4 ms·

I wouldn't have thought this an issue if you're behind an AWS WAF or LB? (and assuming AWS is patched already)

by beny23 5y ago

acdha 5y ago
For an intermediary like a WAF, load-balancer, or CDN to help it needs to do all of the certificate processing and keep your backend app from seeing the certificate at all. If you are terminating HTTPS at the edge and using HTTP internally that's very likely but if you're using an NLB instead of an ALB, it's not. Similarly, you'd also want to see if there's any application-level path where a certificate would be included in an uploaded file but that's far less common.
jamespwilliams 5y ago
I think nearly everything at AWS uses s2n and not OpenSSL these days
- justinsaccount 5y ago
  Has s2n been fips certified? I thought some things (govcloud?) were still using openssl due to fips requirements.
  - znep 5y ago
    I'm not sure what they are using right now, but they currently have the "AWS-LC Cryptographic Module" listed as being in process for validation which may be sufficient for s2n. I know there are other AWS features/services that are lined up behind that validation for FIPS support.
jcims 5y ago
Depends if s2n is vulnerable to the same bug. Of course AWS doesn't protect you against application bugs so if your application has an SSRF vulnerability (presumably otherwise unexploitable) an attacker might be able to DoS you. They don't mention S/MIME or things like signed binaries or SAML assertions but those seem plausibly vulnerable as well.
nazlorenzo 5y ago
This vulnerability affects parsing maliciously crafted certificates, so it will mostly affect clients. If your app is fetching data from a 3rd party and validating its certificate, it may be vulnerable, regardless of how you are fronting requests to your site.
- lmilcin 5y ago
  > This vulnerability affects parsing maliciously crafted certificates, so it will mostly affect clients. Actually, it is the opposite. You seem to be unaware of the fact that servers do receive certificates from the clients which are then parsed. Which is already mentioned in the advisory document: "Thus vulnerable situations include: - TLS clients consuming server certificates - TLS servers consuming client certificates <---- here - Hosting providers taking certificates or private keys from customers - Certificate authorities parsing certification requests from subscribers - Anything else which parses ASN.1 elliptic curve parameters"
  - alexw91 5y ago
    That's not how the TLS handshake works. The TLS server must be configured to request a certificate from the client in order for the client to know that it needs to send a client certificate to the server, and that server-side configuration is disabled for ~99%+ of endpoints. TLS server implementations should be aborting the TLS connection for violating the TLS Handshake state machine if a client attempts to send a client certificate when it wasn't requested. So while this bug affects both clients and servers, 100% of clients are parsing the server's TLS cert during the TLS handshake, but less than ~1% of servers are parsing a client's certificate during a handshake.
    - lmilcin 5y ago
      There is very little reason to DOS a client and a lot of reasons to attack servers. There is a huge number of public facing services that implement mutual auth and all those are potentially vulnerable to DOS. While clients can just decide to not connect to a web service that causes their browser to malfunction (and why have you connected there in the first place?), services are usually not at liberty to ignore a client at this stage. So yes, those servers that do request client certificate are targets and my point still stands that servers are much more affected than the clients. What would be an affected client? You keep connecting to this infected website that causes your browser to die? Somebody embedded some tracking on their page that now points to an infected website? Everybody will just move on and it is hard to say you are very much affected by this problem. Whereas if you are a service and you are affected you absolutely need to implement a fix.

I wouldn't have thought this an issue if you're behind an AWS WAF or LB? (and assuming AWS is patched already)

by beny23 5y ago

I wouldn't have thought this an issue if you're behind an AWS WAF or LB? (and assuming AWS is patched already)

For an intermediary like a WAF, load-balancer, or CDN to help it needs to do all of the certificate processing and keep your backend app from seeing the certificate at all. If you are terminating HTTPS at the edge and using HTTP internally that's very likely but if you're using an NLB instead of an ALB, it's not. Similarly, you'd also want to see if there's any application-level path where a certificate would be included in an uploaded file but that's far less common.

jamespwilliams 5y ago

I think nearly everything at AWS uses s2n and not OpenSSL these days

justinsaccount 5y ago
Has s2n been fips certified? I thought some things (govcloud?) were still using openssl due to fips requirements.
- znep 5y ago
  I'm not sure what they are using right now, but they currently have the "AWS-LC Cryptographic Module" listed as being in process for validation which may be sufficient for s2n. I know there are other AWS features/services that are lined up behind that validation for FIPS support.

jcims 5y ago

Depends if s2n is vulnerable to the same bug. Of course AWS doesn't protect you against application bugs so if your application has an SSRF vulnerability (presumably otherwise unexploitable) an attacker might be able to DoS you. They don't mention S/MIME or things like signed binaries or SAML assertions but those seem plausibly vulnerable as well.

nazlorenzo 5y ago

This vulnerability affects parsing maliciously crafted certificates, so it will mostly affect clients. If your app is fetching data from a 3rd party and validating its certificate, it may be vulnerable, regardless of how you are fronting requests to your site.

lmilcin 5y ago
> This vulnerability affects parsing maliciously crafted certificates, so it will mostly affect clients. Actually, it is the opposite. You seem to be unaware of the fact that servers do receive certificates from the clients which are then parsed. Which is already mentioned in the advisory document: "Thus vulnerable situations include: - TLS clients consuming server certificates - TLS servers consuming client certificates <---- here - Hosting providers taking certificates or private keys from customers - Certificate authorities parsing certification requests from subscribers - Anything else which parses ASN.1 elliptic curve parameters"
- alexw91 5y ago
  That's not how the TLS handshake works. The TLS server must be configured to request a certificate from the client in order for the client to know that it needs to send a client certificate to the server, and that server-side configuration is disabled for ~99%+ of endpoints. TLS server implementations should be aborting the TLS connection for violating the TLS Handshake state machine if a client attempts to send a client certificate when it wasn't requested. So while this bug affects both clients and servers, 100% of clients are parsing the server's TLS cert during the TLS handshake, but less than ~1% of servers are parsing a client's certificate during a handshake.
  - lmilcin 5y ago
    There is very little reason to DOS a client and a lot of reasons to attack servers. There is a huge number of public facing services that implement mutual auth and all those are potentially vulnerable to DOS. While clients can just decide to not connect to a web service that causes their browser to malfunction (and why have you connected there in the first place?), services are usually not at liberty to ignore a client at this stage. So yes, those servers that do request client certificate are targets and my point still stands that servers are much more affected than the clients. What would be an affected client? You keep connecting to this infected website that causes your browser to die? Somebody embedded some tracking on their page that now points to an infected website? Everybody will just move on and it is hard to say you are very much affected by this problem. Whereas if you are a service and you are affected you absolutely need to implement a fix.