libreswan is now older than openswan was at the time of the forced rename to libreswan. Please stop using openswan. It is not safe to use!

I wrote an article over at the Red Hat Security blog about the SLOTH attack and IKE/IPsec.

So the weakdh.org article got some renewed press again, and it keeps coming up with this rather misleading paragraph:

We further estimate that an academic team can break a 768-bit prime and that a nation-state can break a 1024-bit prime. Breaking the single, most common 1024-bit prime used by web servers would allow passive eavesdropping on connections to 18% of the Top 1 Million HTTPS domains. A second prime would allow passive decryption of connections to 66% of VPN servers and 26% of SSH servers. A close reading of published NSA leaks shows that the agency’s attacks on VPNs are consistent with having achieved such a break.

That’s a rather bold statement. How do the authors know the MODP groups of 66% of VPN servers? This is what they write in their research paper:

We measured how IPsec VPNs use Diffie-Hellman in practice by scanning a 1% random sample of the public IPv4 address space for IKEv1 and IKEv2 (the protocols used to initiate an IPsec VPN connection) in May 2015. We used the ZMap UDP probe module to measure support for Oakley Groups 1 and 2 (two popular 768- and 1024-bit, built-in groups) and which group servers prefer. To test support for individual groups, we offered only the single group in question. To detect default behavior, we offered servers a variety of DH groups, with the lowest priority groups being Oakley Groups 1 and 2.

The first problem is that the first packet exchange performs the DiffieHellman and no IDs or credentials are sent. This means that the server in question does not yet know which configuration this connection maps to, unless it is limited by IP. If it is limited by IP, then their probe likely did not even solicit an answer because they came from the wrong IP – Cisco VPN servers tend to not give error messages and will just drop the packet. If the configuration is not limited by IP, because the connection supports roaming users, then the VPN server cannot yet reject the connection based on a weak MODP group. Imagine the following configuration (in SWAN ipsec.conf syntax):

conn regularusers
  left=my.ip.address
  right=%any
  rightid=%fromcert
  ike=aes256-sha1-modp1536

conn oldcisco
  left=my.ip.address
  right=%any
  rightid="CN=knownweakdevice.com, O=OldVendor, C=CA"
  ike=aes256-sha1-modp1024

When the weakdh people’s scanner showed up, it allowed the initial request/response packet to be 1024. But once it got further into the negotiation, if would authenticate and identify the remote peer, and refuse to establish the IKE connection with the modp1024 group for everyone except the one known buggy device. But since the scanner had no credentials, it never got that far. So this is a false positive. This is how freeswan and openswan worked, and how libreswan works. When it receives more information, it tries to switch connection to the best matching connection. Note that with only the above two configurations enabled, libreswan would immediately reject an incoming IKE exchange that tries to use modp768 or modp2048, since no loaded connection specifies that modp group. Note the default when not specifying an IKE line is modp2048 and modp1536 for IKEv2, and modp1536 and modp1024 for IKEv1. With a preference for the higher group.

This method of connection switching requires the IKE daemon to know or go through all the connections to look at the modp group. If I understood the strongswan implementation correctly, it does not even bother to reject the incoming request based on modp group. It will just answer whatever the modp group is on the first packet. Later on in the exchange, once it has committed to a specific configuration, it will check for the modp group and reject the connection. So that is false positive number two for the scanner.

The article continues:

Of the 80K hosts that responded with a valid IKE packet, 44.2% were willing to accept an offered proposal from at least one scan. The majority of the remaining hosts responded with a NO-PROPOSAL-CHOSEN message regardless of our proposal. Many of these may be site-to-site VPNs that reject our source address. We consider these hosts “unprofiled” and omit them from the results here.

Well, that means the authors just excluded all freeswan/openswan/libreswan servers that are configured to not allow modp1024 because that is exactly what they would return. Libreswan per default does not allow modp1024 in IKEv2, so this is a rather big false negative!

