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From 0a8dfc380fe3b210662ba1b1d452fcec2f84841b Mon Sep 17 00:00:00 2001
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From: Greg Hudson <ghudson@mit.edu>
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Date: Tue, 3 Aug 2021 01:15:27 -0400
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Subject: [PATCH] Fix KDC null deref on TGS inner body null server
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After the KDC decodes a FAST inner body, it does not check for a null
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server. Prior to commit 39548a5b17bbda9eeb63625a201cfd19b9de1c5b this
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would typically result in an error from krb5_unparse_name(), but with
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the addition of get_local_tgt() it results in a null dereference. Add
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a null check.
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Reported by Joseph Sutton of Catalyst.
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CVE-2021-37750:
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In MIT krb5 releases 1.14 and later, an authenticated attacker can
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cause a null dereference in the KDC by sending a FAST TGS request with
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no server field.
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ticket: 9008 (new)
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tags: pullup
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target_version: 1.19-next
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target_version: 1.18-next
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RHEL 7.9 backport of commit: d775c95af7606a51bf79547a94fa52ddd1cb7f49
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---
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src/kdc/do_tgs_req.c | 5 +++++
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1 file changed, 5 insertions(+)
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diff --git a/src/kdc/do_tgs_req.c b/src/kdc/do_tgs_req.c
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index 572a2c363..a9a3beb9d 100644
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--- a/src/kdc/do_tgs_req.c
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+++ b/src/kdc/do_tgs_req.c
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@@ -204,6 +204,11 @@ process_tgs_req(krb5_kdc_req *request, krb5_data *pkt,
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status = "FIND_FAST";
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goto cleanup;
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}
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+ if (sprinc == NULL) {
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+ status = "NULL_SERVER";
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+ errcode = KRB5KDC_ERR_S_PRINCIPAL_UNKNOWN;
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+ goto cleanup;
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+ }
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errcode = get_local_tgt(kdc_context, &sprinc->realm, header_server,
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&local_tgt, &local_tgt_storage);
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