libc: Don't use uninitialised string for getnetbyaddr[_r](0) DNS lookupHEADmain

If net is all-zero, the loop to extract all leading non-zero octets will
iterate zero times and leave nn with the value 4, which the following
switch statement to initialise qbuf does not handle. As a result,
_dns_getnetbyaddr will look up the PTR record for this uninitialised
string, which will leak the pre-existing contents of that stack memory
to the DNS resolver and, if remote and not otherwise protected, network.

Note that _dns_getnetbyaddr is only used if nsswitch.conf is configured
to enable the "dns" source for the "networks" database, which is not the
default configuration in FreeBSD.

For glibc this same bug, in code also derived from BIND's, was issued
CVE-2026-0915. This commit adopts the same behaviour as glibc's fix,
which is to regard a net of 0 as being for 0.0.0.0. Apparently NetBSD
will return NS_UNAVAIL instead, which may or may not make more sense,
but in general glibc compatibility tends to cause less friction when
there's not a good reason to avoid it.

Reviewed by:	markj (secteam)
Fixes:		1363f04ce1b8 ("get* rework and new bind code")
MFC after:	1 day
Security:	Same bug as glibc's CVE-2026-0915

0 files changed, 0 insertions, 0 deletions