diff --git a/mssql_python/auth.py b/mssql_python/auth.py
index c7e6683a..b6275e44 100644
--- a/mssql_python/auth.py
+++ b/mssql_python/auth.py
@@ -9,6 +9,67 @@
 from typing import Tuple, Dict, Optional, Union
 from mssql_python.constants import AuthType
 
+def validate_access_token_struct(token_struct: bytes) -> None:
+    """
+    Validate ACCESSTOKEN structure to prevent ODBC driver crashes.
+    
+    The ODBC driver crashes (segfault on macOS/Linux, access violation on Windows)
+    when given malformed access tokens. This function validates the structure
+    before passing to the driver.
+    
+    ACCESSTOKEN structure: typedef struct { DWORD dataSize; BYTE data[]; } ACCESSTOKEN;
+    
+    Args:
+        token_struct (bytes): The ACCESSTOKEN structure to validate
+        
+    Raises:
+        ValueError: If the token structure is invalid
+    """
+    # Check minimum size (4-byte header + data)
+    if len(token_struct) < 4:
+        raise ValueError(
+            f"Invalid access token: minimum 4 bytes required for ACCESSTOKEN structure, got {len(token_struct)} bytes"
+        )
+    
+    # Extract declared size from first 4 bytes
+    declared_size = struct.unpack('<I', token_struct[:4])[0]
+    
+    # Validate structure integrity
+    total_size = len(token_struct)
+    expected_size = declared_size + 4
+    if expected_size != total_size:
+        raise ValueError(
+            f"Invalid access token: size mismatch in ACCESSTOKEN structure. "
+            f"Header declares {declared_size} bytes, but structure has {total_size - 4} bytes of data"
+        )
+    
+    # Validate token data is not empty/all zeros
+    token_data = token_struct[4:]
+    if not any(token_data):
+        raise ValueError("Invalid access token: token data is empty or all zeros")
+    
+    # Validate UTF-16LE encoding (ODBC driver requirement)
+    # JWT tokens in UTF-16LE have null bytes interleaved with ASCII characters
+    if declared_size % 2 != 0:
+        raise ValueError(
+            f"Invalid access token: must be UTF-16LE encoded (got odd byte length {declared_size})"
+        )
+    
+    # Check for UTF-16LE pattern: ASCII characters with interleaved null bytes
+    # Real JWTs start with "eyJ" in UTF-16LE: 65 00 79 00 4A 00
+    if declared_size >= 6:
+        has_utf16_pattern = all([
+            0x20 <= token_data[0] <= 0x7E and token_data[1] == 0,  # First char
+            0x20 <= token_data[2] <= 0x7E and token_data[3] == 0,  # Second char
+            0x20 <= token_data[4] <= 0x7E and token_data[5] == 0   # Third char
+        ])
+        
+        if not has_utf16_pattern:
+            raise ValueError(
+                "Invalid access token: must be UTF-16LE encoded JWT. "
+                "Expected alternating ASCII and null bytes (e.g., 'e\\x00y\\x00J\\x00' for 'eyJ')"
+            )
+
 class AADAuth:
     """Handles Azure Active Directory authentication"""
     
diff --git a/mssql_python/connection.py b/mssql_python/connection.py
index 832d2aac..198f0036 100644
--- a/mssql_python/connection.py
+++ b/mssql_python/connection.py
@@ -148,6 +148,16 @@ def __init__(self, connection_str: str = "", autocommit: bool = False, attrs_bef
             connection_str, **kwargs
         )
         self._attrs_before = attrs_before or {}
+        
+        # Validate access token if provided directly via attrs_before
+        if ConstantsDDBC.SQL_COPT_SS_ACCESS_TOKEN.value in self._attrs_before:
+            from mssql_python.auth import validate_access_token_struct
+            token_struct = self._attrs_before[ConstantsDDBC.SQL_COPT_SS_ACCESS_TOKEN.value]
+            if isinstance(token_struct, (bytes, bytearray)):
+                try:
+                    validate_access_token_struct(bytes(token_struct))
+                except ValueError as e:
+                    raise ValueError(f"Invalid access token in attrs_before: {e}") from e
 
