On the discount days Black Friday and Cyber Monday next week and the week after next, retailers in Germany are threatened with serious cyberattacks.
The same applies to the following Christmas business, as the Federal Office for Information Security (BSI) warned on Wednesday.
The cybersecurity authority advised online retailers in particular to prepare suitable countermeasures.
Above all, the BSI warned of so-called DDoS attacks.
These are concerted, billions of times access to individual websites in a short period of time, overloading them and causing them to crash.
Cyber criminals often use them for blackmail attempts.
And they are upgrading: "In the past few weeks we have observed DDoS attacks with up to 21.8 million requests per second," said BSI President Arne Schönbohm.
These are significantly stronger attacks than were previously measured.
Trade is particularly vulnerable on discount days
On the high-turnover days around the American Thanksgiving holiday, retailers lose a particularly large amount of sales when their websites are inaccessible due to attacks.
That is why the BSI fears a particularly large number of attempts at DDoS extortion.
Previous protective measures against the attacks should be adapted to the new capacities of the cyber criminals, recommended Schönbohm.
For DDoS attacks, hackers often use infected routers at unsuspecting private individuals as well as unprotected servers and systems compromised by Trojans.
Cyber criminals can connect these to so-called botnets and control them remotely.