We found that 31.8% of IKEv1 and 19.7% of IKEv2 servers support Oakley Group 1 (768-bit)

In our sample of IKEv1 servers, 2.6% of profiled servers preferred the 768-bit Oakley Group 1

Now regardless of the strong connections the authors missed, they did find a large number of weak ones. There is no excuse whatsoever for an IKEv2 server to allow modp768 or modp1024. It’s even questionable to allow modp1536 because every IKEv2 implementation supports modp2048. The 31.8% of modp768 is also inexcusable. The freeswan/openswan implementations never supported modp768 unless you specifically recompiled it (and the distro versions never had it compiled in). libreswan removed the compile time option altogether. The modp1536 group made it into freeswan – and preferred over modp1024 – before the modp1536 group was actually standardized in an RFC! So unless you specifically configured any of the *swan IKE implementations to use these weak groups, and your remote clients didn’t start out with a weak group, you would be using modp1536. Even back in 1997. This has not been an IKE problem in opensource IKE software ever. But, there is a catch.

The insecure Cisco administrator

86.1% and 91.0% respectively supported Oakley Group 2 (1024-bit)

This statistic matches my 15 year experience with IKE very well. It is very unfortunate that administrators fear reconfiguring a Cisco’s IPsec configuration. As such, when you need to interop with a Cisco, the other party usually has a SOP document that tells them how to configure the Cisco for use with 3DES-SHA1 modp1024. You are lucky if you are not forced to use md5. This is not a technical problem – Cisco VPN Concentrators can do modp2048. It is the humans that are afraid to touch something they are not familiar with that works.

MODP groups in the near future

The ipsecme working group at the IETF is actually working right now to update the set of mandatory to implement algorithms. This also includes promoting new algorithms to SHOULD or MUST and demoting others to MAY or MUST NOT. But it will only do so for the IKEv2 protocol. Most vendors have locked their IKEv1 stacks and will not make any changes to it anymore. So IKEv1 will always stay using modp1024 and modp1536. So if you can, use IKEv2.

Conclusion

The 66% of VPN number is surely very wrong. Measuring that number is per definition impossible, due to the common implementations of IKE daemons that hide the acceptance and rejection of the modp groups from un-authenticated IKE clients. However, I do agree that there are too many old IKE servers deployed that need to have their configurations upgraded to allow stronger modp groups and stronger encryption protocols.

If you want to go beyond modp1536, you should really upgrade to IKEv2. You will also need to move to IKEv2 if you want to move from AES-SHA1 to AES_GCM (or soon chacha20poly1305). Any IKE implementation that does not support IKEv2 is not actively maintained and should not be used.

See also my earlier article weak DH and LogJam attack impact on IKE

UPDATE: A more detailed explanation of the NO_PROPOSAL_CHOSEN case

The way IKE is specified and the way it works in practice is slightly different. The theory of operation is that a client starts with a list of proposals, that can include multiple DH modp groups. But that initial packet already contains the KE payload for one of the DH modp groups. So the client might send a proposal set containing 1024 + 1536 + 2048, but it only sends one KE payload. That KE payload basically denotes the client’s default. In this example, that would be either 1024 or 1536 or 2048. If the server receives this KE + proposal, it needs to make a decision. If the KE modp group is acceptable, use that. If not, then look if the proposal contains any other modp groups it is willing to use. If it does, it should return INVALID_KE with a payload of the acceptable modp group to use. The client receiving this can then select the other modp group it is willing to use and start the IKE exchange from scratch with a new request. (It’s a little more complicated because this reply is unauthenticated, so the client might wait for a bit to see if a “real server” accepts its modp). In practise, this often does not work. For example iphone ios8/ios9 do not process INVALID_KE payloads at all, as they assume a single correct modp group is configured on both sides using their enrollment program. So in practise, the server will accept the client’s DH if it is one of its allowed groups. This can mean that even though client and server both can do a more secure DH group, they end up using the most commonly used shared group. This is usually 1536 or 1024 for IKEv1 and 2048 for IKEv2.

An additional issue with IKEv1 is that the first packet also contains the OAKLEY_AUTHENTICATION_METHOD. If this is mismatched (eg PSK vs RSA) the IKE server will also return NO_PROPOSAL_CHOSEN.

