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 primitiv

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

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 4UINT _nx_secure_x509_asn1_tlv_block_parse(const UCHAR *buffer, ULONG *buffer_length, USHORT *tlv_type,
 5 
 6                                          USHORT *tlv_tag_class, ULONG *tlv_length,
 7                                          const UCHAR **tlv_data, ULONG *header_length)
 8 
 9 
10{
11 
12 
13 
14UINT   current_index;
15 
16 
17 
18USHORT current_tag;
19 
20 
21 
22ULONG  length;
23 
24 
25 
26ULONG  length_bytes;
27 
28    current_index = 0;
29    current_tag = buffer[current_index];      /* <-- read before the bounds check */
30    if (*buffer_length < 1)
31    {
32        return(NX_SECURE_X509_ASN1_LENGTH_TOO_LONG);
33    }
34 

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.