An analogy similar to Trojan Horse that describes letting in an attractive enemy?

I’m seeking an elusive metaphor, figure of speech, proverb, cliche, or literary allusion that I believe exists but can’t bring to mind. If it exists, it’s also familiar enough to be recognizable by many and not something obscurely poetic.

It expresses the idea of letting the enemy in in disguise—or if not the enemy, exactly, then something that looks attractive but will do you harm, especially by taking over and undermining your will and your defenses.

What’s wrong with the Trojan Horse, for my purposes, is that it’s anything but subtle. I’m thinking of something ingratiating and seemingly beneficial that turns out to be a devil’s bargain—once you’ve seen the cost of letting it in, it’s too late and you can’t get rid of it.

In other words, it’s not about tricks or deception (“Oh, look, the Greeks left us a present! Let’s bring it in.”) so much as about not seeing the potential drawbacks of something because you’re so dazzled by what it can do for you—until you realize that it has you by the throat (for example, promises of easy credit that wind up hamstringing you).

A vampire is close but doesn’t work because the vampire itself doesn’t gradually take over. Instead, it recruits you—all or nothing. The analogy I need expresses the idea of willing complicity with something seemingly benign that doesn’t show its true colors until it’s too deeply entrenched to be eradicated. Something like a parasite, perhaps, or a social institution that comes with a dark side. (Please don’t treat this as an invitation to a political debate.)

Am I dreaming, or is there a well-known analogy or figure of speech that expresses this idea?