Courtesy of our Archives reread thread, Superboy and the Legion #264 showed that Dagon the Avenger possessed a device that could short circuit Dawnstar’s tracking ability. But what other potentially fatal flaws can Legionnaire villains exploit?

Superboy – Magic, Kryptonite
Mon-El – Lead poisoning/ tampering with his anti lead poisoning serum
Dawnstar – Dagon’s device
Karate Kid – Keith Giffen
Ultra Boy – Multi tasking
Chameleon Boy/ Girl – Cancellite (created by that nice Supergirl, and used against Yera in #305)/ grabbing their antennae (used in the Baxter series)
Night Girl – Light
Shrinking Violet - Brainy modified the cancellite to work on Inskians in #305.

Ultra Boy is particularly vulnerable to certain types of radiation.

In his first appearance, Star Boy couldn't see through copper. In the threeboot, he was vulnerable to sugar.

Shadow Lass was initially affected by very bright light.

We've also seen how Star Boy becomes mentally unstable without his medication.

Speaking of unstable, Brainiac 5 seems to go to mush in the presence of certain blondes. (For some reason, Saturn Girl doesn't affect him.)

If cancellite can be modified, watch out everybody!

Tellus is vulnerable without his breathing equipment.

I always thought the Invisible Kids would be quite useless in the rain as their opponents would be able to see them, luckily they only got called for missions when weather control was set for sunshine.

The Invisibles - of course! rain would be disrupted by their presence. Snow would even stick to them. There was a scene in the late reboot in which Lyle's footprints were visible in soft earth, but I can't recall Harbinger's idea ever being used in the comic.

Laryngitis could stop Tyroc, or maybe alter the effect of his cries in unexpected ways. He better stay on good terms with Drura.

The rare element Sigellian is shown to bizarrely affect the Founders in exactly the same way that Kryptonite affects Superboy.

ref: Adventure Comics #267

Thankfully, it has never been seen again.

Superboy rendered Sigellian harmless by vibrating its frequency. Maybe he turned it into another rare element, Siegellian, which eventually destroyed himself.

Non-specific Legionnaire vulnerabilities have included a variety of illnesses, like the Pain Plague or Rigel Fever. There was also Yorggian Fever which caused Durlan R.J. to lose his powers. Maybe Cancellite was derived from Yorggian Fever.

Sun Boy has a vulnerability to brunettes.