Vulnerability Note VU#803539

The Domain Name System (DNS) provides name, address, and other information about Internet Protocol (IP) networks and devices. By issuing queries to and interpreting responses from DNS servers, IP-enabled network operating systems can access DNS information. When an IP network application needs to access or process DNS information, it calls functions in the stub resolver library, which may be part of the underlying network operating system. On BSD-based systems, DNS stub resolver functions are implemented in the system library libc. In ISC BIND, they are implemented in libbind, and on GNU/Linux-based systems, they are implemented in glibc.

The DNS resolver libraries on BSD-based systems (libc), ISC BIND (libbind), GNU/Linux (glibc), and possibly other systems that use code derived from ISC BIND contain buffer overflow vulnerabilities in the way the resolvers handle DNS responses. Quoting from FreeBSD Security Advisory FreeBSD-SA-02:28.resolv:

DNS messages have specific byte alignment requirements, resulting in padding in messages. In a few instances in the resolver code, this padding is not taken into account when computing available buffer space. As a result, the parsing of a DNS message may result in a buffer overrun of up to a few bytes for each record included in the message.

In lib/libc/net/gethnamaddr.c:getanswer() and lib/libc/net/getnetnamadr.c:getnetanswer(), two variables manage packet buffer parsing - a pointer to the byte we are looking at, and the remaining length on the buffer. The remaining length was not updated consistently, and malicious DNS responses are able to write outside the buffer.

An attacker who is able to control DNS responses could exploit arbitrary code or cause a denial of service on vulnerable systems. The attacker would need to be able to spoof DNS responses or control a DNS server that provides responses to a vulnerable system. Any code executed by the attacker would run with the privileges of the process that called the vulnerable resolver function, potentially root.

Apply a Patch

Apply a patch from your vendor. In the case of statically linked binaries, it is necessary to recompile using the patched version of the DNS stub resolver libraries.

Upgrade

Upgrade your system as specified by your vendor.

Use of a local caching DNS server is not an effective workaround

When this document was initially published, it was thought that a caching DNS server that reconstructs DNS responses would prevent malicious code from reaching systems with vulnerable resolver libraries. This workaround does not prevent some DNS responses that contain malicious code from reaching clients, whether or not the responses are reconstructed by a local caching DNS server. Since the server may cache the responses, the malicious code could persist until the server's cache is purged or the entries expire.

Disable Reverse DNS Lookups

Disable the reverse DNS lookup functions in applications that perform DNS name lookups from IP addresses. For example, some HTTP and FTP servers perform reverse DNS lookups to convert IP addresses to hostnames in logs. Disabling reverse DNS lookups will only protect against specific exploit attempts that rely on the reverse lookup as an attack vector.