--- sys/src/libsec/port/tlshand.c
+++ sys/src/libsec/port/tlshand.c
@@ -1412,6 +1412,10 @@ msgSend(TlsConnection *c, Msg *m, int act)
 	if(sendp == nil)
 		sendp = sendbuf;
 	p = sendp;
+	if(p + 4 - sendbuf > sizeof(sendbuf)) {
+		tlsError(c, EInternalError, "output buffer too small for handshake header");
+		goto Err;
+	}
 	if(c->trace)
 		c->trace("send %s", msgPrint((char*)p, (sizeof sendbuf) - (p-sendbuf), m));
 
@@ -1423,6 +1427,15 @@ msgSend(TlsConnection *c, Msg *m, int act)
 		tlsError(c, EInternalError, "can't encode a %d", m->tag);
 		goto Err;
 	case HClientHello:
+		nn = 2 + RandomSize + 1 + m->u.clientHello.sid->len;
+		nn += 2 + 2*m->u.clientHello.ciphers->len;
+		nn += 1 + m->u.clientHello.compressors->len;
+		if(m->u.clientHello.extensions != nil)
+			nn += 2 + m->u.clientHello.extensions->len;
+		if(p + nn - sendbuf > sizeof(sendbuf)) {
+			tlsError(c, EInternalError, "output buffer too small for client hello");
+			goto Err;
+		}
 		// version
 		put16(p, m->u.clientHello.version);
 		p += 2;
@@ -1463,6 +1476,13 @@ msgSend(TlsConnection *c, Msg *m, int act)
 		}
 		break;
 	case HServerHello:
+		nn = 2 + RandomSize + 1 + m->u.serverHello.sid->len + 2 + 1;
+		if(m->u.serverHello.extensions != nil)
+			nn += 2 + m->u.serverHello.extensions->len;
+		if(p + nn - sendbuf > sizeof(sendbuf)) {
+			tlsError(c, EInternalError, "output buffer too small for server hello");
+			goto Err;
+		}
 		put16(p, m->u.serverHello.version);
 		p += 2;
 
@@ -1562,6 +1582,10 @@ msgSend(TlsConnection *c, Msg *m, int act)
 		p += n;
 		break;
 	case HFinished:
+		if(p + m->u.finished.n - sendbuf > sizeof(sendbuf)) {
+			tlsError(c, EInternalError, "output buffer too small for finished");
+			goto Err;
+		}
 		memmove(p, m->u.finished.verify, m->u.finished.n);
 		p += m->u.finished.n;
 		break;
@@ -1598,6 +1622,12 @@ tlsReadN(TlsConnection *c, int n)
 	uchar *p;
 	int nn, nr;
 
+	/* sizeof is signed, so test n < 0 separately */
+	if(n < 0 || n > sizeof(c->buf)) {
+		tlsError(c, EDecodeError, "handshake message too long %d %d", n, sizeof(c->buf));
+		return nil;
+	}
+
 	nn = c->ep - c->rp;
 	if(nn < n){
 		if(c->rp != c->buf){
@@ -1636,11 +1666,6 @@ msgRecv(TlsConnection *c, Msg *m)
 			tlsError(c, EDecodeError, "invalid hello request during handshake");
 			return 0;
 		}
-	}
-
-	if(n > sizeof(c->buf)) {
-		tlsError(c, EDecodeError, "handshake message too long %d %d", n, sizeof(c->buf));
-		return 0;
 	}
 
 	if(type == HSSL2ClientHello){