From 4589352dac3ae111c7621298cf231742209efd9b Mon Sep 17 00:00:00 2001
From: Bill Fenner <fenner@gmail.com>
Date: Fri, 25 Nov 2022 08:41:24 -0800
Subject: [PATCH 1/3] snmp_agent: disallow SET with NULL varbind

---
 agent/snmp_agent.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/agent/snmp_agent.c b/agent/snmp_agent.c
index 867d0c166f..3f678fe2df 100644
--- a/agent/snmp_agent.c
+++ b/agent/snmp_agent.c
@@ -3719,12 +3719,44 @@ netsnmp_handle_request(netsnmp_agent_session *asp, int status)
     return 1;
 }
 
+static int
+check_set_pdu_for_null_varbind(netsnmp_agent_session *asp)
+{
+    int i;
+    netsnmp_variable_list *v = NULL;
+
+    for (i = 1, v = asp->pdu->variables; v != NULL; i++, v = v->next_variable) {
+	if (v->type == ASN_NULL) {
+	    /*
+	     * Protect SET implementations that do not protect themselves
+	     * against wrong type.
+	     */
+	    DEBUGMSGTL(("snmp_agent", "disallowing SET with NULL var for varbind %d\n", i));
+	    asp->index = i;
+	    return SNMP_ERR_WRONGTYPE;
+	}
+    }
+    return SNMP_ERR_NOERROR;
+}
+
 int
 handle_pdu(netsnmp_agent_session *asp)
 {
     int             status, inclusives = 0;
     netsnmp_variable_list *v = NULL;
 
+#ifndef NETSNMP_NO_WRITE_SUPPORT
+    /*
+     * Check for ASN_NULL in SET request
+     */
+    if (asp->pdu->command == SNMP_MSG_SET) {
+	status = check_set_pdu_for_null_varbind(asp);
+	if (status != SNMP_ERR_NOERROR) {
+	    return status;
+	}
+    }
+#endif /* NETSNMP_NO_WRITE_SUPPORT */
+
     /*
      * for illegal requests, mark all nodes as ASN_NULL 
      */

From 7f4ac4051cc7fec6a5944661923acb95cec359c7 Mon Sep 17 00:00:00 2001
From: Bill Fenner <fenner@gmail.com>
Date: Fri, 25 Nov 2022 08:41:46 -0800
Subject: [PATCH 2/3] apps: snmpset: allow SET with NULL varbind for testing

---
 apps/snmpset.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/apps/snmpset.c b/apps/snmpset.c
index 48e14bd513..d542713e1b 100644
--- a/apps/snmpset.c
+++ b/apps/snmpset.c
@@ -182,6 +182,7 @@ main(int argc, char *argv[])
             case 'x':
             case 'd':
             case 'b':
+            case 'n': /* undocumented */
 #ifdef NETSNMP_WITH_OPAQUE_SPECIAL_TYPES
             case 'I':
             case 'U':

From 15f9d7f7e5b90c9b419832ed8e6413feb6570d83 Mon Sep 17 00:00:00 2001
From: Bill Fenner <fenner@gmail.com>
Date: Fri, 25 Nov 2022 10:23:32 -0800
Subject: [PATCH 3/3] Add test for NULL varbind set

---
 .../default/T0142snmpv2csetnull_simple        | 31 +++++++++++++++++++
 1 file changed, 31 insertions(+)
 create mode 100644 testing/fulltests/default/T0142snmpv2csetnull_simple

diff --git a/testing/fulltests/default/T0142snmpv2csetnull_simple b/testing/fulltests/default/T0142snmpv2csetnull_simple
new file mode 100644
index 0000000000..0f1b8f386b
--- /dev/null
+++ b/testing/fulltests/default/T0142snmpv2csetnull_simple
@@ -0,0 +1,31 @@
+#!/bin/sh
+
+. ../support/simple_eval_tools.sh
+
+HEADER SNMPv2c set of system.sysContact.0 with NULL varbind
+
+SKIPIF NETSNMP_DISABLE_SET_SUPPORT
+SKIPIF NETSNMP_NO_WRITE_SUPPORT
+SKIPIF NETSNMP_DISABLE_SNMPV2C
+SKIPIFNOT USING_MIBII_SYSTEM_MIB_MODULE
+
+#
+# Begin test
+#
+
+# standard V2C configuration: testcomunnity
+snmp_write_access='all'
+. ./Sv2cconfig
+STARTAGENT
+
+CAPTURE "snmpget -On $SNMP_FLAGS -c testcommunity -v 2c $SNMP_TRANSPORT_SPEC:$SNMP_TEST_DEST$SNMP_SNMPD_PORT .1.3.6.1.2.1.1.4.0"
+
+CHECK ".1.3.6.1.2.1.1.4.0 = STRING:"
+
+CAPTURE "snmpset -On $SNMP_FLAGS -c testcommunity -v 2c $SNMP_TRANSPORT_SPEC:$SNMP_TEST_DEST$SNMP_SNMPD_PORT .1.3.6.1.2.1.1.4.0 n x"
+
+CHECK "Reason: wrongType"
+
+STOPAGENT
+
+FINISHED
