Flawed cryptography is leaving people using IPsec security protocols vulnerable to hacking, according to the UK's National Infrastructure Security Coordination Centre (NISCC).

The organisation has released an advisory about the discovery of three key flaws in the Encapsulating Security Payload (ESP) that provides base-level encryption of data, typically travelling though virtual private networks.

"An attacker could modify sections of the IPsec packet, causing either the cleartext inner packet to be redirected or a network host to generate an error message," warned NISCC.

"In the latter case, these errors are relayed via the Internet Control Message Protocol. Because of the Protocol's design, these messages directly reveal segments of the header and payload of the inner datagram in cleartext.

"The attacks have been implemented and demonstrated to work under realistic conditions."

The organisation rates the flaws as 'highly critical' and added that the Authentication Header protocols that guarantee the authenticity of data packets are also vulnerable.

The advisory provides three ways to work around the problem, including reconfiguring the ESP system and using Authentication Header and ESP simultaneously to defeat eavesdroppers.