5 ms·

Is there any way to detect that you're being MITM'd?

by mjs 12y ago

bradleyjg 12y ago
You check the certificate chain and compare it to a clean version. Obviously that's going to be a solution limited to very savvy users.
tptacek 12y ago
That's part of the point of certificate pinning.
aslewofmice 12y ago
Would love to know this as well. I only have a high level understanding of the purpose of CA Certs, but beyond that I'm lost. Ignorant questions ahoy: 1. Using Chrome, would you have to manually accept the MITM certificate? 2. Could such a certificate be valid across multiple domains? 3. Would it pose any threat to the computer if it was moved from the MITM network to an outside network? 4. What kind of potential problems could occur if I issued a self-signed certificate for my network?
- minot 12y ago
  Check the certificate store for the browser you are using. Mozilla Firefox has its own. Internet Explorer and Chrome on Windows rely on the Windows certificate store. http://windows.microsoft.com/en-us/windows-vista/view-or-manage-your-certificates http://windows.microsoft.com/en-us/windows-vista/view-or-man... Details on what certificates come with your installation of Mozilla Firefox: https://www.mozilla.org/en-US/about/governance/policies/security-group/certs/ https://www.mozilla.org/en-US/about/governance/policies/secu... Your organization may add their own certificates as described below: https://www.utexas.edu/its/help/user-certs/817 https://www.utexas.edu/its/help/user-certs/817
- Lynbarry 12y ago
  As far as I understand it (please someone correct me if I'm wrong): 1. In this case you would not have to manually accept anything, as the root certificate (the CNNIC cert) is already in your browser/os and the certificate chain for certs created by MCS would be OK (because their cert is signed by CNNIC). 2. As CNNIC issued them an intermediate CA cert, MCS was able to create certificates for any domain they wanted and these certificates would be considered valid by everyone that has CNNIC in the root store. So the MCS cert is not valid accross multiple domains, but it allows MCS to create certificates for every domain which kind of has the same consequences. 3. I think it would pose a threat when leaving the MITM network, but not as a consequence of having been in the MITM network. Only the root certificates are stored locally. Websites have to send a complete certificate chain that anchors their certs in one of the root certs. This means that the cert generated by MCS is not stored and therefore not used when leaving the network anymore. The danger is that this intermediate cert allows MCS to generate certs for any domain and use them outside their network, too. 4. A self signed certificate would have to be installed on the machines in the network. Otherwise users would get a certificate warning and would have to add the cert to their rootstores themselves. Other than that I think that this would grant you the same MITM-powers as this intermediate cert did for MCS, with the only restriction that you couldn't create certs for domains not in your control that would be accepted by users outside your network/that don'd have your self signed cert installed.
itistoday2 12y ago
> Is there any way to detect that you're being MITM'd? Yes, a project we're working on called DNSChain prevents MITM attacks for domains that have their info stored in a blockchain. We're maintaining a list of alternatives and how they compare to DNSChain here: https://github.com/okTurtles/dnschain/blob/master/docs/Comparison.md https://github.com/okTurtles/dnschain/blob/master/docs/Compa...
zaroth 12y ago
It's the golden unsolved, perhaps unsolvable problem in crypto. Trusting trust and such. There's always a key, somewhere, that has to be shared. One way to hack the system is if you have actual knowledge of the person you are talking to, and you assume some limited amount of tampering which can be done in real-time. For example, if I know the sound of your voice, and we want to agree on a key with no MITM, we can setup an audio channel and speak some code words to each other. Baring an adversary which can in real-time intercept and synthesize my voice convincingly speaking a different code, this is pretty secure. [1] [ZRTP] allows the detection of man-in-the-middle (MiTM) attacks by displaying a short authentication string (SAS) for the users to read and verbally compare over the phone. Another imperfect defense is spreading over time the data that an attacker would have to intercept and modify in order to MITM. That's what Chrome is doing with their pin lists. Now an adversary would have to alter the pinning when Chrome is downloaded. Of course in this very thread we're talking about technology which can do exactly that. E.g. technology which has any hope of preventing data exfiltration, would have an easy time altering Chrome's pin-list. Of course the Chrome binaries are signed, so there's another layer to defeat, etc. etc. So the end result is there are a lot of good technologies to prevent MITM. If you can keep the attacker out once, you can generally be confident your future conversations will be secure as well, since good protocols don't start from scratch each time, but rather "ratchet" new keys from the old as you go. [2] One of the big trade-offs is false positives and privacy. For example, it might be nice if my browser remembered the public key of a site I visit, like HN, and let me know if it changed. Two issues are a naive implementation would also serve as a great tracker for every site I've visited, and how do I know if when I get a warning, it's a real attack and not just an expiring certificate rotating out? Now we would need a way for sites to indicate, by signing with their old key, that indeed they are switching to a new key, and complexity explodes from there. [1] - http://blog.cryptographyengineering.com/2012/11/lets-talk-about-zrtp.html http://blog.cryptographyengineering.com/2012/11/lets-talk-ab... [2] - https://whispersystems.org/blog/advanced-ratcheting/ https://whispersystems.org/blog/advanced-ratcheting/
- mjs 12y ago
  > It's the golden unsolved, perhaps unsolvable problem in crypto. Sorry, perhaps my question wasn't clear. I wasn't asking about MITM in the general case, I was asking about this particular case. The certificate chain for Hacker News seems to go AddTrust -> COMODO -> Another COMODO -> *.ycombinator.com. So in this case, if you're MITM'd by MCS Holdings, is MCS Holdings going to be part of the chain (after a CNNIC)?
JeffCyr 12y ago
For web site with authentication (e.g. bank account), protocols like SRP (Secure Remote Password) would prevent the man-in-the-middle if he doesn't know your password. SRP is a mutual authentication protocol with zero knowledge and forward secrecy, it would be nice if major browsers supported it, it's not usable without browser support.

