(Bloomberg) -- Long considered a nuisance, distributed-denial-of-service attacks, or DDoS, are a growing problem for banks and other financial businesses, according to a new report.

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The volume of DDoS attacks targeting financial firms increased 22% year-over-year as of November, according to a new report first provided to Bloomberg News by the Financial Services Information Sharing and Analysis Center, which is known as FS-ISAC. The issue is particularly pronounced in Europe, where financial services saw a 73% increase in DDoS attacks, the report showed.

DDoS attackers marshal an army of connected devices — known as a botnet — and direct internet traffic at a website to disrupt it or shut it down.

The attacks have become a persistent annoyance for financial institutions, causing intermittent downtime and forcing security staffers to repel the activity. Often, hacking groups use DDoS as an extortion technique, demanding money in order to go away. In other cases, political groups aim DDoS attacks at victims in order to send a message, such as when pro-Syrian government hackers aimed their tools at news sites critical of President Bashar al-Assad early during the civil war there.

“I think it’s important to know that in general, when it comes to DDoS attacks, they’re here to stay,” said Teresa Walsh, FS-ISAC’s global head of intelligence.

Last year, DDoS attacks were used to further political aims, targeting those who have taken sides — even indirectly — in the war in Ukraine or other in geopolitical hotspots, including China and Taiwan, according to the report. A group called Killnet, aligned with Russian interests, has waged a campaign of DDoS attacks on websites of businesses, governments and airports in the last year.