Commonwealth Bank's credit quality weakened a little in the September quarter. The bank released a quarterly update yesterday, showing that impairment expenses and impaired assets increased during the quarter.Loan impairment expenses rose from 20 basis points of gross loans in the June quarter to 21 basis points in the September quarter. The value of impaired assets rose from $4.5 billion to $4.8 billion over the same period.The bank said that corporate credit was the area of weakness, while the quality of mortgages and other retail business improved. The value of impaired mortgages fell from $1.08 billion in June to $1.05 billion in September, while the value of impaired corporate and SME loans rose from $3.3 billion to $3.6 billion over the same period.The bank emphasised that it was well provisioned. The ratio of individual provisions to impaired assets was 41.6 per cent - the highest ratio among the big banks.The ratio of total provisions to credit risk-weighted assets was 1.8 per cent - again, the highest ratio among the big banks.The bank reported an unaudited cash profit of $1.85 billion for the quarter - up from $1.8 billion in the June quarter and $1.75 billion in the September quarter last year.