Is there any way to detect that you're being MITM'd?

by mjs 12y ago

Is there any way to detect that you're being MITM'd?

You check the certificate chain and compare it to a clean version. Obviously that's going to be a solution limited to very savvy users.

tptacek 12y ago

That's part of the point of certificate pinning.

aslewofmice 12y ago

Would love to know this as well. I only have a high level understanding of the purpose of CA Certs, but beyond that I'm lost. Ignorant questions ahoy: 1. Using Chrome, would you have to manually accept the MITM certificate? 2. Could such a certificate be valid across multiple domains? 3. Would it pose any threat to the computer if it was moved from the MITM network to an outside network? 4. What kind of potential problems could occur if I issued a self-signed certificate for my network?

minot 12y ago
Check the certificate store for the browser you are using. Mozilla Firefox has its own. Internet Explorer and Chrome on Windows rely on the Windows certificate store. http://windows.microsoft.com/en-us/windows-vista/view-or-manage-your-certificates http://windows.microsoft.com/en-us/windows-vista/view-or-man... Details on what certificates come with your installation of Mozilla Firefox: https://www.mozilla.org/en-US/about/governance/policies/security-group/certs/ https://www.mozilla.org/en-US/about/governance/policies/secu... Your organization may add their own certificates as described below: https://www.utexas.edu/its/help/user-certs/817 https://www.utexas.edu/its/help/user-certs/817
Lynbarry 12y ago
As far as I understand it (please someone correct me if I'm wrong): 1. In this case you would not have to manually accept anything, as the root certificate (the CNNIC cert) is already in your browser/os and the certificate chain for certs created by MCS would be OK (because their cert is signed by CNNIC). 2. As CNNIC issued them an intermediate CA cert, MCS was able to create certificates for any domain they wanted and these certificates would be considered valid by everyone that has CNNIC in the root store. So the MCS cert is not valid accross multiple domains, but it allows MCS to create certificates for every domain which kind of has the same consequences. 3. I think it would pose a threat when leaving the MITM network, but not as a consequence of having been in the MITM network. Only the root certificates are stored locally. Websites have to send a complete certificate chain that anchors their certs in one of the root certs. This means that the cert generated by MCS is not stored and therefore not used when leaving the network anymore. The danger is that this intermediate cert allows MCS to generate certs for any domain and use them outside their network, too. 4. A self signed certificate would have to be installed on the machines in the network. Otherwise users would get a certificate warning and would have to add the cert to their rootstores themselves. Other than that I think that this would grant you the same MITM-powers as this intermediate cert did for MCS, with the only restriction that you couldn't create certs for domains not in your control that would be accepted by users outside your network/that don'd have your self signed cert installed.

