Fix KDC null deref on bad encrypted challenge

The function ec_verify() in src/kdc/kdc_preauth_ec.c contains a check
to avoid further processing if the armor key is NULL.  However, this
check is bypassed by a call to k5memdup0() which overwrites retval
with 0 if the allocation succeeds.  If the armor key is NULL, a call
to krb5_c_fx_cf2_simple() will then dereference it, resulting in a
crash.  Add a check before the k5memdup0() call to avoid overwriting
retval.

CVE-2021-36222:

In MIT krb5 releases 1.16 and later, an unauthenticated attacker can
cause a null dereference in the KDC by sending a request containing a
PA-ENCRYPTED-CHALLENGE padata element without using FAST.

[ghudson@mit.edu: trimmed patch; added test case; edited commit
message]

ticket: 9007 (new)
tags: pullup
target_version: 1.19-next
target_version: 1.18-next

src/kdc/kdc_preauth_ec.c

@@ -87,7 +87,8 @@ ec_verify(krb5_context context, krb5_data *req_pkt, krb5_kdc_req *request,
     }
 
     /* Check for a configured FAST ec auth indicator. */
-    realmstr = k5memdup0(realm.data, realm.length, &retval);
+    if (retval == 0)
+        realmstr = k5memdup0(realm.data, realm.length, &retval);
     if (realmstr != NULL)
         retval = profile_get_string(context->profile, KRB5_CONF_REALMS,
                                     realmstr,

src/tests/Makefile.in

@@ -166,6 +166,7 @@ check-pytests: unlockiter s4u2self
 	$(RUNPYTEST) $(srcdir)/t_cve-2012-1015.py $(PYTESTFLAGS)
 	$(RUNPYTEST) $(srcdir)/t_cve-2013-1416.py $(PYTESTFLAGS)
 	$(RUNPYTEST) $(srcdir)/t_cve-2013-1417.py $(PYTESTFLAGS)
+	$(RUNPYTEST) $(srcdir)/t_cve-2021-36222.py $(PYTESTFLAGS)
 	$(RM) au.log
 	$(RUNPYTEST) $(srcdir)/t_audit.py $(PYTESTFLAGS)
 	$(RUNPYTEST) $(srcdir)/jsonwalker.py -d $(srcdir)/au_dict.json \

src/tests/t_cve-2021-36222.py

@@ -0,0 +1,46 @@
+import socket
+from k5test import *
+
+realm = K5Realm()
+
+# CVE-2021-36222 KDC null dereference on encrypted challenge preauth
+# without FAST
+
+s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
+a = (hostname, realm.portbase)
+
+m = ('6A81A0' '30819D'          # [APPLICATION 10] SEQUENCE
+     'A103' '0201' '05'         #  [1] pvno = 5
+     'A203' '0201' '0A'         #  [2] msg-type = 10
+     'A30E' '300C'              #  [3] padata = SEQUENCE OF
+     '300A'                     #   SEQUENCE
+     'A104' '0202' '008A'       #    [1] padata-type = PA-ENCRYPTED-CHALLENGE
+     'A202' '0400'              #    [2] padata-value = ""
+     'A48180' '307E'            #  [4] req-body = SEQUENCE
+     'A007' '0305' '0000000000' #   [0] kdc-options = 0
+     'A120' '301E'              #   [1] cname = SEQUENCE
+     'A003' '0201' '01'         #    [0] name-type = NT-PRINCIPAL
+     'A117' '3015'              #    [1] name-string = SEQUENCE-OF
+     '1B06' '6B7262746774'      #     krbtgt
+     '1B0B' '4B5242544553542E434F4D'
+                                #     KRBTEST.COM
+     'A20D' '1B0B' '4B5242544553542E434F4D'
+                                #   [2] realm = KRBTEST.COM
+     'A320' '301E'              #   [3] sname = SEQUENCE
+     'A003' '0201' '01'         #    [0] name-type = NT-PRINCIPAL
+     'A117' '3015'              #    [1] name-string = SEQUENCE-OF
+     '1B06' '6B7262746774'      #     krbtgt
+     '1B0B' '4B5242544553542E434F4D'
+                                #     KRBTEST.COM
+     'A511' '180F' '31393934303631303036303331375A'
+                                #   [5] till = 19940610060317Z
+     'A703' '0201' '00'         #   [7] nonce = 0
+     'A808' '3006'              #   [8] etype = SEQUENCE OF
+     '020112' '020111')         #    aes256-cts aes128-cts
+
+s.sendto(bytes.fromhex(m), a)
+
+# Make sure kinit still works.
+realm.kinit(realm.user_princ, password('user'))
+
+success('CVE-2021-36222 regression test')