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krb5: make keyed checksums mandatory where possible

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Make keyed checksums mandatory when generating and verifying checksums, with
the following exceptions:

* the checksum is being generated or verified as part of encrypting data for
  a legacy (DES) encryption type

* the KRB5_CRYPTO_FLAG_ALLOW_UNKEYED_CHECKSUM flag was set on the crypto
  context, used to allow unkeyed checksums in krb5 authenticators

By making unkeyed checksums opt-in, we eliminate a class of potential
vulnerabilities where callers could pass unkeyed checksums.

Any code that uses the mandatory checksum type for a given non-legacy
encryption type should not be affected by this change. It could potentially
break, say, a client trying to do FAST with DES keys but, that should not be
supported (because FAST KDCs also support AES).

Closes: #835
This commit is contained in:
Luke Howard
2021-09-17 11:03:35 +10:00
parent 7fbe7be675
commit 85756bd228
7 changed files with 97 additions and 59 deletions
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67
lib/krb5/mk_req_ext.c
View File

@@ -73,48 +73,37 @@ _krb5_mk_req_internal(krb5_context context,
if (ret)
goto out;
/* it's unclear what type of checksum we can use. try the best one, except:
* a) if it's configured differently for the current realm, or
* b) if the session key is des-cbc-crc
/*
* Use the default checksum type except for some interoperability cases
* with older MIT, DCE and Windows KDCs.
*/
if (in_data) {
if(ac->keyblock->keytype == ETYPE_DES_CBC_CRC) {
/* this is to make DCE secd (and older MIT kdcs?) happy */
ret = krb5_create_checksum(context,
NULL,
0,
CKSUMTYPE_RSA_MD4,
in_data->data,
in_data->length,
&c);
} else if(ac->keyblock->keytype == ETYPE_ARCFOUR_HMAC_MD5 ||
ac->keyblock->keytype == ETYPE_ARCFOUR_HMAC_MD5_56 ||
ac->keyblock->keytype == ETYPE_DES_CBC_MD4 ||
ac->keyblock->keytype == ETYPE_DES_CBC_MD5) {
/* this is to make MS kdc happy */
ret = krb5_create_checksum(context,
NULL,
0,
CKSUMTYPE_RSA_MD5,
in_data->data,
in_data->length,
&c);
} else {
krb5_crypto crypto;
krb5_crypto crypto;
krb5_cksumtype checksum_type = CKSUMTYPE_NONE;
ret = krb5_crypto_init(context, ac->keyblock, 0, &crypto);
if (ret)
goto out;
ret = krb5_create_checksum(context,
crypto,
checksum_usage,
0,
in_data->data,
in_data->length,
&c);
krb5_crypto_destroy(context, crypto);
}
if (ac->keyblock->keytype == ETYPE_DES_CBC_CRC)
checksum_type = CKSUMTYPE_RSA_MD4;
else if (ac->keyblock->keytype == ETYPE_DES_CBC_MD4 ||
ac->keyblock->keytype == ETYPE_DES_CBC_MD5 ||
ac->keyblock->keytype == ETYPE_ARCFOUR_HMAC_MD5 ||
ac->keyblock->keytype == ETYPE_ARCFOUR_HMAC_MD5_56)
checksum_type = CKSUMTYPE_RSA_MD5;
else
checksum_type = CKSUMTYPE_NONE;
ret = krb5_crypto_init(context, ac->keyblock, 0, &crypto);
if (ret)
goto out;
_krb5_crypto_set_flags(context, crypto, KRB5_CRYPTO_FLAG_ALLOW_UNKEYED_CHECKSUM);
ret = krb5_create_checksum(context,
crypto,
checksum_usage,
checksum_type,
in_data->data,
in_data->length,
&c);
krb5_crypto_destroy(context, crypto);
c_opt = &c;
} else {
c_opt = NULL;