The warning from analysts came after up to 40 blue-chip companies were affected by a denial of service attack on servers run by Akamai, a company that specialises in boosting website performance.
Akamai customers Microsoft, Google, Symantec, Trend Micro, Apple and Yahoo were among the websites hit by the attack on Akamai's domain name system, which converts www site names into IP numbers.
The attack will concern users who rely on these suppliers for critical services, patches and updates. Graham Titterington, principal analyst at Ovum, said, "Generally, there is no comeback for users of such sites. You should get the service you require but there may only be one or two firms with service level agreements for their websites."
Google, Microsoft and Symantec could not determine the level of disruption from the attack. Trend Micro's president of European operations, Raimund Genes, said its anti-virus service was not affected, but its website and manual updating might have been.
Tom Leighton, Akamai's chief scientist, said additional security mechanisms would be put in place to prevent similar attacks.
Akamai is used by more than 1,100 global companies.