The _nx_secure_x509_asn1_tlv_block_parse() function parses ASN.1 TLV (tag-length-value) blocks out of DER-encoded data. It is the primitive underneath all X.509 certificate parsing in NetX Secure, and therefore runs on certificates supplied by a remote peer during the TLS handshake.
The function reads the one-byte ASN.1 tag from the caller's buffer *before* checking that the buffer holds at least one byte. When a caller passes a remaining length of zero, the guard correctly returns NX_SECURE_X509_ASN1_LENGTH_TOO_LONG, but the read has already happened one byte past the end of the buffer.
code:
nx_secure/src/nx_secure_x509_asn1_tlv_block_parse.c
UINT _nx_secure_x509_asn1_tlv_block_parse(const UCHAR *buffer, ULONG *buffer_length, USHORT *tlv_type, USHORT *tlv_tag_class, ULONG *tlv_length,
const UCHAR **tlv_data, ULONG *header_length)
{
UINT current_index;
USHORT current_tag;
ULONG length;
ULONG length_bytes;
current_index = 0;
current_tag = buffer[current_index]; /* <-- read before the bounds check */
if (*buffer_length < 1)
{
return(NX_SECURE_X509_ASN1_LENGTH_TOO_LONG);
}
The remainder of the function is correctly ordered. The multi-byte length path is guarded by length_bytes > 4 || length_bytes > *buffer_length before its read loop, the decoded value is checked against length > *buffer_length, and the second single-byte length read follows its own *buffer_length < 1 guard. The tag read is the only load placed ahead of its check.