diff -pu a/nss/lib/ssl/ssl3con.c b/nss/lib/ssl/ssl3con.c
--- a/nss/lib/ssl/ssl3con.c	2014-01-17 18:46:51.999581198 -0800
+++ b/nss/lib/ssl/ssl3con.c	2014-01-17 18:47:05.509804656 -0800
@@ -3473,6 +3473,9 @@ ssl3_HandleAlert(sslSocket *ss, sslBuffe
     case certificate_unknown: 	error = SSL_ERROR_CERTIFICATE_UNKNOWN_ALERT;
 			        					  break;
     case illegal_parameter: 	error = SSL_ERROR_ILLEGAL_PARAMETER_ALERT;break;
+    case inappropriate_fallback:
+        error = SSL_ERROR_INAPPROPRIATE_FALLBACK_ALERT;
+        break;
 
     /* All alerts below are TLS only. */
     case unknown_ca: 		error = SSL_ERROR_UNKNOWN_CA_ALERT;       break;
@@ -4986,6 +4989,7 @@ ssl3_SendClientHello(sslSocket *ss, PRBo
     int              num_suites;
     int              actual_count = 0;
     PRBool           isTLS = PR_FALSE;
+    PRBool           requestingResume = PR_FALSE, fallbackSCSV = PR_FALSE;
     PRInt32          total_exten_len = 0;
     unsigned         paddingExtensionLen;
     unsigned         numCompressionMethods;
@@ -5128,6 +5132,7 @@ ssl3_SendClientHello(sslSocket *ss, PRBo
     }
 
     if (sid) {
+	requestingResume = PR_TRUE;
 	SSL_AtomicIncrementLong(& ssl3stats.sch_sid_cache_hits );
 
 	PRINT_BUF(4, (ss, "client, found session-id:", sid->u.ssl3.sessionID,
@@ -5246,8 +5251,15 @@ ssl3_SendClientHello(sslSocket *ss, PRBo
     	if (sid->u.ssl3.lock) { PR_RWLock_Unlock(sid->u.ssl3.lock); }
     	return SECFailure;	/* count_cipher_suites has set error code. */
     }
+
+    fallbackSCSV = ss->opt.enableFallbackSCSV && (!requestingResume ||
+						  ss->version < sid->version);
+    /* make room for SCSV */
     if (ss->ssl3.hs.sendingSCSV) {
-	++num_suites;   /* make room for SCSV */
+	++num_suites;
+    }
+    if (fallbackSCSV) {
+	++num_suites;
     }
 
     /* count compression methods */
@@ -5389,11 +5389,21 @@ ssl3_SendClientHello(sslSocket *ss, PRBo
	    if (rv != SECSuccess) {
		if (sid->u.ssl3.lock) { NSSRWLock_UnlockRead(sid->u.ssl3.lock); }
		return rv;	/* err set by ssl3_AppendHandshake* */
	    }
	}
    }

+    if (fallbackSCSV) {
+	rv = ssl3_AppendHandshakeNumber(ss, TLS_FALLBACK_SCSV,
+					sizeof(ssl3CipherSuite));
+	if (rv != SECSuccess) {
+	    if (sid->u.ssl3.lock) { PR_RWLock_Unlock(sid->u.ssl3.lock); }
+	    return rv;	/* err set by ssl3_AppendHandshake* */
+	}
+	actual_count++;
+    }
+
    /* if cards were removed or inserted between count_cipher_suites and
     * generating our list, detect the error here rather than send it off to
     * the server.. */
    if (actual_count != num_suites) {
@@ -8084,6 +8105,19 @@ ssl3_HandleClientHello(sslSocket *ss, SS
 	goto loser;		/* malformed */
     }
 
