Association Failures with Legacy Printers due to Management Frame Protection- A Technical Analysis

 Let's understand first, what is Management Frame Protection?

Based on the IEEE 802.11w amendment, Protected Management Frames (PMF), also known as Management Frame Protection (MFP), is a security feature that provides integrity protection for both unicast and broadcast management frames, while also encrypting unicast management frames in the same way as data to provide confidentiality. Without the Protected Management Frames feature, all management frames are sent unprotected in the open. Transmitting open frames makes connections vulnerable to attack. To leverage Protected Management Frames, both the AP and the STA need to be capable of using it, and it must be activated for each encrypted Wi-Fi network of the AP. If those conditions are met, Protected Management Frames are automatically invoked during client association.

Understanding Management Frame Protection Failures

MFP is one of the common challenge when working with legacy devices in modern wireless networks. My recent troubleshooting experience with legacy printers highlights important considerations when deploying MFP in mixed-device environments.

Recently, I was investigating an issue with legacy printers that were stuck in a constant reauthentication loop. 

The common signs of failure:

  • Printers continuously failing authentication attempts
  • Devices unable to maintain stable connections
  • Association failures appearing repeatedly in logs
Using RUCKUS AI's dynamic packet capture feature, I was able to examine the 802.11 frames exchanged between the network and the printers. One can also perform an OTA capture using MacBook or AP Remote Packet captures for the devices in the problem state. The key finding was in the Association Response frames, which showed "Status code: Invalid element, i.e., an element defined in this standard for which the content does not meet the specifications in Clause 9 (Frame formats) (0x0028)"



This status code (0x0028) indicated the device failed to properly implement an element defined in the 802.11 standard's Clause 9, which deals with frame formats.

The Root Cause: Management Frame Protection Requirements

Management Frame Protection have 3 Modes:

  1. Protects against forged management frames: MFP prevents attackers from spoofing disassociation or deauthentication frames that can disconnect clients
  2. Has three implementation modes:
    • No MFP (disabled): No protection for management frames
    • Optional MFP: Clients capable of MFP will use it, while legacy clients without MFP support may still connect
    • Required MFP: Only clients supporting MFP can connect to the network

Looking at the packet captures more closely, I could see the RSN Capabilities field showed following:

For the AP, it is advertised in both Beacon and Probe Response Frames

Wireshark filters for MFP  

"wlan.rsn.capabilities.mfpc"

"wlan.rsn.capabilities.mfpr"



  • Management Frame Protection Required: False
  • Management Frame Protection Capable: True
For the Station, it is advertised in Association Request Frame



  • Management Frame Protection Required: False
  • Management Frame Protection Capable: False
As per the IEEE 802.11 Standards, if an STA is advertising its MFP Capabilities as "False" for both Required and Capable, the actions are described as following for both STA and AP:



AP Action : The AP may associate with the STA
STA Action: The STA may associate with the AP

In this case, as observed in the AP captures, the MFP was set as Capable on the WLAN /SSID. However Station, as stated by the standard, "may" work in such a situation. 

To resolve the issue, here are the recommendations:

  1. Changed the WLAN configuration to use "Optional MFP" instead of "Required MFP"
  2. If "Optional MFP" does not help, Created a separate SSID specifically for legacy devices with MFP disabled
  3. Updated firmware on printers/STA where possible to support newer security standard.

Best Practices for MFP Implementation

Based on this experience, I would recommend:

  1. Inventory your devices and identify those that lack MFP support before enabling MFP
  2. Use "Optional MFP" in environments with mixed client capabilities
  3. Plan for segregated networks for legacy devices that cannot support modern security features

Reference: IEEE Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, CWNP 

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