How did Moon-Gwang get injured?

According to Esquire:

A surprise middle-of-the-rainy-night reappearance from the Park family’s banished housekeeper (Lee Jung-eun, robbed of a Best Supporting Actress nomination) breaks up the Kims’ drunken house party and sends us sprinting into Parasite’s second act, into the basement, into a whole new movie. But we breeze right past an important detail: Moon-gwang’s face has been battered. She has a split lip and a contusion by her right eye. Only her husband asks about it, and she promises to explain later.

In the moment, while we’re still processing all the new information the movie has given us, we might chalk it up to her peach-fuzz allergy. But at the time of her termination, her face is untouched. When she shows back up at the house to feed her cellar-bound husband (after days? A week?) her wounds are fresh. Moon-gwang has been beaten.

When Mrs. Park quietly sends her housekeeper away — an employee who she believes to have tuberculosis — whether she knows it or not, she has banished her to homelessness, to violence, to a life even more precarious than the Kims’. Parasite alludes to an erosion of the South Korean middle class, but here Bong quietly makes it clear: Moon-gwang has gone from the Park’s house directly to the streets, because for her, for too many, there is nothing in between. Increasingly, for those of us watching from the United States, there isn’t either.

It is possible that she got roughed up by the loan sharks who are after her husband.

Note that we might get the real explanation someday:

In an interview with American news outlet TheWrap, Bong Joon Ho explained, “I had all these key ideas accumulated from when I started writing the script. I just couldn’t include all those ideas in the two-hour running time of the film, so they’re all stored in my iPad, and my goal with this limited series is to create a six-hour-long film.”

[...]

The director went on to specifically raise a couple of unanswered questions about the character of Gook Moon Gwang (played by Lee Jung Eun) from the original film.

“For example,” he pointed out, “when the original housekeeper Moon Gwang comes back in the late night, something happened to her face. Even her husband asked about it, but she never answered.”

“I know why she had the bruises on her face,” continued Bong Joon Ho. “I have a story for that, and aside from that, why does she know the existence of this bunker? What relationship does she have with that architect [the original owner of the house] to know of this bunker? So I have all these hidden stories that I have stored.”