Purity: Ruling on Purification of Inside of Shoes and Soles of Socks by Walking
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 184
The inside of a shoe does not become pure by walking. Furthermore, based on obligatory precaution, the soles of socks do not become pure as a result of walking unless the sole is made of leather and suchlike and walking on them on earth is considered normal.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)