Security researchers have discovered nine vulnerabilities in a range of internet-connected light bulbs made by Osram.
The flaws in the Lightify products could give attackers access to a home wi-fi network, and potentially operate the lights without permission.
Osram said a "majority" of the problems would be fixed in a software update in August, but four remained unpatched.
One security expert said Osram had made an "elementary" mistake.
Osram's Lightify range features internet-connected light bulbs that can be controlled using a smartphone app.
Researcher Deral Heiland from Rapid7 discovered nine vulnerabilities in the Home and Pro range and reported them to the manufacturer.
One problem was that the Osram smartphone app stored an unencrypted copy of the user's wi-fi password.
That could give an attacker access to a user's home wi-fi network and the devices connected to it, if the password was extracted from the app.
"In this day and age, you would regard that as an unacceptable security flaw," said Professor Angela Sasse, a cybersecurity expert at University College London.
"It's a well known thing that you don't store passwords like that - it's really elementary."
Another flaw could let an attacker compromise the light bulbs and switch them on or off without permission.
"This is not just about being able to manipulate the light bulbs," said Prof Sasse.
"The vulnerabilities here could give somebody access to control the network itself and that's a very serious issue."
Osram said in a statement: "Since being notified about the vulnerabilities identified by Rapid7, Osram has taken actions to analyse, validate and implement a risk-based remediation strategy.
"The majority of vulnerabilities will be patched in the next version update, currently planned for release in August."
The firm said the remaining unpatched problems involved the ZigBee hub - a device that sits between the light bulbs and a home wi-fi router to relay commands to the lamps.
"Osram is in ongoing coordination with the ZigBee Alliance in relation to known and newly discovered vulnerabilities," the firm told the BBC.
A number of companies including Amazon, Apple, Blackberry and Google are developing platforms to support internet-connected devices in the home.
Prof Sasse said consumers would need to feel confident about the security of smart devices before adopting them.
"What we've seen with many companies that are hardware specialists, is that their quality control may not be on top of the software side of things," she told the BBC.
"They may be able to test that the software does what it's supposed to do - but they don't always test the things it is not supposed to do.
"I think it highlights something that consumers should be concerned about.
"For devices embedded in the home, there should be basic security checks."