diff -up ./lib/pk11wrap/pk11pars.c.add_ems_policy ./lib/pk11wrap/pk11pars.c
--- ./lib/pk11wrap/pk11pars.c.add_ems_policy	2023-06-12 15:37:49.292905411 -0700
+++ ./lib/pk11wrap/pk11pars.c	2023-06-12 17:18:35.129938514 -0700
@@ -389,6 +389,8 @@ static const oidValDef kxOptList[] = {
     { CIPHER_NAME("ECDHE-RSA"), SEC_OID_TLS_ECDHE_RSA, NSS_USE_ALG_IN_SSL_KX },
     { CIPHER_NAME("ECDH-ECDSA"), SEC_OID_TLS_ECDH_ECDSA, NSS_USE_ALG_IN_SSL_KX },
     { CIPHER_NAME("ECDH-RSA"), SEC_OID_TLS_ECDH_RSA, NSS_USE_ALG_IN_SSL_KX },
+    /* not really a key exchange, but it's the closest fit */
+    { CIPHER_NAME("TLS-REQUIRE-EMS"), SEC_OID_TLS_REQUIRE_EMS, NSS_USE_ALG_IN_SSL_KX },
 };
 
 static const oidValDef signOptList[] = {
diff -up ./lib/pk11wrap/secmodti.h.add_ems_policy ./lib/pk11wrap/secmodti.h
--- ./lib/pk11wrap/secmodti.h.add_ems_policy	2023-06-04 01:42:53.000000000 -0700
+++ ./lib/pk11wrap/secmodti.h	2023-06-12 17:18:35.129938514 -0700
@@ -202,4 +202,10 @@ struct PK11GenericObjectStr {
 /* This mask includes all CK_FLAGs with an equivalent CKA_ attribute. */
 #define CKF_KEY_OPERATION_FLAGS 0x000e7b00UL
 
+/* this oid value could change values if it's added after other new
+ * upstream oids. We protect applications by hiding the define in a private
+ * header file that only NSS sees. Currently it's only available through
+ * the policy code */
+#define SEC_OID_TLS_REQUIRE_EMS SEC_OID_PRIVATE_1
+
 #endif /* _SECMODTI_H_ */
diff -up ./lib/ssl/ssl3con.c.add_ems_policy ./lib/ssl/ssl3con.c
--- ./lib/ssl/ssl3con.c.add_ems_policy	2023-06-04 01:42:53.000000000 -0700
+++ ./lib/ssl/ssl3con.c	2023-06-12 17:18:35.130938525 -0700
@@ -36,6 +36,7 @@
 #include "pk11func.h"
 #include "secmod.h"
 #include "blapi.h"
+#include "secmodti.h" /* until SEC_OID_TLS_REQUIRE_EMS is upstream */
 
 #include <stdio.h>
 
@@ -3480,6 +3481,29 @@ ssl3_ComputeMasterSecretInt(sslSocket *s
     CK_TLS12_MASTER_KEY_DERIVE_PARAMS master_params;
     unsigned int master_params_len;
 
+    /* if we are using TLS and we aren't using the extended master secret,
+     * and SEC_OID_TLS_REQUIRE_EMS policy is true, fail. The caller will
+     * send and alert (eventually). In the RSA Server case, the alert
+     * won't happen until Finish time because the upper level code
+     * can't tell a difference between this failure and an RSA decrypt
+     * failure, so it will proceed with a faux key */
+    if (isTLS) {
+        PRUint32 policy;
+        SECStatus rv;
+
+        /* first fetch the policy for this algorithm */
+        rv = NSS_GetAlgorithmPolicy(SEC_OID_TLS_REQUIRE_EMS, &policy);
+        /* we only look at the policy if we can fetch it. */
+        if (rv == SECSuccess) {
+            if (policy & NSS_USE_ALG_IN_SSL_KX) {
+                /* just set the error, we don't want to map any errors
+                 * set by NSS_GetAlgorithmPolicy here */
+                PORT_SetError(SSL_ERROR_UNSUPPORTED_VERSION);
+                return SECFailure;
+            }
+        }
+    }
+
     if (isTLS12) {
         if (isDH)
             master_derive = CKM_TLS12_MASTER_KEY_DERIVE_DH;
diff -up ./lib/util/secoid.c.add_ems_policy ./lib/util/secoid.c
--- ./lib/util/secoid.c.add_ems_policy	2023-06-12 15:37:49.293905422 -0700
+++ ./lib/util/secoid.c	2023-06-12 17:20:29.498142775 -0700
@@ -1795,6 +1795,11 @@ const static SECOidData oids[SEC_OID_TOT
        SEC_OID_EXT_KEY_USAGE_IPSEC_USER,
        "IPsec User",
        CKM_INVALID_MECHANISM, INVALID_CERT_EXTENSION),
+
+    /* this will change upstream. for now apps shouldn't use it */
+    /* we need it for the policy code.  */
+    ODE(SEC_OID_PRIVATE_1,
+        "TLS Require EMS", CKM_INVALID_MECHANISM, INVALID_CERT_EXTENSION),
 };
 
 /* PRIVATE EXTENDED SECOID Table
@@ -2095,6 +2100,8 @@ SECOID_Init(void)
 
     /* turn off NSS_USE_POLICY_IN_SSL by default */
     xOids[SEC_OID_APPLY_SSL_POLICY].notPolicyFlags = NSS_USE_POLICY_IN_SSL;
+    /* turn off TLS REQUIRE EMS by default */
+    xOids[SEC_OID_PRIVATE_1].notPolicyFlags = ~0;
 
     envVal = PR_GetEnvSecure("NSS_HASH_ALG_SUPPORT");
     if (envVal)
diff -up ./lib/util/secoidt.h.add_ems_policy ./lib/util/secoidt.h
--- ./lib/util/secoidt.h.add_ems_policy	2023-06-12 17:18:35.131938535 -0700
+++ ./lib/util/secoidt.h	2023-06-12 17:21:49.675987022 -0700
@@ -501,6 +501,9 @@ typedef enum {
     SEC_OID_EXT_KEY_USAGE_IPSEC_END = 361,
     SEC_OID_EXT_KEY_USAGE_IPSEC_TUNNEL = 362,
     SEC_OID_EXT_KEY_USAGE_IPSEC_USER = 363,
+    /* this will change upstream. for now apps shouldn't use it */
+    /* give it an obscure name here */
+    SEC_OID_PRIVATE_1 = 372,
 
     SEC_OID_TOTAL
 } SECOidTag;