Threat Research

Internet Kill Switch Found?

By Dehui Yin | August 12, 2015

Last week, the Internet Systems Consortium (ISC) released a critical update to its popular software, BIND. BIND is almost everywhere on the Internet, acting as a DNS name server.

DNS is a network service used to translate human-readable domain names to numeric identifiers called IP addresses and vice-versa.

The update was in response to a recently discovered DoS vulnerability (CVE-2015-5477), which can be exploited to take down the vulnerable BIND server remotely. ISC released an urgent patch for this vulnerability due to its severity. Only one packet is required to remotely crash the servers; no authentication is required. That means the attack can be easily automated and some reports already show that it’s been exploited in the wild.

In this post, we are analyzing the root cause of this vulnerability.

This DoS issue is caused by an error in function dns_tkey_processquery,which is used to handle dns TKEY query. It parses the dns TKEY query and extracts the TKEY Resource Record to do the related operation, such as TKEY Deletion and GSS-API Establishment.

The code snippet (version 9.7.7) below shows the vulnerable function dns_tkey_processquery.

isc_result_t dns_tkey_processquery(dns_message_t *msg, dns_tkeyctx_t *tctx,

                              dns_tsig_keyring_t *ring)



omitted for clarity


tkeyset = NULL;

            name = NULL;

            result = dns_message_findname(msg, DNS_SECTION_ADDITIONAL, qname,

                                                      dns_rdatatype_tkey, 0, &name, &tkeyset);

The function dns_tkey_processquery calls dns_message_findname to extract the TKEY Resource Record from the Additional section according to the domain name in the Query section of the DNS request. 

isc_result_t dns_message_findname(dns_message_t *msg, dns_section_t section,

                             dns_name_t *target, dns_rdatatype_t type,

                             dns_rdatatype_t covers, dns_name_t **name,

                             dns_rdataset_t **rdataset)


            ....omitted for clarity...

            if (name != NULL)                 

                        REQUIRE(*name == NULL);   // pass assertion

          ....omitted for clarity...

            result = findname(&foundname, target,


            ....omitted for clarity...

            if (name != NULL)

                        *name = foundname; //not NULL anymore

            ....omitted for clarity...

            result = dns_message_findtype(foundname, type, covers, rdataset);

//result= ISC_R_NOTFOUND here

            if (result == ISC_R_NOTFOUND)

                        return (DNS_R_NXRRSET);

If dns_message_type returns  ISC_R_NOTFOUND, it will go back to dns_tkey_processquery  and call dns_message_findname again to find the TKEY Resource Record from Answer section. Because the parameter “name” for dns_message_findname doesn’t reset to NULL after call the function in the first time, it will cause a Require assertion failure which makes the named progress terminate and exit.

if (result != ISC_R_SUCCESS) {

                        if (dns_message_findname(msg, DNS_SECTION_ANSWER, qname,

                                                             dns_rdatatype_tkey, 0, &name,

                                                             &tkeyset) != ISC_R_SUCCESS) {

As you can see above, the function “dns_message_findname” is called again.

isc_result_t dns_message_findname(dns_message_t *msg, dns_section_t section,

                             dns_name_t *target, dns_rdatatype_t type,

                             dns_rdatatype_t covers, dns_name_t **name,

                             dns_rdataset_t **rdataset)



            if (name != NULL)                

                        REQUIRE(*name == NULL);  //assertion fail this time!

As a result, the named process terminates due to the assertion failure as shown in image below.

The PoCs are also widely available, which may have accelerated the massive adoption by attackers.

Both of these 2 PoCs send the TKEY query via UDP, but actually during our analysis we were able to verify that sending the attack over TCP is just as effective.

Investigation of these PoCs also showed that the CLASS field of the resource record in the Question section is not fixed: it can be either “0xff” or “1”. From our tests, we were able to verify that only these 2 values can be used to conduct the attack.

Fortinet released IPS signature ISC.BIND.TKEY.Queries.DoS for this vulnerability to cover all the attack vectors mentioned above.


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