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From 4e8579f0a41b66ed8029f21a52082e1c27ab3996 Mon Sep 17 00:00:00 2001
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From: Joseph Sutton <josephsutton@catalyst.net.nz>
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Date: Wed, 7 Jul 2021 11:47:44 +1200
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Subject: [PATCH] Fix KDC null deref on bad encrypted challenge
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The function ec_verify() in src/kdc/kdc_preauth_ec.c contains a check
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to avoid further processing if the armor key is NULL. However, this
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check is bypassed by a call to k5memdup0() which overwrites retval
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with 0 if the allocation succeeds. If the armor key is NULL, a call
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to krb5_c_fx_cf2_simple() will then dereference it, resulting in a
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crash. Add a check before the k5memdup0() call to avoid overwriting
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retval.
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CVE-2021-36222:
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In MIT krb5 releases 1.16 and later, an unauthenticated attacker can
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cause a null dereference in the KDC by sending a request containing a
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PA-ENCRYPTED-CHALLENGE padata element without using FAST.
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[ghudson@mit.edu: trimmed patch; added test case; edited commit
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message]
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(cherry picked from commit fc98f520caefff2e5ee9a0026fdf5109944b3562)
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ticket: 9007
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version_fixed: 1.18.4
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(cherry picked from commit c4a406095b3ea4a67ae5b8ea586cbe9abdbae76f)
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---
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src/kdc/kdc_preauth_ec.c | 3 ++-
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src/tests/Makefile.in | 1 +
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src/tests/t_cve-2021-36222.py | 46 +++++++++++++++++++++++++++++++++++
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3 files changed, 49 insertions(+), 1 deletion(-)
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create mode 100644 src/tests/t_cve-2021-36222.py
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diff --git a/src/kdc/kdc_preauth_ec.c b/src/kdc/kdc_preauth_ec.c
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index 7e636b3f9..43a9902cc 100644
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--- a/src/kdc/kdc_preauth_ec.c
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+++ b/src/kdc/kdc_preauth_ec.c
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@@ -87,7 +87,8 @@ ec_verify(krb5_context context, krb5_data *req_pkt, krb5_kdc_req *request,
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}
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/* Check for a configured FAST ec auth indicator. */
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- realmstr = k5memdup0(realm.data, realm.length, &retval);
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+ if (retval == 0)
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+ realmstr = k5memdup0(realm.data, realm.length, &retval);
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if (realmstr != NULL)
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retval = profile_get_string(context->profile, KRB5_CONF_REALMS,
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realmstr,
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diff --git a/src/tests/Makefile.in b/src/tests/Makefile.in
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index 3f88f1713..0ffbebf56 100644
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--- a/src/tests/Makefile.in
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+++ b/src/tests/Makefile.in
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@@ -158,6 +158,7 @@ check-pytests: unlockiter s4u2self
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$(RUNPYTEST) $(srcdir)/t_cve-2012-1015.py $(PYTESTFLAGS)
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$(RUNPYTEST) $(srcdir)/t_cve-2013-1416.py $(PYTESTFLAGS)
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$(RUNPYTEST) $(srcdir)/t_cve-2013-1417.py $(PYTESTFLAGS)
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+ $(RUNPYTEST) $(srcdir)/t_cve-2021-36222.py $(PYTESTFLAGS)
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$(RM) au.log
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$(RUNPYTEST) $(srcdir)/t_audit.py $(PYTESTFLAGS)
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$(RUNPYTEST) $(srcdir)/jsonwalker.py -d $(srcdir)/au_dict.json \
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diff --git a/src/tests/t_cve-2021-36222.py b/src/tests/t_cve-2021-36222.py
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new file mode 100644
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index 000000000..57e04993b
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--- /dev/null
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+++ b/src/tests/t_cve-2021-36222.py
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@@ -0,0 +1,46 @@
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+import socket
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+from k5test import *
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+
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+realm = K5Realm()
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+
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+# CVE-2021-36222 KDC null dereference on encrypted challenge preauth
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+# without FAST
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+
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+s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
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+a = (hostname, realm.portbase)
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+
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+m = ('6A81A0' '30819D' # [APPLICATION 10] SEQUENCE
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+ 'A103' '0201' '05' # [1] pvno = 5
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+ 'A203' '0201' '0A' # [2] msg-type = 10
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+ 'A30E' '300C' # [3] padata = SEQUENCE OF
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+ '300A' # SEQUENCE
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+ 'A104' '0202' '008A' # [1] padata-type = PA-ENCRYPTED-CHALLENGE
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+ 'A202' '0400' # [2] padata-value = ""
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+ 'A48180' '307E' # [4] req-body = SEQUENCE
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+ 'A007' '0305' '0000000000' # [0] kdc-options = 0
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+ 'A120' '301E' # [1] cname = SEQUENCE
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+ 'A003' '0201' '01' # [0] name-type = NT-PRINCIPAL
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+ 'A117' '3015' # [1] name-string = SEQUENCE-OF
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+ '1B06' '6B7262746774' # krbtgt
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+ '1B0B' '4B5242544553542E434F4D'
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+ # KRBTEST.COM
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+ 'A20D' '1B0B' '4B5242544553542E434F4D'
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+ # [2] realm = KRBTEST.COM
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+ 'A320' '301E' # [3] sname = SEQUENCE
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+ 'A003' '0201' '01' # [0] name-type = NT-PRINCIPAL
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+ 'A117' '3015' # [1] name-string = SEQUENCE-OF
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+ '1B06' '6B7262746774' # krbtgt
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+ '1B0B' '4B5242544553542E434F4D'
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+ # KRBTEST.COM
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+ 'A511' '180F' '31393934303631303036303331375A'
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+ # [5] till = 19940610060317Z
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+ 'A703' '0201' '00' # [7] nonce = 0
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+ 'A808' '3006' # [8] etype = SEQUENCE OF
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+ '020112' '020111') # aes256-cts aes128-cts
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+
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+s.sendto(bytes.fromhex(m), a)
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+
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+# Make sure kinit still works.
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+realm.kinit(realm.user_princ, password('user'))
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+
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+success('CVE-2021-36222 regression test')
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