In the Name of Allaah, the Most Merciful…
An honored guest reminded me today that our brothers need to be clear on the issue of ankle socks and the ruling on wiping over them for wudhoo’, since it is an issue that many people may not know about.
Ankle socks: very short socks usually worn with low top sports shoes, they usually do not cover the ankle.
Ankle socks that do not cover the ankle bone entirely do not fit the criteria of a “khuff”, and thus it is not permissible to wipe over them for wudhoo’. Thus, wiping over ankle socks (that do not cover the ankle) is INVALID, and the wudhoo’ is incomplete and thus invalid as well.
Our shaykh, Dr. Saalih al-Fawzaan (may Allaah preserve him) said:
“…And it is a condition (for the validity of wiping over the footwear) that the footwear cover the foot. He may not wipe over footwear that is not sufficiently covering what he is required to wash (i.e. the entire foot, top and bottom, including the ankles), like footwear that is below the ankle, or footwear that is big enough, but does not properly cover the foot because it is too thin, like a thin nylon sock. None of these things may be wiped over since they do not adequately cover (the foot)…” 
 al-Mulakh-khas al-Fiqhee (1/42)
 Original source: http://www.salafitalk.net/st/viewmessages.cfm?forum=10&topic=9456
Compiled and translated by Moosaa ibn John Richardson