Software Engineering Institute

NLnetLabs advisory states:

"Certain types of DNS queries may cause Unbound to respond with a DNS error message. The code generating this error message contains an error whereby Unbound constructs an empty UDP message. That empty UDP message triggers an assertion failsafe in the part of Unbound that is invoked when the interface-automatic option is set. In the case that the interface-automatic option was not set or assertion failsafe is not configured the empty packets would be sent, which is not harmful. The error can only be triggered when Unbound is configured with assertions (--enable-checking or --enable-debug) and has the option interface-automatic turned on.

Versions in which this error occurs: Unbound 1.0 up to and including Unbound 1.4.9

• FreeBSD ports for Unbound are not susceptible by default, as it does not enable assertions.
• RHEL/Fedora systems do compile Unbound with --enable-checking but do not set automatic-interface:yes per default.
• Debian/Ubuntu Unbound packages do not compile with assertions by default."

A remote, unauthenticated attacker can cause the Unbound daemon to crash creating a denial of service condition.

Apply an UpdateThis vulnerability has been addressed in Unbound 1.4.10. The following patch may also be applied to resolve the issue:

Index: daemon/worker.c
===================================================================
--- daemon/worker.c\t(revision 2416)
+++ daemon/worker.c\t(working copy)
@@ -779,6 +779,7 @@
qinfo.qtype == LDNS_RR_TYPE_IXFR) {
verbose(VERB_ALGO, "worker request: refused zone transfer.");
log_addr(VERB_CLIENT,"from",&repinfo->addr, repinfo->addrlen);
+ldns_buffer_rewind(c->buffer);
LDNS_QR_SET(ldns_buffer_begin(c->buffer));
LDNS_RCODE_SET(ldns_buffer_begin(c->buffer),
LDNS_RCODE_REFUSED);