itistoday2 12y ago

> Is there any way to detect that you're being MITM'd? Yes, a project we're working on called DNSChain prevents MITM attacks for domains that have their info stored in a blockchain. We're maintaining a list of alternatives and how they compare to DNSChain here: https://github.com/okTurtles/dnschain/blob/master/docs/Comparison.md https://github.com/okTurtles/dnschain/blob/master/docs/Compa...

zaroth 12y ago

It's the golden unsolved, perhaps unsolvable problem in crypto. Trusting trust and such. There's always a key, somewhere, that has to be shared. One way to hack the system is if you have actual knowledge of the person you are talking to, and you assume some limited amount of tampering which can be done in real-time. For example, if I know the sound of your voice, and we want to agree on a key with no MITM, we can setup an audio channel and speak some code words to each other. Baring an adversary which can in real-time intercept and synthesize my voice convincingly speaking a different code, this is pretty secure. [1] [ZRTP] allows the detection of man-in-the-middle (MiTM) attacks by displaying a short authentication string (SAS) for the users to read and verbally compare over the phone. Another imperfect defense is spreading over time the data that an attacker would have to intercept and modify in order to MITM. That's what Chrome is doing with their pin lists. Now an adversary would have to alter the pinning when Chrome is downloaded. Of course in this very thread we're talking about technology which can do exactly that. E.g. technology which has any hope of preventing data exfiltration, would have an easy time altering Chrome's pin-list. Of course the Chrome binaries are signed, so there's another layer to defeat, etc. etc. So the end result is there are a lot of good technologies to prevent MITM. If you can keep the attacker out once, you can generally be confident your future conversations will be secure as well, since good protocols don't start from scratch each time, but rather "ratchet" new keys from the old as you go. [2] One of the big trade-offs is false positives and privacy. For example, it might be nice if my browser remembered the public key of a site I visit, like HN, and let me know if it changed. Two issues are a naive implementation would also serve as a great tracker for every site I've visited, and how do I know if when I get a warning, it's a real attack and not just an expiring certificate rotating out? Now we would need a way for sites to indicate, by signing with their old key, that indeed they are switching to a new key, and complexity explodes from there. [1] - http://blog.cryptographyengineering.com/2012/11/lets-talk-about-zrtp.html http://blog.cryptographyengineering.com/2012/11/lets-talk-ab... [2] - https://whispersystems.org/blog/advanced-ratcheting/ https://whispersystems.org/blog/advanced-ratcheting/

mjs 12y ago
> It's the golden unsolved, perhaps unsolvable problem in crypto. Sorry, perhaps my question wasn't clear. I wasn't asking about MITM in the general case, I was asking about this particular case. The certificate chain for Hacker News seems to go AddTrust -> COMODO -> Another COMODO -> *.ycombinator.com. So in this case, if you're MITM'd by MCS Holdings, is MCS Holdings going to be part of the chain (after a CNNIC)?

JeffCyr 12y ago

For web site with authentication (e.g. bank account), protocols like SRP (Secure Remote Password) would prevent the man-in-the-middle if he doesn't know your password. SRP is a mutual authentication protocol with zero knowledge and forward secrecy, it would be nice if major browsers supported it, it's not usable without browser support.