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What is Heartbleed? A coding error that caused a security crisis

Josh Fruhlinger | Sept. 14, 2017
The mistake that caused the Heartbleed vulnerability can be traced to a single line of code in OpenSSL, an open source code library. Here's how Heartbleed works, how it was exploited, and how to fix it if you have an unpatched server.

Heartbleed is a vulnerability that came to light in April of 2014; it allowed attackers unprecedented access to sensitive information, and it was present on thousands of web servers, including those running major sites like Yahoo.

Heartbleed was caused by a flaw in OpenSSL, an open source code library that implemented the Transport Layer Security (TLS) and Secure Sockets Layer (SSL) protocols. In short, a malicious user could easily trick a vulnerable web server into sending sensitive information, including usernames and passwords.

The TLS/SSL standards are crucial for modern web encryption, and while the flaw was in the OpenSSL implementation rather than the standards themselves, OpenSSL is so widely used — when the bug was made public, it affected 17% of all SSL servers — that it precipitated a security crisis.


How Heartbleed works

To understand how the Heartbleed vulnerability (CVE-2014-0160) works, you need to know a little bit about how the TLS/SSL protocols operate, and how computers store information in memory.

One important part of the TLS/SSL protocols is what's called a heartbeat. Essentially, this is how the two computers communicating with one another let each other know that they're still connected even if the user isn't downloading or uploading anything at the moment. Occasionally, one of the computers will send an encrypted piece of data, called a heartbeat request, to the other. The second computer will reply back with the exact same encrypted piece of data, proving that the connection is still in place. Crucially, the heartbeat request includes information about its own length.

So, for example, if you're reading your Yahoo mail but haven't done anything in a while to load more information, your web browser might send a signal to Yahoo's servers saying, in essence, "This is a 40 KB message you're about to get. Repeat it all back to me." (The requests can be up to 64 KB long.) When Yahoo's servers receive that message, they allocate a memory buffer — a region of physical memory where it can store information — that's 40 KB long, based on the reported length of the heartbeat request. Next, it stores the encrypted data from the request into that memory buffer, then reads the data back out of it and sends it back to your web browser.

That's how it's supposed to work. The Heartbleed vulnerability arose because OpenSSL's implementation of the heartbeat functionality was missing a crucial safeguard: the computer that received the heartbeat request never checked to make sure the request was actually as long as it claimed to be. So if a request said it was 40 KB long but was actually only 20 KB, the receiving computer would set aside 40 KB of memory buffer, then store the 20 KB it actually received, then send back that 20 KB plus whatever happened to be in the next 20 KB of memory. That extra 20 KB of data is information that the attacker has now extracted from the web server.


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