And both both IKEv1 and IKEv2, the initial packet contains encryption/integrity algorithms too. If these mismatch (eg AES vs 3DES or SHA1 vs MD5) then the IKE server will also return NO_PROPOSAL_CHOSEN.

You can test the NO_PROPOSAL_CHOSEN return directed at vpn.nohats.ca. It is configured to accept connections from anyone, but requires modp1536 for either IKEv1 or IKEv2, using X.509/RSA authentication. You can send it PSK requests for modp1024 and get NO_PROPOSAL_CHOSEN.

A large dutch ISP ran into issues with their Registrar Network Solutions over the weekend. (why anyone would be using NetworkSolutions like it is 1993 is beyond me)

A misunderstanding over a payment caused hundreds of domains to be entered into PENDING-DELETE state. One such example is puiterwijk.org. Some of these domains have been in this state now for days.

What made things worse is that said NetworkSolutions took over running DNS for these domains, including an MX record that points to an actual mail server!

So if you were lucky, your email just bounced. If unlucky, someone else got your emails. The TLS certificate on that mail server doesn’t even match their hostname, so anyone on path can also just MITM it and a traceroute shows 26+ hops all over the place.

However, they did not modify the DS records after taking over the NS records and MX/A records. So those domains that used DNSSEC, including the above mentioned puiterwijk.org, are not resolving at all because the validators are rejecting the NetworkSolutions stolen DNS zones as bogus. So, my emails to this person were not delivered to the rogue MX servers because both he and I deployed DNSSEC. Hoorah, I guess….

Now, taking over MX and causing email failures like this is pretty evil. I would hope this violates some ICANN or PIR agreement but as said Registrar has been a sad registrar since uhm about 1993, I guess nothing is going to change. Let’s hope the Dutch ISP has learned from this and will move to another Registrar soon after this mess gets resolved.

TL;DR The LogJam downgrade attack does not apply to MODP groups in the IKE protocol, only to TLS, so IKE or IPsec is not impacted.

If you are using libreswan you are not vulnerable to weak MODP groups and using MODP2048 per default unless specifically configured for a lower MODP group.
If you are using openswan with IKEv2 you are using MODP2048, but if you are using IKEv1 you are using MODP1536 which is still much stronger than MODP768 or MODP1024.

Libreswan as a client to a weak server will allow MODP1024 in IKEv1 as the least secure option, and MODP1536 in IKEv2 as the least secure option.
Openswan does not properly implement INVALID_KE, so it cannot connect to another DH group than the one it started out as, so it runs the risk of getting locked out if the server side bumps their minimum MODP group to 2048. openswan defaults to MODP1536 in IKEv1 and MODP2048 in IKEv2

(minor update: Hugh Redelmeier corrected me that freeswan never supported modp768 – the smallest modp was 1024)

A bunch of smart crypto research released a paper describing the LogJam attack. It is a cryptographic attack on weak DiffieHellman groups. It talks about DH in various protocols. It is an interesting read. When you break the DH layer, you obtain the secret session keys needed to decrypt the various protocol (TLS, IPsec, SSH, etc) sessions. The conclusion from the paper is that any DH group smaller than 1024 should not be used as even non-nationstate attackers can obtain the resources needed to break these. They further conclude that they believe it very likely that the standarized 1024 bit DH MODP group has been broken as well by the NSA. It involves large scale computing clusters and sieving and doing a lot of heating up the planet. They can do this because in crypto standards, everyone uses the same MODP prime values for the DH exchange.

First, a small clarification. When the paper talks about IPsec, they mostly mean the IKE keying protocol, not the ESP packet encryption protocol. IKE is responsible for the secure keying operation that generates symmetric session keys used for the actual packet encryption in the ESP protocol. IKE is the userland daemon, ESP is the kernel level encryption/decryption.

The DH values for IKE are listed in various RFC’s, and can be found in the IANA registry for the older (but more used) IKEv1 GROUP values and for the newer but less used IKEv2 GROUP values. For the published attack, the only type of groups relevant are the MODP groups. The ECP groups do not use exponentiation and thus are not vulnerable to factoring primes.

