About AirPort Base Station Firmware Update 7.6.4
This update is available for all Apple 802.11n Wi-Fi base stations including AirPort Express, AirPort Extreme, and AirPort Time Capsule. It contains fixes that:

• Resolve an issue that may cause an IPv6 Tunnel error
  (ref: v7.6.3 IPv6 Tunnel Error in router/NAT mode)
• Resolve an issue that may cause AirPort Time Capsule to report that the backup disk is in use (i.e. "sparsebundle in use")
• Resolve an issue that may intermittently cause a loss in Wi-Fi connectivity with iOS devices

Note: This firmware update is for all 802.11n AirPort base stations.
(New 802.11ac AE/TC models had v7.7.1 FW rel'd June 10th, 2013.)

For information on the security content of this update, visit: http://support.apple.com/kb/HT1222
(Apple security updates page, no doc there on this update until Sept 6th - see below)

For detailed information on AirPort base station firmware updates, including instructions how to install the update, please visit: http://support.apple.com/kb/HT1218 ("About AirPort base station firmware updates")

Update: It wasn't posted on Aug 13th when 7.6.4 was first released, but apple later posted the doc with details About the security content of AirPort Base Station Firmware Update 7.6.4.

You can check Apple's Airport and Time Capsule forums for feedback before updating.
(I've got a 2nd gen Dual-Band 802.11N AE base still using 7.6.1 firmware. Not had any reason to update yet.)
  

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For links to older news pages, see the Archives.