A fix to the software code behind bitcoin won't be ready until next week, according to the lead programmer charged with solving the problem, highlighting the vulnerabilities of the computer-driven virtual currency.

Hackers on Monday began attacking the world's largest bitcoin exchanges to exploit a flaw in the code that could allow users to send false requests to resend payments. The attacks forced the exchanges to suspend customer withdrawals.

The Tokyo-based Mt. Gox exchange on Monday said it had received fraudulent requests. Bitcoin's core developers later that day discovered a bug within a part of its digital infrastructure. The next morning, hackers exploited the bug and swarmed other exchanges.

The delay in solving the code problem could undermine confidence in the five-year-old virtual currency, which has seen a dramatic run-up in prices in the past year and has begun to gain adoption by a broad swath of merchants.

"It will take two or three days to write the code and longer than that to test it and convince ourselves that we didn't break something else," said Gavin Andresen, the chief scientist of the Bitcoin Foundation, a trade organization representing bitcoin-related businesses. Mr. Andresen is head of the five-member team of programmers attempting to fix the problem.