Bug is in the OpenSSL's implementation of the TLS/DTLS (transport layer security protocols) heartbeat extension (RFC6520). When it is exploited it leads to the leak of memory contents from the server to the client and from the client to the server. What makes the Heartbleed Bug unique? The bug allows for reading memory of systems protected by the vulnerable OpenSSL versions and could allow for disclosure of otherwise encrypted confidential information as well as the encryption keys themselves.

  1. Openssl Bug Bites
  2. Openssl Big Number
  3. Openssl Bug Spray

For a more detailed analysis of this catastrophic bug, see this update, which went live about 18 hours after Ars published this initial post.

Researchers have discovered an extremely critical defect in the cryptographic software library an estimated two-thirds of Web servers use to identify themselves to end users and prevent the eavesdropping of passwords, banking credentials, and other sensitive data.

Heartbleed OpenSSL Bug Reveals the True Cost of Open-Source Software What checking that does happen can take years. That’s what happened with OpenSSL, and is the cause of the Heartbleed bug. To exploit the bug, a TLS client asks for renegotiation but deliberately leaves out one of the settings it used when it first connected. The OpenSSL server code fails to notice that the needed data was not supplied this time, and incorrectly tries to use the non-existent data anyway, given that it was used last time. 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.

The warning about the bug in OpenSSL coincided with the release of version 1.0.1g of the open-source program, which is the default cryptographic library used in the Apache and nginx Web server applications, as well as a wide variety of operating systems and e-mail and instant-messaging clients. The bug, which has resided in production versions of OpenSSL for more than two years, could make it possible for people to recover the private encryption key at the heart of the digital certificates used to authenticate Internet servers and to encrypt data traveling between them and end users. Attacks leave no traces in server logs, so there's no way of knowing if the bug has been actively exploited. Still, the risk is extraordinary, given the ability to disclose keys, passwords, and other credentials that could be used in future compromises.

'Bugs in single software or library come and go and are fixed by new versions,' the researchers who discovered the vulnerability wrote in a blog post published Monday. 'However this bug has left a large amount of private keys and other secrets exposed to the Internet. Considering the long exposure, ease of exploitations and attacks leaving no trace this exposure should be taken seriously.'

The researchers, who work at Google and software security firm Codenomicon, said even after vulnerable websites install the OpenSSL patch, they may still remain vulnerable to attacks. The risk stems from the possibility that attackers already exploited the vulnerability to recover the private key of the digital certificate, passwords used to administer the sites, or authentication cookies and similar credentials used to validate users to restricted parts of a website. Fully recovering from the two-year-long vulnerability may also require revoking any exposed keys, reissuing new keys, and invalidating all session keys and session cookies. Members of the Tor anonymity project have a brief write-up of the bug here, and a this analysis provides useful technical details.


OpenSSL is by far the Internet's most popular open-source cryptographic library and TLS implementation. It is the default encryption engine for Apache, nginx, which according to Netcraft runs 66 percent of websites. OpenSSL also ships in a wide variety of operating systems and applications, including the Debian Wheezy, Ubuntu, CENTOS, Fedora, OpenBSD, FreeBSD, and OpenSUSE distributions of Linux. The missing bounds check in the handling of the Transport Layer Security (TLS) heartbeat extension affects OpenSSL 1.0.1 through 1.0.1f.

The bug, which is officially referenced as CVE-2014-0160, makes it possible for attackers to recover up to 64 kilobytes of memory from the server or client computer running a vulnerable OpenSSL version. Nick Sullivan, a systems engineer at CloudFlare, a content delivery network that patched the OpenSSL vulnerability last week, said his company is still evaluating the likelihood that private keys appeared in memory and were recovered by attackers who knew how to exploit the flaw before the disclosure. Based on the results of the assessment, the company may decide to replace its underlying TLS certificate or take other actions, he said.

Attacking from the outside

The researchers who discovered the vulnerability, however, were less optimistic about the risks, saying the bug makes it possible for attackers to surreptitiously bypass virtually all TLS protections and to retrieve sensitive data residing in the memory of computers or servers running OpenSSL-powered software.

'We attacked ourselves from outside, without leaving a trace,' they wrote. 'Without using any privileged information or credentials we were able steal from ourselves the secret keys used for our X.509 certificates, user names and passwords, instant messages, emails and business critical documents and communication.'

They called on white-hat hackers to set up 'honeypots' of vulnerable TLS servers designed to entrap attackers in an attempt to see if the bug is being actively exploited in the wild. The researchers have dubbed the vulnerability Heartbleed because the underlying bug resides in the OpenSSL implementation of the TLS heartbeat extension as described in RFC 6520 of the Internet Engineering Task Force.

