A message on the Cisco website shows the company has reset passwords as a precautionary measure.
"This incident does not appear to be due to a weakness in Cisco products or technologies," said the company on its website.
An email has been circulating claiming Cisco's password file for its customer portal had been compromised.
Some fear thousands of Cisco customers' details could be disclosed and ousiders may have access to restricted information and tools.
SC received notification of the email this morning and its circulation has been confirmed by four Cisco customers.
The email has started a mass scramble amongst Cisco users to prove its authenticity and plot contingency plans if the breach is proved real.
John Meakin, group head of information security at Standard Chartered Bank, confirmed that his organization has received the email. "I've asked for an analysis what might happen if it were true," he said.
The news arrives in a bad couple of weeks for Cisco. Last week the company was forced to release a security alert following the revelation of major Cisco software flaws by one of its former researchers at the Black Hat conference in Las Vegas.
Michael Lynn quit hours before revealing flaws in Cisco IOS Shellcode. The company immediately sued Lynn for not conforming to "established industry practises."