Abstract: Three attacks that apply to certain configurations of IPsec have been identified. These configurations use Encapsulating Security Payload (ESP) in tunnel mode with confidentiality only, or with integrity protection being provided by a higher layer protocol. Some configurations using AH to provide integrity protection are also vulnerable.

Vendors affected:

Operating Systems affected:

Applications/Services affected: IPSEC

Title

NISCC Vulnerability Advisory IPSEC - 004033

Detail

NISCC Vulnerability Advisory 004033/NISCC/IPSEC

Vulnerability Issues with IPsec Configurations

Version Information

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Advisory Reference 004033/NISCC/IPSEC
Release Date 9 May 2005
Last Revision 9 May 2005
Version Number 1.0

What is affected?

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Potentially any configuration of IPsec that uses Encapsulating Security Payload (ESP) in tunnel
mode with confidentiality only, or with integrity protection being provided by a higher layer
protocol. Some configurations using AH to provide integrity protection are also vulnerable.

Impact

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If exploited, it is possible for an active attacker to obtain the plaintext version of the IPsec-
protected communications using only moderate effort.

Severity

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This is rated as high.

Summary

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IP Security (IPsec) is a set of protocols developed by the Internet Engineering Task Force (IETF)
to support secure exchange of packets at the IP layer; IPsec has been deployed widely to implement
Virtual Private Networks (VPNs).

Three attacks that apply to certain configurations of IPsec have been identified. These
configurations use Encapsulating Security Payload (ESP) in tunnel mode with confidentiality only,
or with integrity protection being provided by a higher layer protocol. Some configurations using
AH to provide integrity protection are also vulnerable. In these configurations, an attacker can
modify sections of the IPsec packet, causing either the cleartext inner packet to be redirected or
a network host to generate an error message. In the latter case, these errors are relayed via the
Internet Control Message Protocol (ICMP); because of the design of ICMP, these messages directly
reveal segments of the header and payload of the inner datagram in cleartext. An attacker who can
intercept the ICMP messages can then retrieve plaintext data. The attacks have been implemented and

demonstrated to work under realistic conditions.

[Please note that revisions to this advisory will not be notified by email. All
subscribers are advised to regularly check the UNIRAS website for updates to this notice.]

Details

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CVE number: CAN-2005-0039

IPsec consists of several separate protocols; these include:

* Authentication Header (AH): provides authenticity guarantees for packets, by attaching strong 

  cryptographic checksum to packets.

* Encapsulating Security Payload (ESP): provides confidentiality guarantees for packets, by 
  encrypting packets with encryption algorithms. ESP also provides optional authentication 

services
for packets.

* Internet Key Exchange (IKE): provide ways to securely negotiate shared keys.

AH and ESP has two modes of use: transport mode and tunnel mode. With ESP in tunnel mode, an IP
packet (called the inner packet) is encrypted in its entirety and is used to form the payload of
a new packet (called the outer packet); ESP typically uses CBC-mode encryption to provide
confidentiality. However, without some form of integrity protection, CBC-mode encrypted
data is vulnerable to modification by an active attacker.

By making careful modifications to selected portions of the payload of the outer packet, an
attacker can effect controlled changes to the header of the inner (encrypted) packet. The modified
inner packet is subsequently processed by the IP software on the receiving security gateway or the
endpoint host; the inner packet, in cleartext form, may be redirected or certain error messages
may be produced and communicated by ICMP. Because of the design of ICMP, these messages directly
reveal cleartext segments of the header and payload of the inner packet. If these messages can be
intercepted by an attacker, then plaintext data is revealed.

Attacks exploiting these vulnerabilities rely on the following:

* Exploitation of the well-known bit flipping weakness of CBC mode encryption.

* Lack of integrity protection for inner packets.

* Interaction between IPsec processing and IP processing on security gateways and end hosts.

These attacks can be fully automated so as to recover the entire contents of multiple
IPsec-protected inner packets.

