According to Russian IT security site securitylabs.ru, hackers have obtained 800MB of Cisco's source code for IOS 12.3 after breaching Cisco's corporate network.
A hacker called "Franz" apparently bragged about the deed and posted a sample of the code in an IRC chat room.
In a statement, Cisco said: "Cisco is aware that a potential compromise of its proprietary information occurred and was reported on a public Web site just prior to the weekend. The Cisco information security team is looking into this matter and investigating what happened."
Firms around the world rely on Cisco operating systems for routers and network infrastructure. If the article is true, those firms and the entire internet could be affected. Hackers who know the source code could be able to make denial-of-service attacks on Cisco networks.
As yet, it is uncertain how dangerous the leak could be.
In February, Microsoft experienced a similar leak of its Windows 2000 code.