KIEV — Like most good actors, Volodymyr Zelenskiy relishes a challenging role.

He sought Ukraine’s presidency despite a complete lack of political experience. Then, after a stunning landslide win, he called early parliamentary elections, leading his new political movement to an absolute majority last weekend with the largest seat count of any party in Ukraine’s post-Soviet history.

Now, with the levers of power firmly in his grasp, the Ukrainian actor-turned-president faces a challenge many here consider a mission impossible: reforming the country’s chronically corrupt domestic spy service.

Known by its acronym SBU (Sluzhba bezpeky Ukrayiny), Ukraine’s secret service is the successor organization to the Soviet-era KGB, from which it inherited its original staff, structure and modus operandi. Over the years, its spies have been implicated in a long series of corruption scandals, large and small, involving crimes ranging from murder to illegal arms sales.

The SBU remains closely entwined with both Ukraine’s powerful oligarchs and its political class, Western officials say.

The SBU, which employs more than 30,000 people across the country, has been nearly impervious to reform since the collapse of the Soviet Union.

Zelenskiy, a former comedian whose unlikely victory in the presidential election has given Ukrainians new hope for their country’s future, was elected on a promise to end Ukraine’s endemic culture of corruption. Many political observers see restructuring the security service as a crucial test of both his will and ability to follow through on that pledge.

It’s also an issue that could determine Zelenskiy’s success in bringing his country closer to the West, a step he has said is one of his chief aims as president.

For years, both the EU and the U.S. have been prodding Ukraine’s leadership to address the problems at the agency. Indeed, encouraging the government to fix the SBU has been a central priority for the EU’s advisory mission in Ukraine since it established operations there in 2014. So far, it hasn’t worked.

“We never had a proper response from the political elite,” a senior European diplomat said. “The SBU is responsible to one man — the president — and that makes the system very politicized.”

Translation: It was often in Zelenskiy’s predecessors’ personal interest to keep the secret service happy.

Even as Ukraine has endured successive bouts of political upheaval in the nearly 30 years since the collapse of the Soviet Union, the SBU, which employs more than 30,000 people across the country, has been nearly impervious to reform. That record is all the more extraordinary considering that corruption in the service is an open secret.

“The SBU has never been touched,” said Olena Halushka, head of international relations at the Kiev-based Anti-Corruption Action Center. “It’s the only institution where there’s been no reform.”

One big obstacle has been Ukraine’s ongoing war with Russia in the east of the country. Petro Poroshenko, Zelenskiy’s immediate predecessor as president, vowed to combat corruption at the SBU and restructure the agency when he was elected in 2014. He even asked NATO and the EU to help the SBU develop a reform plan, but the war with Russia soon put the issue on the back burner.

Poroshenko’s critics also suspect he didn’t want to relinquish presidential control of the powerful service, as recommended by NATO and the EU, especially as he faced reelection.

Whatever the reason, the SBU has continued to conduct business as usual.

Ukrainian anti-corruption activists and Western diplomats say the SBU’s economic crime unit is notorious for shaking down local businesses.

The main thrust of the EU and Washington’s critiques of the agency is that its structure breeds corruption.

In addition to its classic intelligence-gathering and counterintelligence functions, the SBU is also responsible for areas that in most democracies fall under the purview of law enforcement. These include pursuing corruption cases, economic crime and organized crime. The agency also has the exclusive right to issue wiretaps.

The SBU’s sweeping mandate makes it Europe’s largest intelligence agency. With an estimated 33,000 employees, it’s more than eight times the size of the U.K.’s domestic intelligence agency, MI5, for example.

“This is clearly not a service that would come even close to Western standards,” a Kiev-based European diplomat said.

Ukrainian investigative reporters, meanwhile, continue to uncover blatant evidence of corruption in the service.

SBU officers who earn $300-$500 per month have been caught on camera driving expensive cars, including in one case a $70,000 Toyota Landcruiser. One senior SBU official was discovered living in a $3 million apartment in Kiev. The property was registered under the name of a relative who lived in a small rural village.

Ukrainian anti-corruption activists and Western diplomats say the SBU’s economic crime unit is notorious for shaking down local businesses.

“Businesses are really suffering from the corruption,” a senior EU official said.

Concerns about corruption have also scared away foreign investors, hobbling Ukraine’s economic recovery, the official added.

Ukrainian officials insist that they’ve gotten the message and are willing to restructure the SBU, even against the inevitable resistance.

“We agree that the security service needs to be reformed,” said Deputy Interior Minister Tetiana Kovalchuk in a recent interview. “We need to see a new, reformed security force of Ukraine.”

Ukrainian anti-corruption activists say if the reform doesn’t happen soon, it may never happen. While the country continues to face serious challenges with corruption in its judiciary and other sectors, many here are convinced that fixing the SBU is the key to turning the tide.

In the end, it will be up to the president. With his unprecedented democratic mandate, Zelenskiy is the only one with the influence to take on the vested interests and restructure the spy service.

The other key factor — Western pressure.

“If we didn’t have pressure from the U.S. and EU, we would have zero reforms here,” said Oleksandr Liemienov, the founder of State Watch, an anti-corruption watchdog.