From 7aea9fc73fb508e3168581990eb2e2ff7a1aea31 Mon Sep 17 00:00:00 2001

From: Isaac Boukris <iboukris@gmail.com>

Date: Mon, 9 Mar 2020 16:04:21 +0100

Subject: [PATCH] Implement KERB_AP_OPTIONS_CBT (server side)

Add server support for Microsoft's KERB_AP_OPTIONS_CBT as described in

MS-KILE.  If the client includes the AP option in the authenticator

authdata and the server passed channel bindings, require the bindings

to match.

[ghudson@mit.edu: refactored to put more logic in the helper function;

added a comment; clarified commit message]

ticket: 8900 (new)

(cherry picked from commit 4f7c77b64a048ca5e3199b26b31493698c777a9c)

(cherry picked from commit 6407bf087fe53088d91efd09df736e979cd4e8db)

---

 src/include/krb5/krb5.hin                |  1 +

 src/lib/gssapi/krb5/accept_sec_context.c | 45 +++++++++++++++++++++++-

 2 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/src/include/krb5/krb5.hin b/src/include/krb5/krb5.hin

index f8269fb17..9264bede1 100644

--- a/src/include/krb5/krb5.hin

+++ b/src/include/krb5/krb5.hin

@@ -1915,6 +1915,7 @@ krb5_verify_checksum(krb5_context context, krb5_cksumtype ctype,

 #define KRB5_AUTHDATA_SIGNTICKET        512     /**< formerly 142 in krb5 1.8 */

 #define KRB5_AUTHDATA_FX_ARMOR 71

 #define KRB5_AUTHDATA_AUTH_INDICATOR 97

+#define KRB5_AUTHDATA_AP_OPTIONS 143

 /** @} */ /* end of KRB5_AUTHDATA group */

 /* password change constants */

diff --git a/src/lib/gssapi/krb5/accept_sec_context.c b/src/lib/gssapi/krb5/accept_sec_context.c

index 9d3e2f4fe..175a24c4e 100644

--- a/src/lib/gssapi/krb5/accept_sec_context.c

+++ b/src/lib/gssapi/krb5/accept_sec_context.c

@@ -430,6 +430,32 @@ kg_process_extension(krb5_context context,

 /* A zero-value channel binding, for comparison */

 static const uint8_t null_cb[CB_MD5_LEN];

+/* Look for AP_OPTIONS in authdata.  If present and the options include

+ * KERB_AP_OPTIONS_CBT, set *cbt_out to true. */

+static krb5_error_code

+check_cbt(krb5_context context, krb5_authdata **authdata,

+          krb5_boolean *cbt_out)

+{

+    krb5_error_code code;

+    uint32_t ad_ap_options;

+    const uint32_t KERB_AP_OPTIONS_CBT = 0x4000;

+

+    *cbt_out = FALSE;

+

+    code = krb5_find_authdata(context, NULL, authdata,

+                              KRB5_AUTHDATA_AP_OPTIONS, &authdata);

+    if (code || authdata == NULL)

+        return code;

+    if (authdata[1] != NULL || authdata[0]->length != 4)

+        return KRB5KRB_AP_ERR_MSG_TYPE;

+

+    ad_ap_options = load_32_le(authdata[0]->contents);

+    if (ad_ap_options & KERB_AP_OPTIONS_CBT)

+        *cbt_out = TRUE;

+

+    return 0;

+}

+

 /*

  * The krb5 GSS mech appropriates the authenticator checksum field from RFC

  * 4120 to store structured data instead of a checksum, indicated with checksum

@@ -454,7 +480,7 @@ process_checksum(OM_uint32 *minor_status, krb5_context context,

     krb5_error_code code = 0;

     OM_uint32 status, option_id, token_flags;

     size_t cb_len, option_len;

-    krb5_boolean valid, token_cb_present = FALSE, cb_match = FALSE;

+    krb5_boolean valid, client_cbt, token_cb_present = FALSE, cb_match = FALSE;

     krb5_key subkey;

     krb5_data option, empty = empty_data();

     krb5_checksum cb_cksum;

@@ -582,6 +608,23 @@ process_checksum(OM_uint32 *minor_status, krb5_context context,

         }

     }

+    /*

+     * If the client asserts the KERB_AP_OPTIONS_CBT flag (from MS-KILE) in the

+     * authenticator authdata, and the acceptor passed channel bindings,

+     * require matching channel bindings from the client.  The intent is to

+     * prevent an authenticator generated for use outside of a TLS channel from

+     * being used inside of one.

+     */

+    code = check_cbt(context, authenticator->authorization_data, &client_cbt);

+    if (code) {

+        status = GSS_S_FAILURE;

+        goto fail;

+    }

+    if (client_cbt && acceptor_cb != GSS_C_NO_CHANNEL_BINDINGS && !cb_match) {

+        status = GSS_S_BAD_BINDINGS;

+        goto fail;

+    }

+

     status = GSS_S_COMPLETE;

 fail: