Let us start by HTTP
When we are browsing internet by typing a web address (URL/Server address) in a browser (client) starting with http we are using HTTP protocol. It functions as a request-response protocol in a client server model. That means client (say, browser) sends a request and server sends back a response. Note that browser is just one type of client. And a client is not necessarily has to be a browser.
HTTP is built over TCP
HTTP protocol is implemented at the top of a reliable transport layer protocol, mostly TCP.
HTTP has security problem
However an attacker can sit on the line between the client and the server and can read what has been transpired between the two. Not only that, an attacker can impersonate a server, i.e., act as if it is the server and can collect all client information that might be sensitive to be revealed.
There comes TLS/SSL to build HTTPS
Hence it is important to make sure that client is talking to the real server. It is also important that all communication between the client and the server is kept secret between the two. TLS/SSL comes into picture here. It implements transport layer security to protect the HTTP line and then the HTTP is called HTTPS. When you are typing a URL starting with https you are using a secured HTTP line.
TLS/SSL uses asymmetric encryption
Asymmetric encryption also known as public-key cryptography uses a key pair; a private key and a public key. The private key remains known only to the server while the public key is open/known to all. Data can be encrypted by any of the two keys. And then only the other key can decrypt that.
There are two things in public-key cryptography:
· Public key encryption – public key encrypts, private key decrypts
· Digital signature – private key encrypts, public key decrypts
When a server gets a certificate (X.509) from a certificate authority (CA), it essentially generates/gets a key pair. Then the certificate contains the public key of the website and a signature of the CA (i.e., signed by the private key of the CA) that issued the certificate among other identity information. When browser talks to a website, the website has to first provide its certificate.
Browser has a list of trusted CAs and their public keys. Browser can use the right CA’s (as mentioned in the certificate) public key to verify the certificate (signature).
And symmetric encryption
Now that client trust’s the website, it uses the public key of the website to encrypt a key that it would like to use as the symmetric key for further communication. Well, in reality there can be a number of steps here that essentially decides how the symmetric key to be generated/negotiated/used. Typically a disposable session key is generated from an initially (at the beginning of secure line creation) generated master secret to implement forward secrecy. That means, even if at any point the session key is compromised, only the data transferred using that session key will be compromised. But the attacker won’t be able to figure out the next session key that will be generated after a while.
Unlike asymmetric key where any of the two private and public key can encrypt data that only the other key can decrypt, in symmetric encryption it is a single key that does both the encryption and decryption. Symmetric encryption is better for performance, key distribution and helps to implement forward secrecy by continuously changing session keys.
Here comes OpenSSL
So if you want a server that would support TLS/SSL support you need a component to implement that. OpenSSL is such an open source implementation. There are many other implementations like GnuTLS etc.
But TCP has an issue
So now we have an HTTPS connection built over TCP where client is sending a request and server is giving back a response. But how long would the client wait for the server to respond? At times we do need to wait in a blocking call till the response comes back. But then at times TCP can fail to detect whether the other side is still alive. Especially, if the other side closes the connection without following a normal tear down process.
Also at times, firewalls sitting on the line close idle TCP connection.
Here comes heartbeat
To make sure client is not waiting for a response to come from a server sitting on the other side of a closed connection. And to make sure firewalls are not closing the connection when it is idle, a packet is periodically being sent to server. And the server sends back a response.
OpenSSL’s heartbeat implementation went wrong
OpenSSL’s hearbeat implementation went wrong and this bug was introduced in 2012. The buggy code could read up to around 64KB memory of the server process and send that back to client as each heartbeat response. Client could construct the message in a way that could to some extent dictate where/how much it wants to read from server. That would it turn allow the client to steal some sensitive information to exploit.
How to exploit Heartbleed bug?
A client as said need not be a browser. Rather one could write one’s own HTTPS client and construct the heartbeat message on his/her own. By using multiple and especially simultaneous heartbeat messages one can accumulate a large section of memory of the server process. After analysing the data one can possibly even get the server’s private key. If the server belongs to a e-commerce site one could possibly get the credit card information, user ids and passwords of the users who were using that site that time.
Even though as of now we were mostly focused on web servers it is not the only thing that gets affected. OpenSSL (heartbeat extension) is also used in email server, VPN and other TLS/SSL secured client server systems. Also it is not only the server but the client as well that can be affected.
There are few sites like ssllabs where one can paste the URL and check whether it is vulnerable. OpenSSL already released a fixed version. If you own a server that is vulnerable it is better to assume that it has been compromised. It is then best to change the certificate. If you are using a server where you have an Id and password, it is better to change your password.