CVE-2021-36222 (Python, published 2021-07-22T18:15Z)
Description: value: ec_verify in kdc/kdc_preauth_ec.c in the Key Distribution Center (KDC) in MIT Kerberos 5 (aka krb5) before 1.18.4 and 1.19.x before 1.19.2 allows remote attackers to cause a NULL pointer dereference and daemon crash. This occurs because a return value is not properly managed in a certain situation.
Severity: MEDIUM (CVSS v3 base score: 7.5)
Weakness: CWE-476 \u2014 NULL Pointer Dereference
The product dereferences a pointer that it expects to be valid but is NULL.
Repository: https://github.com/krb5/krb5
Files changed:
src/kdc/kdc_preauth_ec.c
src/tests/Makefile.in
src/tests/t_cve-2021-36222.py
Commit message: 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


Vulnerable code:
    realmstr = k5memdup0(realm.data, realm.length, &retval);

Fixed code:
    if (retval == 0)
        realmstr = k5memdup0(realm.data, realm.length, &retval);
	$(RUNPYTEST) $(srcdir)/t_cve-2021-36222.py $(PYTESTFLAGS)
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')