diff --git a/certs/blacklist.c b/certs/blacklist.c
index 1cdf309a5b08..b254c87ceb3a 100644
--- a/certs/blacklist.c
+++ b/certs/blacklist.c
@@ -19,6 +19,16 @@
 #include "blacklist.h"
 #include "common.h"
 
+/*
+ * According to crypto/asymmetric_keys/x509_cert_parser.c:x509_note_pkey_algo(),
+ * the size of the currently longest supported hash algorithm is 512 bits,
+ * which translates into 128 hex characters.
+ */
+#define MAX_HASH_LEN	128
+
+static const char tbs_prefix[] = "tbs";
+static const char bin_prefix[] = "bin";
+
 static struct key *blacklist_keyring;
 
 #ifdef CONFIG_SYSTEM_REVOCATION_LIST
@@ -32,24 +42,40 @@ extern __initconst const unsigned long revocation_certificate_list_size;
  */
 static int blacklist_vet_description(const char *desc)
 {
-	int n = 0;
+	int i, prefix_len, tbs_step = 0, bin_step = 0;
 
-	if (*desc == ':')
-		return -EINVAL;
-	for (; *desc; desc++)
-		if (*desc == ':')
-			goto found_colon;
+	/* The following algorithm only works if prefix lengths match. */
+	BUILD_BUG_ON(sizeof(tbs_prefix) != sizeof(bin_prefix));
+	prefix_len = sizeof(tbs_prefix) - 1;
+	for (i = 0; *desc; desc++, i++) {
+		if (*desc == ':') {
+			if (tbs_step == prefix_len)
+				goto found_colon;
+			if (bin_step == prefix_len)
+				goto found_colon;
+			return -EINVAL;
+		}
+		if (i >= prefix_len)
+			return -EINVAL;
+		if (*desc == tbs_prefix[i])
+			tbs_step++;
+		if (*desc == bin_prefix[i])
+			bin_step++;
+	}
 	return -EINVAL;
 
 found_colon:
 	desc++;
-	for (; *desc; desc++) {
+	for (i = 0; *desc && i < MAX_HASH_LEN; desc++, i++) {
 		if (!isxdigit(*desc) || isupper(*desc))
 			return -EINVAL;
-		n++;
 	}
+	if (*desc)
+		/* The hash is greater than MAX_HASH_LEN. */
+		return -ENOPKG;
 
-	if (n == 0 || n & 1)
+	/* Checks for an even number of hexadecimal characters. */
+	if (i == 0 || i & 1)
 		return -EINVAL;
 	return 0;
 }