Bug#991374: unblock: krb5/1.18.3-6
Package: release.debian.org
Severity: normal
User: release.debian.org@packages.debian.org
Usertags: unblock
X-Debbugs-Cc: kaduk@mit.edu
Please unblock package krb5
Upstream krb5 recently fixed a CVE in the KDC (server) process
whereby an unauthenticated request can trigger a NULL dereference.
The krb5 maintainer notes that systemd will restart the KDC if this
happens, but of course an attacker can send a flood of such crafted
packets and effectively DoS the process.
[ Reason ]
The NULL dereference is a service crash triggerable by a remote
unauthenticated attacker, and we should not have this DoS vulnerability in the
stable release.
[ Impact ]
An unpatched KDC is vulnerable to denial of service by an unauthenticated
remote attacker.
[ Tests ]
Upstream included a unit test to verify that a malformed packet does not crash
the KDC. Upstream also has an extensive test suite that serves as a
regression test.
[ Risks ]
The risk here is pretty minimal; the issue stems from a function that was
written in an "if-ladder" style so that any operation that might write to
"retval" has to be wrapped in a conditional so that the operation is skipped
if an error had occurred previously. The fix is to add such a "retval == 0"
check to a function call that takes a pointer to retval as a parameter and
writes to it, causing any previous errors in the function to be ignored.
(Upstream has since re-written the function to use a more robust coding style,
but that is not a minimal fix suitable for unblock.)
[ Checklist ]
[x] all changes are documented in the d/changelog
[x] I reviewed all changes and I approve them
[x] attach debdiff against the package in testing
unblock krb5/1.18.3-6
diff -Nru krb5-1.18.3/debian/changelog krb5-1.18.3/debian/changelog
--- krb5-1.18.3/debian/changelog 2021-03-28 10:43:01.000000000 -0700
+++ krb5-1.18.3/debian/changelog 2021-07-21 11:07:07.000000000 -0700
@@ -1,3 +1,10 @@
+krb5 (1.18.3-6) unstable; urgency=high
+
+ * Pull in upstream patch to fix CVE-2021-36222 (KDC NULL dereference),
+ Closes: #991365
+
+ -- Benjamin Kaduk <kaduk@mit.edu> Wed, 21 Jul 2021 11:07:07 -0700
+
krb5 (1.18.3-5) unstable; urgency=medium
* Update breaks on libk5crypto3 toward other internal libraries because
diff -Nru krb5-1.18.3/debian/patches/0010-Fix-KDC-null-deref-on-bad-encrypted-challenge.patch krb5-1.18.3/debian/patches/0010-Fix-KDC-null-deref-on-bad-encrypted-challenge.patch
--- krb5-1.18.3/debian/patches/0010-Fix-KDC-null-deref-on-bad-encrypted-challenge.patch 1969-12-31 16:00:00.000000000 -0800
+++ krb5-1.18.3/debian/patches/0010-Fix-KDC-null-deref-on-bad-encrypted-challenge.patch 2021-07-21 11:06:53.000000000 -0700
@@ -0,0 +1,112 @@
+From: Joseph Sutton <josephsutton@catalyst.net.nz>
+Date: Wed, 7 Jul 2021 11:47:44 +1200
+Subject: 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]
+
+(cherry picked from commit fc98f520caefff2e5ee9a0026fdf5109944b3562)
+
+ticket: 9007
+version_fixed: 1.18.4
+
+(cherry picked from commit c4a406095b3ea4a67ae5b8ea586cbe9abdbae76f)
+---
+ src/kdc/kdc_preauth_ec.c | 3 ++-
+ src/tests/Makefile.in | 1 +
+ src/tests/t_cve-2021-36222.py | 46 +++++++++++++++++++++++++++++++++++++++++++
+ 3 files changed, 49 insertions(+), 1 deletion(-)
+ create mode 100644 src/tests/t_cve-2021-36222.py
+
+diff --git a/src/kdc/kdc_preauth_ec.c b/src/kdc/kdc_preauth_ec.c
+index 7e636b3..43a9902 100644
+--- a/src/kdc/kdc_preauth_ec.c
++++ b/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,
+diff --git a/src/tests/Makefile.in b/src/tests/Makefile.in
+index 3f88f17..0ffbebf 100644
+--- a/src/tests/Makefile.in
++++ b/src/tests/Makefile.in
+@@ -158,6 +158,7 @@ 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 \
+diff --git a/src/tests/t_cve-2021-36222.py b/src/tests/t_cve-2021-36222.py
+new file mode 100644
+index 0000000..57e0499
+--- /dev/null
++++ b/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')
diff -Nru krb5-1.18.3/debian/patches/series krb5-1.18.3/debian/patches/series
--- krb5-1.18.3/debian/patches/series 2021-03-22 14:02:44.000000000 -0700
+++ krb5-1.18.3/debian/patches/series 2021-07-21 11:06:53.000000000 -0700
@@ -7,3 +7,4 @@
debian-local/0007-Fix-pkg-config-library-include-paths.patch
debian-local/0008-Use-isystem-for-include-paths.patch
0009-Add-.gitignore.patch
+0010-Fix-KDC-null-deref-on-bad-encrypted-challenge.patch
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