krb5 commit [krb5-1.12]: Prevent requires_preauth bypass [CVE-2015-2694]

[Tom Yu]

https://github.com/krb5/krb5/commit/b0c571e709c72da799ccc15fb5755f7910170e33
commit b0c571e709c72da799ccc15fb5755f7910170e33
Author: Greg Hudson <ghudson at mit.edu>
Date:   Tue Mar 24 12:02:37 2015 -0400

    Prevent requires_preauth bypass [CVE-2015-2694]
    
    In the OTP kdcpreauth module, don't set the TKT_FLG_PRE_AUTH bit until
    the request is successfully verified.  In the PKINIT kdcpreauth
    module, don't respond with code 0 on empty input or an unconfigured
    realm.  Together these bugs could cause the KDC preauth framework to
    erroneously treat a request as pre-authenticated.
    
    CVE-2015-2694:
    
    In MIT krb5 1.12 and later, when the KDC is configured with PKINIT
    support, an unauthenticated remote attacker can bypass the
    requires_preauth flag on a client principal and obtain a ciphertext
    encrypted in the principal's long-term key.  This ciphertext could be
    used to conduct an off-line dictionary attack against the user's
    password.
    
        CVSSv2 Vector: AV:N/AC:M/Au:N/C:P/I:P/A:N/E:POC/RL:OF/RC:C
    
    (cherry picked from commit e3b5a5e5267818c97750b266df50b6a3d4649604)
    (cherry picked from commit df8afc60d970a7176a55ffe7ce21cfd57ba423cd)
    
    ticket: 8194 (new)
    version_fixed: 1.12.4
    status: resolved

 src/plugins/preauth/otp/main.c          |   10 +++++++---
 src/plugins/preauth/pkinit/pkinit_srv.c |    4 ++--
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/src/plugins/preauth/otp/main.c b/src/plugins/preauth/otp/main.c
index bf9c6a8..7941b4a 100644
--- a/src/plugins/preauth/otp/main.c
+++ b/src/plugins/preauth/otp/main.c
@@ -42,6 +42,7 @@ static krb5_preauthtype otp_pa_type_list[] =
 struct request_state {
     krb5_kdcpreauth_verify_respond_fn respond;
     void *arg;
+    krb5_enc_tkt_part *enc_tkt_reply;
 };
 
 static krb5_error_code
@@ -159,6 +160,9 @@ on_response(void *data, krb5_error_code retval, otp_response response)
     if (retval == 0 && response != otp_response_success)
         retval = KRB5_PREAUTH_FAILED;
 
+    if (retval == 0)
+        rs.enc_tkt_reply->flags |= TKT_FLG_PRE_AUTH;
+
     rs.respond(rs.arg, retval, NULL, NULL, NULL);
 }
 
@@ -263,8 +267,6 @@ otp_verify(krb5_context context, krb5_data *req_pkt, krb5_kdc_req *request,
     krb5_data d, plaintext;
     char *config;
 
-    enc_tkt_reply->flags |= TKT_FLG_PRE_AUTH;
-
     /* Get the FAST armor key. */
     armor_key = cb->fast_armor(context, rock);
     if (armor_key == NULL) {
@@ -298,12 +300,14 @@ otp_verify(krb5_context context, krb5_data *req_pkt, krb5_kdc_req *request,
         goto error;
     }
 
-    /* Create the request state. */
+    /* Create the request state.  Save the response callback, and the
+     * enc_tkt_reply pointer so we can set the TKT_FLG_PRE_AUTH flag later. */
     rs = k5alloc(sizeof(struct request_state), &retval);
     if (rs == NULL)
         goto error;
     rs->arg = arg;
     rs->respond = respond;
+    rs->enc_tkt_reply = enc_tkt_reply;
 
     /* Get the principal's OTP configuration string. */
     retval = cb->get_string(context, rock, "otp", &config);
diff --git a/src/plugins/preauth/pkinit/pkinit_srv.c b/src/plugins/preauth/pkinit/pkinit_srv.c
index 1179216..279415a 100644
--- a/src/plugins/preauth/pkinit/pkinit_srv.c
+++ b/src/plugins/preauth/pkinit/pkinit_srv.c
@@ -306,7 +306,7 @@ pkinit_server_verify_padata(krb5_context context,
 
     pkiDebug("pkinit_verify_padata: entered!\n");
     if (data == NULL || data->length <= 0 || data->contents == NULL) {
-        (*respond)(arg, 0, NULL, NULL, NULL);
+        (*respond)(arg, EINVAL, NULL, NULL, NULL);
         return;
     }
 
@@ -318,7 +318,7 @@ pkinit_server_verify_padata(krb5_context context,
 
     plgctx = pkinit_find_realm_context(context, moddata, request->server);
     if (plgctx == NULL) {
-        (*respond)(arg, 0, NULL, NULL, NULL);
+        (*respond)(arg, EINVAL, NULL, NULL, NULL);
         return;
     }