         # Initialize encoding settings with defaults for Python 3
         # Python 3 only has str (which is Unicode), so we use utf-16le by default
diff --git a/mssql_python/constants.py b/mssql_python/constants.py
index 05df3e14..b64b0ee4 100644
--- a/mssql_python/constants.py
+++ b/mssql_python/constants.py
@@ -136,6 +136,9 @@ class ConstantsDDBC(Enum):
     SQL_QUICK = 0
     SQL_ENSURE = 1
 
+    # Connection Attributes
+    SQL_COPT_SS_ACCESS_TOKEN = 1256
+
 class GetInfoConstants(Enum):
     """
     These constants are used with various methods like getinfo().
diff --git a/tests/test_008_auth.py b/tests/test_008_auth.py
index 6bf6c410..633906d0 100644
--- a/tests/test_008_auth.py
+++ b/tests/test_008_auth.py
@@ -219,4 +219,179 @@ def test_error_handling():
 
     # Test non-string input
     with pytest.raises(ValueError, match="Connection string must be a string"):
-        process_connection_string(None)
\ No newline at end of file
+        process_connection_string(None)
+
+
+def test_short_access_token_protection_blocks_short_tokens():
+    """
+    Test protection against ODBC driver crashes with malformed access tokens.
+    
+    Microsoft ODBC Driver 18 has a bug where it crashes (segfault on macOS/Linux,
+    access violation on Windows) when given malformed access tokens. This test
+    verifies that our defensive validation properly rejects invalid tokens before
+    they reach the ODBC driver.
+    
+    The validation is implemented in Connection::setAttribute() in connection.cpp
+    and checks:
+    1. Minimum size (4 bytes for ACCESSTOKEN header)
+    2. Structure integrity (declared size matches actual size)
+    3. Non-empty data (not all zeros)
+    
+    This test runs in a subprocess to isolate potential crashes.
+    """
+    import os
+    import subprocess
+    
+    # Get connection string and remove UID/Pwd to force token-only mode
+    conn_str = os.getenv("DB_CONNECTION_STRING")
+    if not conn_str:
+        pytest.skip("DB_CONNECTION_STRING environment variable not set")
+    
+    # Remove authentication to force pure token mode
+    conn_str_no_auth = conn_str
+    for remove_param in ["UID=", "Pwd=", "uid=", "pwd="]:
+        if remove_param in conn_str_no_auth:
+            parts = conn_str_no_auth.split(";")
+            parts = [p for p in parts if not p.lower().startswith(remove_param.lower())]
+            conn_str_no_auth = ";".join(parts)
+    
+    # Escape connection string for embedding in subprocess code
+    escaped_conn_str = conn_str_no_auth.replace('\\', '\\\\').replace('"', '\\"')
+    
+    # Test cases for problematic tokens
+    test_cases = [
+        (b"", "empty token"),
+        (b"x" * 3, "too small (< 4 bytes)"),
+        (b"\x00\x00\x00\x00", "header only, no data"),
+        (b"\x10\x00\x00\x00" + b"\x00" * 16, "size mismatch (declares 16, total 20)"),
+        (b"\x10\x00\x00\x00" + b"\x00" * 12, "size mismatch (declares 16, has 12)"),
+        (b"\x08\x00\x00\x00" + b"\x00" * 8, "all zeros data"),
+    ]
+    
+    for token, description in test_cases:
+        # Convert bytes to hex string for safe embedding in subprocess code
+        token_hex = token.hex()
+        
+        code = f"""
+import sys
+from mssql_python import connect
+
+conn_str = "{escaped_conn_str}"
+fake_token = bytes.fromhex("{token_hex}")
+attrs_before = {{1256: fake_token}}  # SQL_COPT_SS_ACCESS_TOKEN = 1256
+
+try:
+    connect(conn_str, attrs_before=attrs_before)
+    print("ERROR: Should have raised exception for {description}")
+    sys.exit(1)
+except Exception as e:
+    error_msg = str(e)
+    # Check for our validation error messages
+    if "Invalid access token" in error_msg:
+        print(f"PASS: Got expected validation error for {description}")
+        sys.exit(0)
+    else:
+        print(f"ERROR: Got unexpected error for {description}: {{error_msg}}")
+        sys.exit(1)
+"""
+        
+        result = subprocess.run(
+            [sys.executable, "-c", code],
+            capture_output=True,
+            text=True
+        )