+    /* If the ClientHello version is less than our maximum version, check for a
+     * TLS_FALLBACK_SCSV and reject the connection if found. */
+    if (ss->vrange.max > ss->clientHelloVersion) {
+	for (i = 0; i + 1 < suites.len; i += 2) {
+	    PRUint16 suite_i = (suites.data[i] << 8) | suites.data[i + 1];
+	    if (suite_i != TLS_FALLBACK_SCSV)
+		continue;
+	    desc = inappropriate_fallback;
+	    errCode = SSL_ERROR_INAPPROPRIATE_FALLBACK_ALERT;
+	    goto alert_loser;
+	}
+    }
+
     /* grab the list of compression methods. */
     rv = ssl3_ConsumeHandshakeVariable(ss, &comps, 1, &b, &length);
     if (rv != SECSuccess) {
diff -pu a/nss/lib/ssl/ssl3prot.h b/nss/lib/ssl/ssl3prot.h
--- a/nss/lib/ssl/ssl3prot.h	2014-01-17 17:59:03.242109996 -0800
+++ b/nss/lib/ssl/ssl3prot.h	2014-01-17 18:47:05.509804656 -0800
@@ -98,6 +98,7 @@ typedef enum {
     protocol_version        = 70,
     insufficient_security   = 71,
     internal_error          = 80,
+    inappropriate_fallback  = 86,	/* could also be sent for SSLv3 */
     user_canceled           = 90,
     no_renegotiation        = 100,
 
diff -pu a/nss/lib/ssl/sslerr.h b/nss/lib/ssl/sslerr.h
--- a/nss/lib/ssl/sslerr.h	2014-01-17 17:59:03.242109996 -0800
+++ b/nss/lib/ssl/sslerr.h	2014-01-17 18:47:05.509804656 -0800
@@ -196,6 +196,7 @@ SSL_ERROR_INCORRECT_SIGNATURE_ALGORITHM
 SSL_ERROR_BAD_CHANNEL_ID_DATA = (SSL_ERROR_BASE + 129),
 SSL_ERROR_INVALID_CHANNEL_ID_KEY = (SSL_ERROR_BASE + 130),
 SSL_ERROR_GET_CHANNEL_ID_FAILED = (SSL_ERROR_BASE + 131),
+SSL_ERROR_INAPPROPRIATE_FALLBACK_ALERT = (SSL_ERROR_BASE + 132),
 
 SSL_ERROR_END_OF_LIST	/* let the c compiler determine the value of this. */
 } SSLErrorCodes;
diff -pu a/nss/lib/ssl/SSLerrs.h b/nss/lib/ssl/SSLerrs.h
--- a/nss/lib/ssl/SSLerrs.h	2014-01-17 17:59:03.242109996 -0800
+++ b/nss/lib/ssl/SSLerrs.h	2014-01-17 18:47:05.509804656 -0800
@@ -421,3 +421,8 @@ ER3(SSL_ERROR_INVALID_CHANNEL_ID_KEY, (S
 
 ER3(SSL_ERROR_GET_CHANNEL_ID_FAILED, (SSL_ERROR_BASE + 131),
 "The application could not get a TLS Channel ID.")
+
+ER3(SSL_ERROR_INAPPROPRIATE_FALLBACK_ALERT, (SSL_ERROR_BASE + 132),
+"The connection was using a lesser TLS version as a result of a previous"
+" handshake failure, but the server indicated that it should not have been"
+" needed.")
diff -pu a/nss/lib/ssl/ssl.h b/nss/lib/ssl/ssl.h
--- a/nss/lib/ssl/ssl.h	2014-01-17 18:46:51.999581198 -0800
+++ b/nss/lib/ssl/ssl.h	2014-01-17 18:48:54.971613341 -0800
@@ -183,6 +183,8 @@ SSL_IMPORT PRFileDesc *DTLS_ImportFD(PRF
 
 /* Request Signed Certificate Timestamps via TLS extension (client) */
 #define SSL_ENABLE_SIGNED_CERT_TIMESTAMPS 27
+#define SSL_ENABLE_FALLBACK_SCSV       28 /* Send fallback SCSV in
+                                           * handshakes. */
 
