CVE-2026-102758 in NetX Duo
Summary
by MITRE • 09/29/2026
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.
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Analysis
by VulDB Data Team • 09/29/2026
The vulnerability resides in the _nx_secure_x509_asn1_tlv_block_parse function within NetX Secure, a critical component responsible for parsing ASN.1 TLV structures from DER-encoded data during X.509 certificate validation. This function serves as the foundational primitive for all TLS handshake certificate processing, meaning it is invoked whenever a remote peer presents a digital certificate. The core technical flaw involves an out-of-bounds read caused by incorrect instruction ordering within the parsing logic. Specifically, the code attempts to retrieve the one-byte ASN.1 tag from the input buffer at index zero before verifying that the provided buffer length is greater than or equal to one byte. While subsequent checks correctly validate remaining lengths and multi-byte fields, this initial load operation occurs unconditionally prior to any bounds verification.
When a malicious actor supplies a DER-encoded certificate with an empty TLV block or triggers a state where the remaining buffer length is zero, the function proceeds to execute the tag read instruction immediately upon entry. Because no guard exists before this specific memory access, the processor reads one byte past the end of the allocated buffer. This constitutes a classic out-of-bounds read vulnerability, classified under CWE-125: Out-of-bounds Read in standard security taxonomies. The consequence is that arbitrary data from adjacent memory regions may be loaded into the current_tag variable. Although the function subsequently detects the length violation and returns an error code NX_SECURE_X509_ASN1_LENGTH_TOO_LONG, the side effect of reading unauthorized memory has already occurred during execution.
The operational impact of this vulnerability depends heavily on the surrounding context and compiler optimizations. In many scenarios, such as when using standard optimization levels or specific architectures, the read value might be discarded if it is not used before the error return. However, modern compilers may optimize code in ways that preserve the side effects of memory accesses even if the result appears unused. Furthermore, reading out-of-bounds can trigger hardware exceptions on systems with strict memory protection, leading to a denial of service through application crash or segmentation fault. In more complex exploitation scenarios involving speculative execution vulnerabilities like Spectre variants, this unguarded read could potentially leak sensitive information from kernel space or other protected memory regions into user-space registers, although the primary risk remains the potential for instability and crashes during high-volume TLS negotiations.
From a threat modeling perspective using MITRE ATT&CK techniques, this flaw aligns with T1059: Command and Scripting Interpreter if it leads to code execution via subsequent vulnerabilities, but more accurately reflects information disclosure or denial of service vectors depending on the outcome. The lack of input validation before memory access is a fundamental design error that violates secure coding principles regarding defensive programming. To mitigate this issue, developers must enforce strict bounds checking prior to any pointer dereferencing operations. The fix involves moving the check for *buffer_length < 1 to precede the assignment current_tag = buffer[current_index]. Additionally implementing static analysis tools configured to detect out-of-bounds accesses and enforcing compiler flags that enable stack canaries or address sanitization during testing phases will help identify similar patterns in other parts of the codebase. Regular security audits focusing on ASN.1 parsing libraries are essential given their frequent exposure to untrusted network inputs.