A vested interest.

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For example:

Meaning: If you have a vested interest in something, you have a strong personal interest in it because you stand to gain from it.

The Minister of Commerce is suspected of having vested interests in several companies that have won big government contracts recently.
As both a shareholder in a publishing company and a published author, the newspaper's book reviewer is seen as having vested interests in certain publishers.

Note: This idiom is mostly used in reference to people who use their power or influence unethically for their personal benefit.

Quick Quiz:

Which type of person is most often accused of corruption involving vested interests in companies?

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