The OpenSSL vulnerability is the latest to threaten the HTTPS scheme that's the default and often only method for cryptographically protecting websites, e-mail, an other Internet communications from attacks that allow hackers to eavesdrop on end users or impersonate trusted websites. Last month, developers of the GnuTLS library disclosed an equally catastrophic bug that left hundreds of open-source applications open to similar attacks. And in February, Apple fixed an extremely critical vulnerability in the iOS and OS X operating systems that also made it possible for hackers to bypass HTTPS protections.

This post has been updated throughout to add newly available links and details.

We’re sure you’ve heard of OpenSSL, and even if you aren’t a coder yourself, you’ve almost certainly used it.

OpenSSL is one of the most popular open-source cryptography libraries out there, and lots of well-known products rely on it, especially on Linux, which doesn’t have a standard, built-in encryption toolkit of its own.

Even on Windows and macOS, which do have encryption toolkits built into their distributions, you may have software installed that includes and uses OpenSSL instead of the operating system’s standard cryptographic libraries.

As its name suggests, OpenSSL is very commonly used for supporting network-based encryption using TLS, which is the contemporary name for what used to be called SSL.

TLS, or transport layer security, is what puts the padlock into your browser, and it’s probably what encrypts your email in transit these days, along with protecting many other online communications initiated by your computer.

So, when an OpenSSL security advisory reports exploitable vulnerabilities in the software…

…it’s worth paying attention, and upgrading as soon as you can.

The latest patches, which came out in OpenSSL 1.1.1k on 2021-03-25, fix two high-severity bugs that you should definitely know about:

  • CVE-2021-3449: Crash can be provoked when connecting to a vulnerable server.
  • CVE-2021-3450: Vulnerable client can be tricked into accepting a bogus TLS certificate.

Vulnerabilities compared

Even though we think the second bug is the more interesting of the two, we’ve seen several reports that have focused their attention on the first one, perhaps because it threatens immediate and disruptive drama.

The bug can be triggered by a TLS feature called renegotiation, where two computers that are already connected over TLS agree to set up a new secure connection, typically with different (supposedly more secure) settings.

To exploit the bug, a TLS client asks for renegotiation but deliberately leaves out one of the settings it used when it first connected.

The OpenSSL server code fails to notice that the needed data was not supplied this time, and incorrectly tries to use the non-existent data anyway, given that it was used last time…

…thus reading from a non-existent memory location, causing the server software to crash.

This means that a malicious client could, in theory, deliberately crash a vulnerable web server or email server at will, leading to a dangerous Denial of Service (DoS) situation that could be repeated ad nauseam every time the server came back up.

Session renegotiation, which is complex and considered error-prone (an opinion that is only strengthened by the appearance of this bug), was removed from TLS 1.3, the latest version of the protocol. However, very few web servers we know of have switched entirely to TLS 1.3 yet, and will still happily accept TLS 1.2 connections for reasons of backwards compatibility. You can turn off renegotiation for TLS 1.2 if you want, but it’s enabled by default in OpenSSL. Many servers that rely on OpenSSL may therefore be vulnerable to this flaw.

The second bug, CVE-2021-3450, is slightly more complex to exploit, but could end up being more damaging than a DoS attack, because it allows security checks to be circumvented.

After all, in many ways, a server that stops working altogether, as disruptive as that sounds, is better than a server that keeps on running but that behaves insecurely.

When STRICT means less secure

The CVE-2021-3450 vulnerability involves a special setting that an OpenSSL client program can turn on called X509_V_­FLAG_­X509_STRICT. (We’ll shorten this from now on to just X509_STRICT.)

This setting, which is not enabled by default, tells the OpenSSL code to perform additional checks when it is establishing a TLS connection.

Ironically, however, turning it on activates a dangerous bug.

As you probably know, the server side of a TLS connection usually submits a so-called digital certificate right at the start of proceedings.

This certificate asserts that the holder of the certificate has the right to operate the domain name that you just connected to, e.g. www.sophos.com, and includes a digital signature from a third party, known as a CA, that vouches for that assertion.

CA is short for certificate authority, a company that is supposed to check up on newly-created certificates to verify that the certificate creator does indeed have the authority over the domain name that they claim, after which the CA signs and issues the certificate, as depicted here:

Without CA verification, literally anyone could issue certificates for literally any domain name, including those for well-known brands and services, and you would have no way of telling that they were an imposter.

So, your browser, or whatever program is setting up the TLS connection, typically checks the certificates it receives to ensure that they are correctly signed by a CA, and then looks up that CA in a list of “trusted authorities” that either the browser or your operating system considers competent to sign certificates.

If the signature checks out and the CA checks out, then the TLS connection is considered verified; if not, you will see one of those “certificate warning” pages that fraudulent or misconfigured sites provoke.

Certificate checking in OpenSSL

Very greatly simplified, OpenSSL has code that looks like this to verify the CA of a certificate before it validates a connection:

However, as mentioned above, there’s a non-default X509_STRICT option to do some extra certificate checks, including a special check that was introduced recently (in OpenSSL 1.1.1h, just three versions ago) to detect the use of non-standard cryptographic settings.

