentioned above, contains a stack overflow.

In order to trigger this overflow, an attacker needs to get BIND to cache an NS record with a very large length. Furthermore, the attacker needs to cache a record for the resolution of the NS record that contains one of the problem conditions for the logging. This is achievable by sending a query to a recursive name server, asking it to resolve a large name that is under the authority of a malicious name server. The malicious name server then needs to refer the request to another name server also with a large name, and provide an additional record giving an invalid address for that name server.

The limitations placed upon the character set allowed in domain names makes the construction of a viable return address difficult. However, there is a potential for an attacker to make the name server return into memory that the attacker has forced the name server to allocate. In this case, vulnerability is contingent upon the location of the heap and the amount of memory available, as well as whether or not the operating system has a policy of lazy swap page allocation as opposed to an eager reservation policy. COVERT has verified that it is possible to exploit named running under Linux by growing the heap to sizes that far exceed that amount of memory and swap available. This was performed by utilizing specific patterns of memory allocation that maximize untouched memory.

The situation may be further complicated by the overwriting of two other stack based buffers, nsbuf and abuf, which are read from within the same sprintf that overflows the stack based buffer. This does not come in to play, however, if the value chosen to overwrite the saved return address does not utilize the terminating null byte of the string. It is worth noting that this behavior could make it easier for an attacker to exploit the problem under operating systems that implement sprintf such that overlapping copies are handled correctly.

The format string vulnerability in BIND 4 occurs in the syslog call in nslookupComplain(). This vulnerability is in the same section of code as the previously described buffer overflow, and thus can be triggered in a similar fashion by using an authoritative name server under malicious control. This vulnerability was corrected in bind-4.9.5-P1, although certain vendors' named implementations based upon this code remain vulnerable.

o Resolution

ISC has produced patches to address these issues. Except as otherwise noted, BIND version 4.9.8 and 8.2.3 resolve the vulnerabilities described in this advisory.

For ISC's description of these problems:


To download updated versions of BIND:


In cooperation with COVERT Labs, the CERT/CC is coordinating the collection of information on vulnerable distributions from third party vendors. For the most current vendor information, please read CERT Advisory CA-2001-02 "Multiple Vulnerabilities in BIND" available at:


o Credits

Discovery and documentation of these vulnerabilities was conducted by Anthony Osborne and John McDonald of the COVERT Labs at PGP Security.

o Contact Information

For more information about the COVERT Labs at PGP Security, visit our website at http://www.pgp.com/covert or send e-mail to covert@nai.com

o Legal Notice

The information contained within this advisory is Copyright (C) 2000 Networks Associates Technology Inc. It may be redistributed provided that no fee is charged for distribution and that the advisory is not modified in any way.

Network Associates and PGP are registered Trademarks of Network Associates, Inc. and/or its affiliated companies in the United States and/or other Countries. All other registered and unregistered trademarks in this document are the sole property of their respective owners.