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UK.gov warns over VPN crypto flaw

Remote access snooping risk prompts yellow alert

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Firms using IPsec VPNs for remote access could be vulnerable to hacker attack because of cryptographic weaknesses in key sub-protocols, a UK government UNIRAS alert warns.

The UK's National Infrastructure Security Coordination Centre (NISCC) describes weaknesses in the certain configurations of IPsec VPNs as "high risk". The issue is not product specific: instead it revolves around how systems are set up. Three attacks that apply to certain configurations of IPsec VPNs have been identified. These configurations use Encapsulating Security Payload (ESP)1 in tunnel mode with confidentiality only, or with integrity protection being provided by a higher layer protocol. Some configurations using Authentication Header (AH)2 to provide integrity protection are also vulnerable.

Attacks based on the cryptographic weaknesses of the sub-protocols involve modify sections of the IPsec packet to either redirect packets or generate error messages, in both cases exposing the clear text inner packet of supposedly encrypted communications. "If exploited, it is possible for an active attacker to obtain the plaintext version of the IPsec-protected communications using only moderate effort," NISCC warns.

It advises users to Configure ESP for use in both confidentiality and integrity protection in order to guard against possible exploitation. Other workarounds are explained in NISCC's detailed advisory here.

Roy Hills, technical director of security testing firm NTA Monitor, and an expert on VPN security, said the problem is unlikely to affect most users in practice but for some it poses a severe security risk.

"This looks like a real issue. However, it should only affect people who use ESP without integrity protection. Most people use ESP with both confidentiality (encryption) and integrity (HMAC), and so should not be affected. CESG guidelines for UK Government are to use either ESP plus AH or ESP with integrity, neither of which would be susceptible to this problem," he explained.

"Although it shouldn't affect many people, it will doubtless affect some, and these people probably won't realise that they are exposed because an insecure IPsec tunnel looks just like a secure IPsec tunnel to the user."

Hills has found similar types of security problems with VPN security in the field as explained in a white paper on the topic here. ®

1 ESP specifies how packets of data are encrypted to guard against snooping. The protocol can also be used to provide guarantees over confidentiality of data sent over VPNs. 2 AH attaches a cryptographic checksum to packets giving assurances that data has not been tampered with en-route between two points.

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