Heap corruption via crafted DN strings

Description

A DN may be represented in string form with arbitrary amounts of space
around the component values. These spaces are supposed to be ignored,
but invalid DNs strings with spaces may instead cause a zero byte to
be written into out-of-bounds memory.

An LDAP bind request can send a string DN as a username. This DN is
necessarily parsed before the password is checked, so an attacker
without real credentials can anonymously trigger this bug.

The location of zero byte is a negative offset relative to the
location of a dynamically allocated heap buffer; the exact offset
depends on the DN string. While it is possible for an attacker to
cause non-fatal data corruption, usefully targeting this is likely to
be difficult and the most likely outcome is a crash.

The affected parsing routine is widely used. LDAP bind is not the only
way to trigger the bug remotely, though it appears to be the only
unauthenticated method.

For technical details of the vulnerability, see the patch and
the bug at https://bugzilla.samba.org/show_bug.cgi?id=14595.

Patch Availability

Patches addressing both these issues have been posted to:

https://www.samba.org/samba/security/

Additionally, Samba 4.14.2 (4.14.1), 4.13.7 (4.13.6) and 4.12.14 (4.12.13) have
been issued as security releases to correct the defect. Samba administrators
are advised to upgrade to these releases or apply the patch as soon as possible.

CVSSv3 calculation

CVSSv3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H (7.5)

Workaround and mitigation

None.

Credits

Found and fixed by Douglas Bagnall of Catalyst and the Samba Team,
using Honggfuzz.

Advisory written by Douglas Bagnall.

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