diff --git a/drivers/auth/auth_mod.c b/drivers/auth/auth_mod.c
index fa9509a..1bf03d4 100644
--- a/drivers/auth/auth_mod.c
+++ b/drivers/auth/auth_mod.c
@@ -243,7 +243,7 @@
 		      unsigned int *cert_nv_ctr,
 		      bool *need_nv_ctr_upgrade)
 {
-	char *p;
+	unsigned char *p;
 	void *data_ptr = NULL;
 	unsigned int data_len, len, i;
 	unsigned int plat_nv_ctr;
@@ -258,16 +258,24 @@
 
 	/* Parse the DER encoded integer */
 	assert(data_ptr);
-	p = (char *)data_ptr;
-	if (*p != ASN1_INTEGER) {
+	p = (unsigned char *)data_ptr;
+
+	/*
+	 * Integers must be at least 3 bytes: 1 for tag, 1 for length, and 1
+	 * for value.  The first byte (tag) must be ASN1_INTEGER.
+	 */
+	if ((data_len < 3) || (*p != ASN1_INTEGER)) {
 		/* Invalid ASN.1 integer */
 		return 1;
 	}
 	p++;
 
-	/* NV-counters are unsigned integers up to 32-bit */
-	len = (unsigned int)(*p & 0x7f);
-	if ((*p & 0x80) || (len > 4)) {
+	/*
+	 * NV-counters are unsigned integers up to 31 bits.  Trailing
+	 * padding is not allowed.
+	 */
+	len = (unsigned int)*p;
+	if ((len > 4) || (data_len - 2 != len)) {
 		return 1;
 	}
 	p++;