 #ifdef SSL_DEPRECATED_FUNCTION 
 /* Old deprecated function names */
diff -pu a/nss/lib/ssl/sslimpl.h b/nss/lib/ssl/sslimpl.h
--- a/nss/lib/ssl/sslimpl.h	2014-01-17 18:46:51.999581198 -0800
+++ b/nss/lib/ssl/sslimpl.h	2014-01-17 18:51:17.963962287 -0800
@@ -338,6 +338,7 @@ typedef struct sslOptionsStr {
     unsigned int enableNPN              : 1;  /* 26 */
     unsigned int enableALPN             : 1;  /* 27 */
     unsigned int enableSignedCertTimestamps : 1;  /* 28 */
+    unsigned int enableFallbackSCSV     : 1;  /* 29 */
 } sslOptions;
 
 typedef enum { sslHandshakingUndetermined = 0,
diff -pu a/nss/lib/ssl/sslproto.h b/nss/lib/ssl/sslproto.h
--- a/nss/lib/ssl/sslproto.h	2014-01-17 18:10:16.793281867 -0800
+++ b/nss/lib/ssl/sslproto.h	2014-01-17 18:47:05.509804656 -0800
@@ -172,6 +172,11 @@
  */
 #define TLS_EMPTY_RENEGOTIATION_INFO_SCSV	0x00FF
 
+/* TLS_FALLBACK_SCSV is a signaling cipher suite value that indicates that a
+ * handshake is the result of TLS version fallback. This value is not IANA
+ * assigned. */
+#define TLS_FALLBACK_SCSV			0x5600
+
 /* Cipher Suite Values starting with 0xC000 are defined in informational
  * RFCs.
  */
diff -pu a/nss/lib/ssl/sslsock.c b/nss/lib/ssl/sslsock.c
--- a/nss/lib/ssl/sslsock.c	2014-01-17 18:46:52.009581364 -0800
+++ b/nss/lib/ssl/sslsock.c	2014-01-17 18:59:17.931852364 -0800
@@ -88,7 +88,8 @@ static sslOptions ssl_defaults = {
     PR_FALSE,   /* enableOCSPStapling */
     PR_TRUE,    /* enableNPN          */
     PR_FALSE,   /* enableALPN         */
-    PR_FALSE    /* enableSignedCertTimestamps */
+    PR_FALSE,   /* enableSignedCertTimestamps */
+    PR_FALSE    /* enableFallbackSCSV */
 };
 
 /*
@@ -792,6 +793,10 @@ SSL_OptionSet(PRFileDesc *fd, PRInt32 wh
 	ss->opt.enableSignedCertTimestamps = on;
 	break;
 
+      case SSL_ENABLE_FALLBACK_SCSV:
+       ss->opt.enableFallbackSCSV = on;
+       break;
+
       default:
 	PORT_SetError(SEC_ERROR_INVALID_ARGS);
 	rv = SECFailure;
@@ -867,6 +872,7 @@ SSL_OptionGet(PRFileDesc *fd, PRInt32 wh
     case SSL_ENABLE_SIGNED_CERT_TIMESTAMPS:
 	on = ss->opt.enableSignedCertTimestamps;
 	break;
+    case SSL_ENABLE_FALLBACK_SCSV: on = ss->opt.enableFallbackSCSV; break;
 
     default:
 	PORT_SetError(SEC_ERROR_INVALID_ARGS);
@@ -933,6 +939,9 @@ SSL_OptionGetDefault(PRInt32 which, PRBo
     case SSL_ENABLE_SIGNED_CERT_TIMESTAMPS:
 	on = ssl_defaults.enableSignedCertTimestamps;
 	break;
+    case SSL_ENABLE_FALLBACK_SCSV:
+	on = ssl_defaults.enableFallbackSCSV;
+	break;
 
     default:
 	PORT_SetError(SEC_ERROR_INVALID_ARGS);
@@ -1112,6 +1121,10 @@ SSL_OptionSetDefault(PRInt32 which, PRBo
 	ssl_defaults.enableSignedCertTimestamps = on;
 	break;
 
+      case SSL_ENABLE_FALLBACK_SCSV:
+       ssl_defaults.enableFallbackSCSV = on;
+       break;
+
       default:
 	PORT_SetError(SEC_ERROR_INVALID_ARGS);
 	return SECFailure;