We won’t go into detail here, but you need to know that one sort of TLS certificate uses what is called Elliptic Curve Cryptography (ECC), which is an algorithm based on mathematical computations using equations that define what are known as elliptic curves.

If you did high school mathematics, you may rememer x2 + y2 = 1 as the equation for a conventional circle, which is just an ellipse that is perfectly round, and (x/A)2 + (y/B)2 = 1 as the equation for ellipses that look more like rugby balls.

In this formula, A and B are parameters that determine the width and the height of the resulting shape.

The elliptical formulas and calculations used in ECC are somewhat more complex and include a greater number of curve parameters, which aren’t meant to be secret, but that must nevertheless be chosen wisely.

For an analogy of why parameters matter in elliptical formulas, consider the “oval” ellipses you studied at school. In the formula we gave above, for example, you mustn’t let A or B be zero or the formula won’t work at all. And if you make A very tiny and B very large then you will end up with a super-stretched ellipse that will look like a stick if you draw a graph, and will be much harder to work with than if you simply chose, say, A=3 and B=2.

Unfortunately, choosing ECC parameters carelessly could result in weakened encryption.

Even worse, attackers could deliberately choose bad parameters to weaken the encryption on purpose, in order to boost their chances of hacking into your network traffic later on.

As a result, various standards bodies have come up with lists of supposedly “known good” ECC parameters that you are expected to choose from in order to avoid this problem.

And, from OpenSSL 1.1.1h and later, turning on OpenSSL’s X509_STRICT mode causes the code to ensure that any TLS connections that rely on ECC use only standard elliptic curve settings.

Openssl Bug Bites

The updated code goes something like this:

If you read the code above carefully, you will see that if an attacker wants to present a fake certificate that is not correctly verified by a CA, and knows you have strict checks enabled…

…then if they configure their server to use a bog-standard elliptic curve certificate with standard parameters, the certificate test above will always succeed at the end, even if the CA verification step failed earlier on.

Openssl Big Number

Almost all web browsers these days will accept either RSA or Elliptic Curve Cryptography certificates. ECC certificates are increasingly popular because they’re typically a lot smaller than RSA certificates with a comparable security strength. That’s a simple side-effect of the size of the numbers used in the mathematical calculations that go on behind the scenes in ECC and RSA cryptography.

In the code, you can see that if the CA check fails then the variable result is set to BAD in order to remember that there was an error.

But if the certificate is using ECC with standard parameters, and strict checking is turned on, then the variable result later gets “upgraded” to GOOD when the ECC check is done, and the previous error simply gets overwritten.

So the code correctly detects that the certificate is fake, but then “forgets” that fact and reports that the certificate is valid instead.

Openssl bug 2021

The CVE-2021-3450 bug is strangely reminiscent of Apple’s infamous “goto fail” flaw (CVE-2014-1266) from 2014. In that vulnerability, careless coding in the TLS connection verification function meant that by deliberately provoking what should have been an error, you could skip over the certificate validation process entirely. At the 2014 Black Hat conference, that bug notoriously won Apple the Pwnie Award for Most Epic Fail, one of a number of humorous “prizes” given out each year for security blunders made during the previous 12 months.

What to do?

  • Upgrade to OpenSSL 1.1.1k. If you are still using earlier versions that are no longer supported, you will need to examine the code yourself to see if these vulnerabilities apply to your software, and if so to make your own patches if needed.
  • Turn off TLS 1.2 renegotiation. A client can only exploit CVE-2021-3449 if TLS renegotiation is allowed. It’s enabled by default but if your server doesn’t require it, turning it off will sidestep the Denial of Service bug described above.
  • Don’t use X509_STRICT mode. The CVE-2021-3450 bug gets sidestepped if strict certificate checking is turned off. If you can manage without the additional certificate checks (they are, after all, not on by default) then this may be the lesser of two evils until you can upgrade to version 1.1.1k.

Also, if you are a programmer, try not to write error-checking code the way that it was done in OpenSSL’s certificate verification routines.

There are several other approaches you can take:

Openssl Bug Spray

  • Bail out at the first error you detect. If you aren’t interested in accumulating and reporting a complete list of errors, but merely in ensuring that there aren’t any, you reduce the chance of mistakes by returning BAD as soon as you know something is wrong.
  • Only allow one type of assignment to your result value. If you start by assuming no errors, set your result variable to GOOD at the start and change its value to BAD every time you find an error. It’s easier to review your error-checking function if you don’t have anywhere in the code path where the value can get reset to GOOD.
  • Count the number of errors encountered, starting from zero. If you want to report all the errors as you find them, increment a counter every time instead of using a simple GOOD/BAD (boolean) variable. That way, you can’t accidentally lose track of errors you previously encountered. At the end, simply test that there were zero errors in total before declaring the overall outcome as GOOD.
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