We did a review of this code path. In my opinion it is a pure memory

leak. The flow is krb5_gss_inquire_cred() -> kg_get_defcred() ->
krb5_gss_acquire_cred() with NULL handle, for GSS_C_INITIATE and no
specific parameters -> accept_cred_context(). This means it would only
use defaults from existing ccache and may be a default client keytab if
the default ccache was not initialized yet. As a result, it is the same
information an application already posseses access to.

Upstream krb5 1.18.4 has a minimal variant of the same fix:
----------------------------------------------------------------------------
commit b92be484630b38e26f5ee4bd67973fbd7627009c
Author: Greg Hudson <ghudson@mit.edu>
Date:   Wed Jul 21 13:44:30 2021 -0400   
{{
     Fix defcred leak in krb5 gss_inquire_cred()

     Commit 1cd2821c19b2b95e39d5fc2f451a035585a40fa5 altered the memory}}
     management of krb5_gss_inquire_cred(), introducing defcred to act as
     an owner pointer when the function must acquire a default credential.
     The commit neglected to update the code to release the default cred
     along the successful path.  The old code does not trigger because
     cred_handle is now reassigned, so the default credential is leaked.

     Reported by Pavel Březina.

     (a minimal alternative to commit 593e16448e1af23eef74689afe06a7bcc86e79c7)

     ticket: 9016
     version_fixed: 1.18.4

diff --git a/src/lib/gssapi/krb5/inq_cred.c b/src/lib/gssapi/krb5/inq_cred.c
index a8f254110..cd8384d08 100644
--- a/src/lib/gssapi/krb5/inq_cred.c
+++ b/src/lib/gssapi/krb5/inq_cred.c
@@ -197,9 +197,7 @@ krb5_gss_inquire_cred(minor_status, cred_handle, name, lifetime_ret,
          mechs = GSS_C_NO_OID_SET;

{

}}

-    if (cred_handle == GSS_C_NO_CREDENTIAL)
-        krb5_gss_release_cred(minor_status, (gss_cred_id_t *)&cred);
-
+    krb5_gss_release_cred(minor_status, &defcred);
      krb5_free_context(context);
      *minor_status = 0;
      return((lifetime == 0)?GSS_S_CREDENTIALS_EXPIRED:GSS_S_COMPLETE);
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