diff --git a/ChangeLog b/ChangeLog
index 672a158..2e8f16c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -23,6 +23,8 @@
      overflow. #1179
    * Fix memory allocation corner cases in memory_buffer_alloc.c module. Found
      by Guido Vranken. #639
+   * Fix X509 CRT parsing that would potentially accept an invalid tag when
+     parsing the subject alternative names.
 
 Changes
    * Clarify the documentation of mbedtls_ssl_setup.
diff --git a/include/mbedtls/asn1.h b/include/mbedtls/asn1.h
index fde328a..96c1c9a 100644
--- a/include/mbedtls/asn1.h
+++ b/include/mbedtls/asn1.h
@@ -88,6 +88,21 @@
 #define MBEDTLS_ASN1_PRIMITIVE               0x00
 #define MBEDTLS_ASN1_CONSTRUCTED             0x20
 #define MBEDTLS_ASN1_CONTEXT_SPECIFIC        0x80
+
+/*
+ * Bit masks for each of the components of an ASN.1 tag as specified in
+ * ITU X.690 (08/2015), section 8.1 "General rules for encoding",
+ * paragraph 8.1.2.2:
+ *
+ * Bit  8     7   6   5          1
+ *     +-------+-----+------------+
+ *     | Class | P/C | Tag number |
+ *     +-------+-----+------------+
+ */
+#define MBEDTLS_ASN1_TAG_CLASS_MASK          0xC0
+#define MBEDTLS_ASN1_TAG_PC_MASK             0x20
+#define MBEDTLS_ASN1_TAG_VALUE_MASK          0x1F
+
 /* \} name */
 /* \} addtogroup asn1_module */
 
diff --git a/library/x509_crt.c b/library/x509_crt.c
index 0e28dac..daa3163 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -473,9 +473,12 @@
         if( ( ret = mbedtls_asn1_get_len( p, end, &tag_len ) ) != 0 )
             return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
 
-        if( ( tag & MBEDTLS_ASN1_CONTEXT_SPECIFIC ) != MBEDTLS_ASN1_CONTEXT_SPECIFIC )
+        if( ( tag & MBEDTLS_ASN1_TAG_CLASS_MASK ) !=
+                MBEDTLS_ASN1_CONTEXT_SPECIFIC )
+        {
             return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS +
                     MBEDTLS_ERR_ASN1_UNEXPECTED_TAG );
+        }
 
         /* Skip everything but DNS name */
         if( tag != ( MBEDTLS_ASN1_CONTEXT_SPECIFIC | 2 ) )
diff --git a/tests/suites/test_suite_x509parse.data b/tests/suites/test_suite_x509parse.data
index 73ccead..8e17163 100644
--- a/tests/suites/test_suite_x509parse.data
+++ b/tests/suites/test_suite_x509parse.data
@@ -1144,6 +1144,10 @@
 X509 Certificate ASN1 (invalid version overflow)
 x509parse_crt:"301A3018a00602047FFFFFFF8204deadbeef30080604cafed00d0500":"":MBEDTLS_ERR_X509_UNKNOWN_VERSION
 
+X509 Certificate ASN1 (invalid SubjectAltNames tag)
+depends_on:MBEDTLS_RSA_C:MBEDTLS_SHA1_C
+x509parse_crt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
+
 X509 CRL ASN1 (Incorrect first tag)
 x509parse_crl:"":"":MBEDTLS_ERR_X509_INVALID_FORMAT