KDC null deref due to referrals [CVE-2013-1417]

An authenticated remote client can cause a KDC to crash by making a
valid TGS-REQ to a KDC serving a realm with a single-component name.
The process_tgs_req() function dereferences a null pointer because an
unusual failure condition causes a helper function to return success.

While attempting to provide cross-realm referrals for host-based
service principals, the find_referral_tgs() function could return a
TGS principal for a zero-length realm name (indicating that the
hostname in the service principal has no known realm associated with
it).

Subsequently, the find_alternate_tgs() function would attempt to
construct a path to this empty-string realm, and return success along
with a null pointer in its output parameter.  This happens because
krb5_walk_realm_tree() returns a list of length one when it attempts
to construct a transit path between a single-component realm and the
empty-string realm.  This list causes a loop in find_alternate_tgs()
to iterate over zero elements, resulting in the unexpected output of a
null pointer, which process_tgs_req() proceeds to dereference because
there is no error condition.

Add an error condition to find_referral_tgs() when
krb5_get_host_realm() returns an empty realm name.  Also add an error
condition to find_alternate_tgs() to handle the length-one output from
krb5_walk_realm_tree().

The vulnerable configuration is not likely to arise in practice.
(Realm names that have a single component are likely to be test
realms.)  Releases prior to krb5-1.11 are not vulnerable.

Thanks to Sol Jerome for reporting this problem.

CVSSv2: AV:N/AC:M/Au:S/C:N/I:N/A:P/E:H/RL:O/RC:C

ticket: 7668 (new)
tags: pullup
target_version: 1.11.4

1 files changed, 3 insertions, 1 deletions

diff --git a/src/kdc/do_tgs_req.c b/src/kdc/do_tgs_req.c
index 7ddb84a420..2d5fceed94 100644
--- a/src/kdc/do_tgs_req.c
+++ b/src/kdc/do_tgs_req.c
@@ -1035,6 +1035,8 @@ find_alternate_tgs(kdc_realm_t *kdc_active_realm, krb5_principal princ,
         goto cleanup;
     }
 cleanup:
+    if (retval == 0 && server_ptr == NULL)
+        retval = KRB5_KDB_NOENTRY;
     if (retval != 0)
         *status = "UNKNOWN_SERVER";
 
@@ -1137,7 +1139,7 @@ find_referral_tgs(kdc_realm_t *kdc_active_realm, krb5_kdc_req *request,
         goto cleanup;
     }
     /* Don't return a referral to the empty realm or the service realm. */
-    if (realms == NULL || realms[0] == '\0' ||
+    if (realms == NULL || realms[0] == NULL || *realms[0] == '\0' ||
         data_eq_string(srealm, realms[0])) {
         retval = KRB5KDC_ERR_S_PRINCIPAL_UNKNOWN;
         goto cleanup;