djbdns：セキュリティホール

djbこと D. J. Bernstein 氏は、かねがねdjbdnsの最初のセキュリティホールを発見した人には$1000の賞金を出すと公言して、はや数年。
この度、とうとう Matthew Dempsky 氏によって djbdns のセキュリティ問題 (djbdns misformats some long response packets)が発見され、djb は問題であることを認め、Dempsky 氏は賞金$1000を獲得となった。

ちなみに問題の内容は、DNSパケットは有効なオフセット値は16384より小さくなくてはならないのだが、djbdns-1.05では応答パケットに対してこの制限が掛かっていない。
従って、不正な応答パケットを使って16384バイト以降に現れる名前で上書きすることができる(キャッシュ汚染させることができる)、というものらしい。
既に修正方法も公開されており、単純な修正方法なので、未対応の人は対応しておきましょう。
ただ、セキュリティリスクとしては、それほど高くない模様ですが。

以下、詳細を転載しておきます。

--
The DNS packet format allows names to be compressed by replacing the
suffix of a name with an encoded offset to another location in the
packet where the suffix already exists. Because of the encoding
scheme, valid offsets are limited to < 16384.

In djbdns 1.05, response.c handles name compression. Line 12 has a
comment "each < 16384" on the name_ptr array, but response_addname()
from the same file does not enforce this limitation. The result is
that when encoding names with a suffix that first appears >= 16384
bytes into the packet, response_addname() incorrectly tries to encode
an offset to that name and produces a misformatted response packet.
(At the bottom of this email, there is a patch for this.)

You can reproduce an exploit of this bug as follows:

    # Download and build ucspi-tcp-0.88.
    $ curl -O http://cr.yp.to/ucspi-tcp/ucspi-tcp-0.88.tar.gz
    $ tar -zxf ucspi-tcp-0.88.tar.gz
    $ echo 'gcc -include /usr/include/errno.h -O' > ucspi-tcp-0.88/conf-cc
    $ make -C ucspi-tcp-0.88
 
    # Download and build djbdns-1.05.
    $ curl -O http://cr.yp.to/djbdns/djbdns-1.05.tar.gz
    $ tar -zxf djbdns-1.05.tar.gz
    $ echo 'gcc -include /usr/include/errno.h -O' > djbdns-1.05/conf-cc
    $ make -C djbdns-1.05
 
    # Use tcpclient and axfr-get to do a zone transfer for
    # burlap.dempsky.org from shinobi.dempsky.org.
    $ ./ucspi-tcp-0.88/tcpclient shinobi.dempsky.org 53 \
          ./djbdns-1.05/axfr-get burlap.dempsky.org data data.tmp
 
    # Use tinydns-data to compile data into data.cdb.
    $ ./djbdns-1.05/tinydns-data
 
    # Simulate an A query for www.x.burlap.dempsky.org using the data
    # from the zone transfer.
    $ ./djbdns-1.05/tinydns-get a www.x.burlap.dempsky.org

The last command will include these two lines in the output:

    additional: foo 8388608 NS a.ns.bar
    additional: foo 8388608 NS b.ns.bar

i.e., poisonous NS records for foo, delegating the domain to a.ns.bar
and b.ns.bar; with my patch applied, only records within
burlap.dempsky.org are output. Also, there's significant freedom in
what poisonous records the attacker can produce.

The security hole here is that an administrator that uses djbdns 1.05
to serve DNS content does not expect that configuring his name server
as above will cause it to send records for names outside of
burlap.dempsky.org. I.e., an attacker can trick the administrator's
name servers to include arbitrary DNS records in response to queries
for names within domains he controls. Note that axfr-get is doing the
right thing here: it already strips out names from outside of the
specified zone; it's just that tinydns-get (and so tinydns and
axfrdns) misformat the response packet. A direct NS query for foo
would not generate these poisonous records.

As a real life example, I registered burlap.dempsky.org as a secondary
domain with EveryDNS pulling data from my server. I was able to trick
their name servers into serving the above poisonous NS records.
EveryDNS's name servers have no authority over the hypothetical foo
TLD, but I could have included poisonous NS records for everydns.net
instead. The DNS cache from djbdns 1.05, dnscache, would have
rejected these records as poison, but it's possible other DNS caches
might accept them. (Either way, EveryDNS installed my patch earlier
today, so this is no longer a risk.)

As another example, I registered burlap.afraid.org as a secondary
domain with FreeDNS (freedns.afraid.org). They don't use djbdns, but
if they had, this would have allowed me to include poisonous NS
records for afraid.org that DNS caches like dnscache and BIND would
have accepted.

Some caveats: this bug only affects domains that serve DNS content
using tinydns and axfrdns (only for DNS queries over TCP; clients do
not need AXFR permissions) from djbdns 1.05 and allow untrusted users
to include arbitrary records (at least about 100 records, totalling
about 30KB of space) within some zone.

In summary: if you use tinydns/axfrdns from djbdns 1.05 to serve
authoritative DNS content and give untrusted users control over
records you serve, I strongly suggest you install this patch. I don't
believe other users are at risk, but they are encouraged to install
this patch as well to be safe.

Finally, here's the promised patch:

 --- response.c.orig     2009-02-24 21:04:06.000000000 -0800
 +++ response.c  2009-02-24 21:04:25.000000000 -0800
 @@ -34,7 +34,7 @@
         uint16_pack_big(buf,49152 + name_ptr[i]);
         return response_addbytes(buf,2);
       }
 -    if (dlen <= 128)
 +    if ((dlen <= 128) && (response_len < 16384))
       if (name_num < NAMES) {
        byte_copy(name[name_num],dlen,d);
        name_ptr[name_num] = response_len;