A flaw was discovered in the way Libreswan's IKE daemon processed certain IKEv1 payloads. A remote attacker could send specially crafted IKEv1 payloads that, when processed, would lead to a denial of service (daemon crash).

By setting unassigned bits of the IPSEC DOI value, an error message
string would be printed with string names as bit numbers. Printing 32 of
these would cause the internal buffer "bitnamesbuf" to be too small. This
buffer is truncated properly in the non-vulnerable versions. A generic
jam_str() function was added to these protections, but it would passert()
if not given at least a buffer length of 1 (to add a NULL to terminate
the string). However, the filled in string would have no more space for
the additional 1 character to be added. The passert() would cause the IKE
daemon to restart.

By setting the next payload value to ISAKMP_NEXT_SAK (used by old Cisco
VPN servers to signal NAT-Traversal payloads), the libreswan daemon would
attempt to interpret this payload as a NAT-D payload. However, it did not
properly do so, causing a passert() which would restart the IKE daemon.

This denial of service can be launched by anyone using a single IKE packet.
No authentication credentials are required. No remote code execution is
possible through this vulnerability. Libreswan automatically restarts when
it crashes.

Acknowledgements:

Red Hat would like to thank Javantea for reporting this issue.