Donald Trump’s decision to dismantle the Deferred Action for Childhood Arrivals (Daca) program from next March will leave almost a million young immigrants in fear that the personal information they voluntarily handed the federal government in signing up for the scheme could now be turned against them to facilitate their own deportations.

Tuesday’s announcement by the attorney general, Jeff Sessions, that the Obama-era protections against deportation for the young undocumented immigrants known as “Dreamers” would begin to be dismantled in March 2018 puts the spotlight on the huge government database that has been compiled over the past five years. The records contain highly sensitive information on every individual who has applied for legal protection under the scheme, as well as relatives who might also be vulnerable to removal from the US.

The latest federal statistics show that 936,394 young undocumented immigrants, all of whom were brought to the US as children, have applied for temporary permission to live and work in the US under Daca. Almost 800,000 have been accepted.

In the process, those young people were required to divulge a large range of personal data, including home addresses, educational history, fingerprints and photographs. Such rich intelligence would provide the Trump administration with a potentially powerful surveillance tool were it to decide to round up and deport people as their Daca permits expire from next spring.

The storage of the home addresses of hundreds of thousands of Dreamers alone could make the work of federal immigration officers vastly easier in deporting them. That knowledge in turn is likely to spread alarm among Daca recipients that their voluntarily divulged information may now be exploited against them, rendering them vulnerable even in their own homes.

The database is currently under the control of the US Citizenship and Immigration Services (USCIS). Under present rules, the data is not shared with the agency responsible for deportations – Immigration and Customs Enforcement (Ice) – but as both outfits fall under the same Department of Homeland Security, the potential for data sharing remains.

“It would not be easy, but nor would it be impossible, for Ice to get hold of this data,” said Cecilia Muñoz, vice-president of policy and technology at the New America thinktank.

Muñoz was director of Barack Obama’s domestic policy council and in that role she took the lead within the White House in 2012 during the introduction of Daca. She told the Guardian that at the time Obama officials were fully aware of the vulnerabilities inherent in the Daca database, which they attempted to communicate clearly to Dreamers as they considered whether to apply for the scheme.

“When President Obama announced Daca in 2012, he made it clear that it was not a permanent protection. We offered as much assurance to people as we could – telling them that we would never use the data on our watch – but we couldn’t assure them about what future administrations would do with it.”

An added concern for many Daca recipients is that in filling out the requisite forms, they were forced to give information that could reveal the locations of their undocumented parents, thus putting them also at risk. Julián Gustavo Gómez of the advocacy group Define American applied for protection under the program soon after it was set up, and in so doing gave the federal government intelligence on his school history, bank accounts, birth certificate and every home address for the past 20 years.

At the time he was living in his parents’ home, and thus revealed their home addresses in his Daca form. As it happens, his parents have since become US citizens and he has acquired a green card, so the risk to the family has been removed, but many other Dreamers have not been so fortunate.

“We outed ourselves – we presented ourselves to the government and said: ‘Look, I’m here in the country illegally, here’s all my information.’ Until then, the government didn’t have any way of knowing who I was or whether or not I was documented,” Gomez said.

In December 2016, shortly before leaving office, Obama’s homeland security secretary Jeh Johnson released a public letter in which he set out the historic convention adhered to by successive administrations that erected a wall between USCIS and immigration enforcement. In the letter, Johnson pointed out that the US government had promised Daca applicants that “the personal information they provided will not later be used for immigration enforcement” unless there were evidence of a threat to national security or public safety.

Johnson’s letter could provide a way for civil rights and immigration advocacy groups to sue the Trump administration should it begin to use the Daca database to help Ice round up Dreamers for deportation. Several prominent legal groups are already preparing strategies for trying to protect the young immigrants should the database be breached in this way.

Lorella Praeli, Hillary Clinton’s national Latino vote director in the 2016 presidential race and now director of immigration policy and campaigns at the ACLU, said that the Johnson letter underscored what a radical departure it would be were Trump to take the personal information that Dreamers had voluntarily given the federal government and use it against them. “We will take all action necessary to hold the government to its word and ensure the rights of Dreamers are protected,” she said.