IKEv1 MODP groups

In IKEv1, the most common modp groups are modp1024 (DH2) and modp1536 (DH5). Some people still want to use modp768 (DH1) but it has strongly decreased over time. Most IKEv1 stacks also support modp2048 (DH14) up to modp8192 (DH18) although notably Cisco administrators in general seem to have stuck with modp1024 and modp1536, just as they have stuck with 3DES and MD5/SHA1. This is mostly because the technical knowledge to configure Cisco’s for VPN is so difficult, that once a Standard Operating Procedure(SOP) is in place, no one wants to, or is allowed to, modify the configuration used. (It’s also the tail end of Cisco asking for extra money to support AES many years ago)

FreeS/WAN never supported modp768 – John Gilmore had good ideas about scaling and NSA computing powers, he was one of the people behind Deep Crack, the EFF 1DES cracker. Openswan which was forked from FreeS/WAN in 2003 had support for modp768 (and 1DES) but did not compile it in per default and neither did any of the Linux distributions that shipped it. But people needed it frequently enough to interoperate that the code was added, but disabled per default. Notably, embedded people using the *swan’s would enable it due to customer demand. It annoyed us so much that sometime around 2005 (I can’t find it in the changelog) we just ripped it out. Of course, it did not prevent people from patching it back in, which some did. Most likely, many unattended embedded devices out there are still doing modp768 bassed VPNs, and as LogJammer shows, that’s within reach of academia and banana republics – so count on these being decrypted routinely by the Five Eyes, Russia, China, and probably most other Western TLA’s.

The default IKEv1 modp group for openswan (and libreswan) was 1536. In July 2014, libreswan changed this to MODP2048. I believe openswan is still using modp1536 (and if you are still using openswan, I recommend that you switch ASAP, see this git commit activity comparison

IKEv2 MODP groups

IKEv2 has seen a very very slow start. I’ve only seen it really being deployed in the last two to three years. Most of these deployments use at least MODP2048, do not support MODP768, but are usually willing to do MODP1536 or even MODP1024. The latter is mostly done for fear of compatibility issues. But also to support an upgrade path for all devices. If your VPN connection supports “either IKEv1 or IKEv2” and your clients are still connecting with IKEv1, then most surely you will need to support MODP1536 and even MODP1024, or you will be losing your older VPN clients.

In April of 2008, as the then release manager of Openswan, we released version 2.6.12 which changed the IKEv2 default to MODP2048. This was a controversial decision – one developer quit the openswan project over this change because he deemed it would cause too many interoperability failures because many IKE implementations did not properly implement INVALID_KE renegotiation or simply did not support MODP2048. No problems emerged after the change, but most likely because simply no one but the certification checkbox people were even testing or running IKEv2 at the time. Since then, libreswan (but not openswan) has implemented proper INVALID_KE renegotiation.

RFC 5114 MODP groups

There is a semi-mysterious RFC 5114 – Additional Diffie-Hellman Groups document. It introduces new MODP groups not with higher sizes, but just with different primes. The justification in the RFC states

The purpose of this document is to provide the parameters and test
data for eight additional groups, in a format consistent with existing RFCs.

It lists one new 1024 bit MODP group and two 2048 bit MODP groups, when we already had RFC 3526, which has MODP groups up to 8192 bit. The odd thing is that when I talked to people in the IPsec community, no one really knew why this document was started. Nothing triggered this document, no one really wanted these, but no one really objected to it either, so the document (originating from Defense contractor BBN) made it to RFC status. Due to this strange history, the RFC 5114 MODP groups are not included in any default proposal set and MUST be manually configured before libreswan will use these groups. We had to support these groups due to demands for RFC compliance and certifications.

IKE and IPsec are not that bad you know

Not that I want to go into a nahnahnahnahnahnah mode, but to those people that keep dissing IKE and IPsec, note that we were again not vulnerable unlike TLS. And when it comes down to cipher mode attacks, IKE has proven to be far more secure than TLS has been in the last three years. So please stop beating up on IKE/IPsec citing some old papers from the late 90’s. Kthanks