+        
+        # Should not crash (exit code 139 on Linux, 134 on macOS, -11 on some systems)
+        assert result.returncode not in [134, 139, -11], \
+            f"Crash detected for {description}! STDERR: {result.stderr}"
+        
+        # Should exit cleanly with our validation error
+        assert result.returncode == 0, \
+            f"Expected validation error for {description}. Exit code: {result.returncode}\nSTDOUT: {result.stdout}\nSTDERR: {result.stderr}"
+        
+        assert "PASS" in result.stdout, \
+            f"Expected PASS message for {description}, got: {result.stdout}"
+
+
+def test_short_access_token_protection_allows_valid_tokens():
+    """
+    Test that properly formatted access tokens are NOT blocked by validation.
+    
+    This verifies that our defensive validation only blocks malformed tokens,
+    and allows properly structured tokens to proceed (even though they may fail
+    authentication if the token is invalid, which is expected behavior).
+    
+    Runs in separate subprocess to avoid ODBC driver state pollution from earlier tests.
+    """
+    import os
+    import subprocess
+    import struct
+    
+    # Get connection string and remove UID/Pwd to force token-only mode
+    conn_str = os.getenv("DB_CONNECTION_STRING")
+    if not conn_str:
+        pytest.skip("DB_CONNECTION_STRING environment variable not set")
+    
+    # Remove authentication to force pure token mode
+    conn_str_no_auth = conn_str
+    for remove_param in ["UID=", "Pwd=", "uid=", "pwd="]:
+        if remove_param in conn_str_no_auth:
+            parts = conn_str_no_auth.split(";")
+            parts = [p for p in parts if not p.lower().startswith(remove_param.lower())]
+            conn_str_no_auth = ";".join(parts)
+    
+    # Escape connection string for embedding in subprocess code
+    escaped_conn_str = conn_str_no_auth.replace('\\', '\\\\').replace('"', '\\"')
+    
+    # Test that properly formatted tokens don't get blocked (but will fail auth)
+    # Create a properly formatted UTF-16LE encoded ACCESSTOKEN structure
+    code = f"""
+import sys
+import struct
+from mssql_python import connect
+
+conn_str = "{escaped_conn_str}"
+
+# Create properly formatted ACCESSTOKEN with UTF-16LE encoded data
+# Use a fake JWT-like string that encodes properly
+fake_jwt = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9"  # Base64-like JWT header
+token_data = fake_jwt.encode('utf-16-le')  # Properly encode as UTF-16LE
+token_struct = struct.pack(f'<I{{len(token_data)}}s', len(token_data), token_data)
+
+attrs_before = {{1256: token_struct}}
+
+try:
+    connect(conn_str, attrs_before=attrs_before)
+    print("ERROR: Should have failed authentication")
+    sys.exit(1)
+except Exception as e:
+    error_msg = str(e)
+    # Should NOT get our validation errors
+    if "Invalid access token" in error_msg:
+        print(f"ERROR: Valid token structure was incorrectly blocked: {{error_msg}}")
+        sys.exit(1)
+    # Should get an authentication/connection error instead
+    elif any(keyword in error_msg.lower() for keyword in ["login", "auth", "tcp", "connect", "token"]):
+        print(f"PASS: Valid token structure not blocked, got expected connection/auth error")
+        sys.exit(0)
+    else:
+        print(f"WARN: Got unexpected error (but structure passed validation): {{error_msg}}")
+        sys.exit(0)  # Still pass - structure validation worked
+"""
+    
+    result = subprocess.run(
+        [sys.executable, "-c", code],
+        capture_output=True,
+        text=True
+    )
+    
+    # Should not crash
+    assert result.returncode not in [134, 139, -11], \
+        f"Segfault detected for legitimate token! STDERR: {result.stderr}"
+    
+    # Should pass the test
+    assert result.returncode == 0, \
+        f"Legitimate token test failed. Exit code: {result.returncode}\nSTDOUT: {result.stdout}\nSTDERR: {result.stderr}"
+    
+    assert "PASS" in result.stdout, \
+        f"Expected PASS message for legitimate token, got: {result.stdout}"