The size of big banks and the possibility of cybersecurity breaches are among the key threats facing the financial system, government researchers warned Congress Tuesday.

In a report submitted to Congress, the Office of Financial Research, a research group within the Treasury tasked with looking out for potential crises, said that overall financial risks are in a "moderate" range but outlined a handful of possible risks.

Of those, the report said, the "'systemic footprint' of the largest banks — their size, complexity and interconnectedness — constitutes a key threat to financial stability."

Although the report notes that the post-crisis reforms carried out under President Obama have made the big banks safer, the office said, "a number of metrics suggest the systemic footprint of [the eight biggest banks] in the U.S. financial system has changed little since the crisis." Those eight banks now hold almost three-quarters of all assets in the banking system, according to the report, and have constantly changing business profiles. The report also noted that some big banks have failed to write credible "living wills" that spell out how they would go bankrupt during a failure without causing a crisis.

The eight banks are: JPMorgan Chase, Citigroup, Bank of America, Goldman Sachs, Morgan Stanley, Bank of New York Mellon, State Street and Wells Fargo.

Cyber breaches also "pose a clear potential risk to financial stability," the report noted. The threats from such attacks are hard to measure and quantify, according to the researchers. Increasingly, banks are including warnings about possible losses from breaches in disclosures to investors.

The report sent to Congress Tuesday, which is required annually, also included five other key threats: The risk that Brexit or other political events in Europe might cause a contagious panic, the rising indebtedness of U.S. businesses, the possibility of panics spreading through clearinghouses that process trades, poor data management, and the pressures that low interest rates have placed on life insurers.