Heartbleed

Summary

This page explains the background information of the "Heartbleed"- bug. The purpose of this documentation is not to guide attackers to reproduce the heartbleed attack, it should only highlight the functionality.

Requirements

• Webserver e.g. (nginx / apache)
• OpenSSL between version 1.0.1 and 1.0.1f (Download from https://github.com/openssl/openssl/branches/all)

Background

• HTTP communication usually takes place over TCP/IP connections
• TCP is reliable but, a continuous data exchange is needed.
• HTTPS Websites are encrypted with SSL
• The Webserver has available sockets which allows connections to clients
• Webserver have timers, after a set time of x - seconds the session gets closed if the client is inactive
• To keep the session alive, the web-browser sends heartbeat requests which are processed by the server

Heartbeat

• Is an extension in OpenSSL
• Special data package
• Possible size: 16 Bytes < Heartbeat Message < 64Kb
• “Heartbeat Requests” are sent from Webbrowser to Webserver to keep the session alive
• The server answers with a “Heartbeat Respond” which contains the exact same dataset

Example

Web browsers (client) sends a length and a message

• Web browser sends message with length 5: ‘hello’
• Web server responds with the exact same message : ‘hello’

Heartbleed

• Choose heartbeat request size as a minimum and claim it to the maximum
• The heartbeat request can be executed multiple times in a row
• Rebuild picture of servers RAM

Example

• Web browser sends message with length 65536: ‘hello’
• The server read a full 64 KiB from memory and sends it to the client
• Web server allocates a dataslot of 64 KiB and copies the message ‘hello’ in the first place. The rest of the 65536 - ‘hello’ size is appended with data in the RAM.
• mThe Web server responds with the filled 64 KiB dataslot (including content of RAM).

Attack

The result of a Hearbleed Bug is shown in the following picture.

Interpretation

The left hand column represents the HEAP address, the middle column contains the fetched data in a hexadecimal representation. The right hand column is the actual readable output. Here may stand user credentials, banking informations or even the encryption method.

(Reminder: the attacked server must implement an OpenSSL version between 1.0.1 and 1.0.1.f)