Companies use virtual private networks to provide secure connections for employees who need to access work applications from outside the office, but a newly discovered vulnerability may allow hackers to turn those secure connections into a channel for launching cyberattacks.

The U.S. Department of Homeland Security today issued a warning about the flaw, which is named VU#192371. The vulnerability has so far been found in enterprise VPN products from four companies: Cisco Systems Inc., F5 Networks Inc., Palo Alto Networks Inc. and Pulse Secure LLC. The researchers who discovered the flaw cautioned that it’s “likely” that similar problems exist in more VPN products.

At issue are the files that the affected programs store on users’ devices. The VPNs generate cookies to let workers access company applications without having to enter their password at every login, as well as to manage data created during browsing sessions. That wouldn’t be an issue by itself, except the cookies are stored in plain text.

The lack of encryption means hackers could theoretically breach a company just by compromising a single employee device. Once they’ve taken control of a computer or handset, they could extract the plain-text VPN cookies and use them to open a connection to the organization’s internal network.

“If an attacker has persistent access to a VPN user’s endpoint or exfiltrates the cookie using other methods, they can replay the session and bypass other authentication methods,” the DHS’ CERT cybersecurity group detailed in a memo. “An attacker would then have access to the same applications that the user does through their VPN session.”

That poses a particularly serious threat for bigger organizations that have a large number of devices connected to their networks. In an enterprise with tens of thousands of employees, every worker endpoint that has a VPN installed could represent a potential attack vector.

On the bright side, the problem appears to be easier to fix than certain other broad-reaching vulnerabilities, such as the notorious Spectre processor exploits uncovered last year. Palo Alto Networks and F5, two of the four companies so far found to have an affected VPN product, have already released patches for their software. Cisco and Pulse Secure are expected to follow suit further down the road.

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