Date: Wed, 16 Aug 2017 10:50:33 +0200 From: Florian Weimer <fweimer@...hat.com> To: oss-security@...ts.openwall.com Subject: Insecure DNS dependency in many Kerberos deployments By default, Kerberos clients perform host name canonicalization (search path resolution, CNAME chain chasing and PTR lookups) to obtain a service principal name. This allows service impersonification: https://ssimo.org/blog/id_015.html As a rule of thumb, the impact is similar to running TLS with CA-based certificate validation, but without host name checks (but perhaps slightly less because the trust domains could be much smaller). The Kerberos client library enables this canonicalization by default: dns_canonicalize_hostname Indicate whether name lookups will be used to canonicalize hostnames for use in service principal names. Setting this flag to false can improve security by reducing reliance on DNS, but means that short hostnames will not be canoni‐ calized to fully-qualified host‐ names. The default value is true. rdns If this flag is true, reverse name lookup will be used in addition to forward name lookup to canonicaliz‐ ing hostnames for use in service principal names. If dns_canonical‐ ize_hostname is set to false, this flag has no effect. The default value is true. Some deployments have implemented compatibility with dns_canonicalize_hostname = false by moving the canonicalization to the application instead, which is of course equally insecure: https://pagure.io/koji/c/fc8a8c6582c5e3b7a8a3a4b887061ba7a3f150a1 https://bugzilla.redhat.com/show_bug.cgi?id=1481983 Kerberos upstream does not want to enable secure behavior by default because of backwards compatibility concerns. Thanks, Florian
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