diff --git a/Modules/cjkcodecs/multibytecodec.c b/Modules/cjkcodecs/multibytecodec.c
index bbd4c1a..a1b0ca9 100644
--- a/Modules/cjkcodecs/multibytecodec.c
+++ b/Modules/cjkcodecs/multibytecodec.c
@@ -163,13 +163,17 @@
 static int
 expand_encodebuffer(MultibyteEncodeBuffer *buf, Py_ssize_t esize)
 {
-	Py_ssize_t orgpos, orgsize;
+	Py_ssize_t orgpos, orgsize, incsize;
 
 	orgpos = (Py_ssize_t)((char *)buf->outbuf -
 				PyString_AS_STRING(buf->outobj));
 	orgsize = PyString_GET_SIZE(buf->outobj);
-	if (_PyString_Resize(&buf->outobj, orgsize + (
-	    esize < (orgsize >> 1) ? (orgsize >> 1) | 1 : esize)) == -1)
+	incsize = (esize < (orgsize >> 1) ? (orgsize >> 1) | 1 : esize);
+
+	if (orgsize > PY_SSIZE_T_MAX - incsize)
+		return -1;
+
+	if (_PyString_Resize(&buf->outobj, orgsize + incsize) == -1)
 		return -1;
 
 	buf->outbuf = (unsigned char *)PyString_AS_STRING(buf->outobj) +orgpos;
@@ -473,6 +477,12 @@
 	buf.excobj = NULL;
 	buf.inbuf = buf.inbuf_top = *data;
 	buf.inbuf_end = buf.inbuf_top + datalen;
+
+	if (datalen > (PY_SSIZE_T_MAX - 16) / 2) {
+		PyErr_NoMemory();
+		goto errorexit;
+	}
+
 	buf.outobj = PyString_FromStringAndSize(NULL, datalen * 2 + 16);
 	if (buf.outobj == NULL)
 		goto errorexit;
@@ -735,6 +745,11 @@
 	origpending = ctx->pendingsize;
 
 	if (origpending > 0) {
+		if (datalen > PY_SSIZE_T_MAX - ctx->pendingsize) {
+			PyErr_NoMemory();
+			/* inbuf_tmp == NULL */
+			goto errorexit;
+		}
 		inbuf_tmp = PyMem_New(Py_UNICODE, datalen + ctx->pendingsize);
 		if (inbuf_tmp == NULL)
 			goto errorexit;
@@ -797,9 +812,10 @@
 	Py_ssize_t npendings;
 
 	npendings = (Py_ssize_t)(buf->inbuf_end - buf->inbuf);
-	if (npendings + ctx->pendingsize > MAXDECPENDING) {
-		PyErr_SetString(PyExc_UnicodeError, "pending buffer overflow");
-		return -1;
+	if (npendings + ctx->pendingsize > MAXDECPENDING ||
+		npendings > PY_SSIZE_T_MAX - ctx->pendingsize) {
+			PyErr_SetString(PyExc_UnicodeError, "pending buffer overflow");
+			return -1;
 	}
 	memcpy(ctx->pending + ctx->pendingsize, buf->inbuf, npendings);
 	ctx->pendingsize += npendings;
@@ -1001,7 +1017,7 @@
 		  PyObject *args, PyObject *kwargs)
 {
 	MultibyteDecodeBuffer buf;
-	char *data, *wdata;
+	char *data, *wdata = NULL;
 	Py_ssize_t wsize, finalsize = 0, size, origpending;
 	int final = 0;
 
@@ -1017,6 +1033,10 @@
 		wdata = data;
 	}
 	else {
+		if (size > PY_SSIZE_T_MAX - self->pendingsize) {
+			PyErr_NoMemory();
+			goto errorexit;
+		}
 		wsize = size + self->pendingsize;
 		wdata = PyMem_Malloc(wsize);
 		if (wdata == NULL)
@@ -1235,6 +1255,10 @@
 			PyObject *ctr;
 			char *ctrdata;
 
+			if (PyString_GET_SIZE(cres) > PY_SSIZE_T_MAX - self->pendingsize) {
+				PyErr_NoMemory();
+				goto errorexit;
+            }
 			rsize = PyString_GET_SIZE(cres) + self->pendingsize;
 			ctr = PyString_FromStringAndSize(NULL, rsize);
 			if (ctr == NULL)