All 802.11 WLAN products, whether they use a, b, g, or n radios, must comply with exactly the same 802.11 security measures. Currently, those measures are defined by the IEEE 802.11i-2004 Amendment to the IEEE 802.11-1999 Standard -- Medium Access Control (MAC) Security Enhancements. So, technically, there should be no difference in security features found in 802.11a/b/g products and 802.11n products.
In practice, you will see some temporal differences between new and old products. For example, it is still possible to buy some old 802.11b products that support only the original WEP, and many not-so-old 802.11g products that support both WEP and WPA (an early snapshot of the 802.11i standard that used TKIP encryption). But the Wi-Fi Alliance now requires all new Wi-Fi certified products to support WPA2 (the final 802.11i standard that includes both TKIP and AES). So new 802.11n products (and pre-n products) certified by the Wi-Fi Alliance can all be expected to provide no less than full 802.11i security.
Another difference will be seen in related WLAN analysis, monitoring, and prevention products used to detect and respond to security incidents. Existing products that use a/b/g cards, decode engines, and analysis modules will need to be upgraded to hear, parse, and analyze 802.11n transmissions. If you don't upgrade your monitoring system, you may end up with a security "blind spot" when it comes to 802.11n or pre-n products.
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