In more detail, the three identified attacks on ESP in tunnel mode when integrity protection is not

present work as follows:

1. Destination Address Rewriting

   • An attacker modifies the destination IP address of the encrypted (inner) packet by bit-
     flipping in the payload of the outer packet.
   • The security gateway decrypts the outer payload to recover the (modified) inner packet.
   • The gateway then routes the inner packet according to its (modified) destination IP address.
   • If successful, the "plaintext" inner datagram arrives at a host of the attacker's choice.

2. IP Options

   • An attacker modifies the header length of the encrypted (inner) packet by bit-flipping in the

     payload of the outer packet.

   • The security gateway decrypts the outer payload to recover the (modified) inner packet.

   • The gateway then performs IP options processing on the inner packet because of the modified
     header length, with the first part of the inner payload being interpreted as options bytes.

   • With some probability, options processing will result in the generation of an ICMP "parameter

     problem" message.

   • The ICMP message is routed to the now modified source address of the inner packet.

   • An attacker intercepts the ICMP message and retrieves the "plaintext" payload of the inner
     packet.

3. Protocol Field

   • An attacker modifies the protocol field and source address field of the encrypted (inner)
     packet by bit-flipping in the payload of the outer packet.
   • The security gateway decrypts the outer payload to recover the (modified) inner packet.
   • The gateway forwards the inner packet to the intended recipient.
   • The intended recipient inspects the protocol field of the inner packet and generates an ICMP
     "protocol unreachable" message.
   • The ICMP message is routed to the now modified source address of the inner packet.
   • An attacker intercepts the ICMP message and retrieves the "plaintext" payload of the inner
     packet.

The attacks are probabilistic in nature and may need to be iterated many times in a first phase in
order to be successful. Once this first phase is complete, the results can be reused to efficiently
recover the contents of further inner packets.

Naturally, the attacker must be able to intercept traffic passing between the security gateways in
order to mount the attacks. For the second and third attacks to be successful, the attacker must be

able intercept the relevant ICMP messages. Variants of these attacks in which the destination of
the ICMP messages can be controlled by the attacker are also possible.

Solution

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Any of the following methods can be used to rectify this issue:

1. Configure ESP to use both confidentiality and integrity protection. This is the recommended
   solution.

2. Use the AH protocol alongside ESP to provide integrity protection. However, this must be done
   carefully: for example, the configuration where AH in transport mode is applied end-to-end and
   tunnelled inside ESP is still vulnerable.

3. Remove the error reporting by restricting the generation of ICMP messages or by filtering
   these messages at a firewall or security gateway.

Vendor Information

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A list of vendors affected by this vulnerability is not currently available. Please visit the web
site in order to check for updates.

Credits

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The NISCC Vulnerability Team would like to thank all vendors for their co-operation with
the handling of this vulnerability.

Contact Information

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The NISCC Vulnerability Management Team can be contacted as follows:

Email [email protected]
Please quote the advisory reference in the subject line

Telephone +44 (0)870 487 0748 Ext 4511
Monday - Friday 08:30 - 17:00

Fax +44 (0)870 487 0749

Post Vulnerability Management Team
NISCC
PO Box 832
London
SW1P 1BG

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What is NISCC?

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For further information regarding the UK National Infrastructure Security Co-ordination Centre,
please visit http://www.niscc.gov.uk/.

Reference to any specific commercial product, process, or service by trade name, trademark
manufacturer, or otherwise, does not constitute or imply its endorsement, recommendation, or
favouring by NISCC. The views and opinions of authors expressed within this notice shall not
be used for advertising or product endorsement purposes.

Neither shall NISCC accept responsibility for any errors or omissions contained within this
advisory. In particular, they shall not be liable for any loss or damage whatsoever,
arising from or in connection with the usage of information contained within this notice.

C 2005 Crown Copyright
<End of NISCC Vulnerability Advisory>

Acknowledgements

UNIRAS wishes to acknowledge the contributions of NISCC Vulnerability Team for the information contained in this Briefing.
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