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Cloud Security: Blog Feed Post

Where You Mitigate Heartbleed Matters

There’s the impact on gadgets and devices we might not immediately think of being vulnerable

There are a variety of opinions on the seriousness of Heartbleed being put forth ranging from "it's not the end of the world" to "the sky is falling, duck and cover." Usually the former cites the relatively low percentage of sites impacted by Heartbleed, pegged at about 17% or 500,000 sites by Netcraft. The latter cite the number of consumers impacted, which is a way bigger number to be sure. Sites tracking the impact to users suggest many of the largest sites have potentially been impacted, translating into many millions of users.

And then there’s the impact on gadgets and devices we might not immediately think of being vulnerable. A wide variety of smart phones, IP phones, switches and routers have been identified as being vulnerable. Home internet routers and that nifty system you had put in that lets you mess with your house’s temperature from any device, anywhere are likely impacted. With the Internet of Things connecting more and more devices it’s likely that list will only continue to grow. The growing consensus is that a plurality of the impacted devices will never be updated; leaving organizations that may interact with those devices vulnerable and in need of a mitigating solution that doesn’t rely on updates or changes to the device.

There will be, as everyone scrambles to protect customers and consumers from Heartbleed, a variety of mitigating solutions offered up to address this pesky bug. Network devices will enable organizations with the visibility necessary to detect and reject requests attempting to exploit the vulnerability.

There are a variety of points within the data path where solutions could be put into place to mitigate this (and similar) vulnerabilities. Thus customers must choose the most strategic point in the network at which to deploy their selected mitigation. To choose that point, organizations should ask how the exploit is detected by given solutions. To see why that's needful, consider how the attack works.

How Heartbleed Works



Heartbleed takes advantage of a missing length check in the OpenSSL code handling a relatively innocuous extension to the TSL/SSL protocol (defined in RFC 6520). It comprises two simple messages: a request and a response. The request can be sent be either the client or the server as a means to keep the connection alive. The sender ships off a HeartbeatMessage with a small amount of data, expecting the receiver to send back that same data. What's important about the protocol interaction is that whichever party sends the request determines the length of the response. The sender tells the receiver how much data it's sending - and thus how much should be returned.

Now, the OpenSSL code should be making sure the length the attacker says he's sending is actually what's available. The code, however, does not. It simply trusts the sender and grabs whatever amount of data was specified out of memory. This is how an attacker can access data that's in memory and wind up with all sorts of sensitive data like passwords and private keys.

Mitigation Options

Because this exploit takes advantage of a vulnerability in encrypted communications, any mitigating solution must be in the path of that communication. That's a given. In that path are three points where this exploit can be mitigated:

1. Client. You can check the client operating system and device type and match that against known usage of the impacted OpenSSL versions. Once detected, the client can be rejected - preventing the offending request from ever being sent in the first place. Rejection of clients based on the possibility they might be an attacker can result in angry legitimate consumers, employees or partners, however.

2. On Request. Inspect client requests and upon discovery of a HeartbeatMessage, reject it. This prevents the request from being forwarding to vulnerable systems and servers.

3. On Response. Inspect responses and upon seeing a HeartbeatMessage response, check its length. If it's greater than a length you feel comfortable with, discard it. This method will prevent attackers from receiving sensitive data, but it should be noted that at the point of discovery, the server - and data - has already been compromised.

Location in the Network Matters
You have to be in communication path to implement these solutions. That means some solutions being put forth are architecturally misplaced to be able to completely mitigate this vulnerability. For example, the firewall landscape is bifurcating and separating inbound (application delivery) and outbound (next generation firewall) duties. That means while next-generation firewalls (NGFW) are capable of the inspection and interaction necessary to detect and mitigate Heartbleed on response, they generally only do so in the outbound use case. That's an important capability, but it won't catch inbound attempts, just outbound. Further complicating the situation is a growing delineation of security responsibilities between inbound and outbound in the firewall market. Growth and scale of security has led to separate inbound and outbound security solutions. NGFW are an outbound solution, generally positioned only as protection for corporate users. They’re intended to protect organizations from malware and malicious code entering the corporate data center by means of its employees accessing infected sites. They aren’t deployed in a position to protect servers and applications on the inbound path. Those that are can provide inbound protection but only on response, which means your servers have already been compromised.

The right place to implement a mitigating solution is one that will afford you the choice of your mitigating solution - or allow all three, if you really want comprehensive coverage. It must be in the data path and have visibility into both the client and the server side of the equation. In most networks, that strategic point of control is the application delivery firewall.


Using the right tool in the right place in the network means you can implement any (or all) of the three mitigating solutions in not only a one place, but in the most effective place. The right tool is not just one that has the right position in the network. It takes visibility and programmability to dig deeply into the network stack and find the data indicative of an attack – intentional or not. The right tool will be able to distinguish between client side and server side traffic and apply the applicable logic. The logic that detects Heartbleed on the client side is different than that of the server side. In the case of the client it must look for a specific message indicating a Heartbeat request or inspecting the client device environment itself. On the server side, it’s checking the size of the response. Each of these cases requires unique code. That means the right tool must have a programmatic environment that can execute with surgical-like precision the logic necessary at the right time – at the time of connection, on request and on response.

The right tool, then, is positioned on the inbound path – in front of vulnerable services – and offers an event-driven, programmatic way to execute the right logic at the right time to detect vulnerable clients, malicious requests and responses carrying unauthorized sensitive data. An F5 ADC offers that event-driven, programmatic interface with iRules and is strategically positioned in the network to support all three mitigation solutions.
Consider again how Heartbleed works and the three mitigation options:


(1) Client. In most network architectures this means it is connecting to an application delivery controller (ADC) that provides load balancing services. When that ADC is F5, it also acts as an application delivery firewall (ADF) and can be programmatically controlled.  That means it can inspect the request and, if it's vulnerable, reject the connection.

(2) On Request. Because an ADC sits between the client and server and acts as a proxy, it sees every request and response. It can be programmatically instructed using iRules to inspect those requests and, upon finding a Heartbeat request message, can reject it.  It is not necessary to decrypt the request to detect the Heartbeat message.

(3) On Response. As noted, the strategic point of control in which an F5 ADC is deployed in the network means it sees every response, too. It can programmatically inspect responses and if found to be over a specified length, discard it to prevent the attacker from getting a hold of sensitive data.

F5 suggests the "On Request" mitigation for dealing with Heartbleed. This approach minimizes the impact to clients and prevents legitimate requests from being rejected, and further assures that servers are not compromised. Customers have the option, of course, to implement any or all three of these options in order to protect their applications, customers and data as they see fit. F5 supports customer choices in every aspect of application delivery whether related to security, orchestration or architectural model.

Action Items

At this point, nearly a week after the exposure of Heartbleed, organizations should have a good handle on how it works and what the impact is to their business. There's no question the response to Heartbleed involves server patches and upgrades and the procurement of new keys, with consumer password change processes to come soon thereafter.

In the meantime, servers (and thus customers) remain vulnerable. Organizations should be looking at putting into place a mitigation solution to protect both while longer-term plans are put into action.

No matter which approach you choose, F5 has got you covered.

[Edited: 11:11am PT with new graphic]

Read the original blog entry...

More Stories By Lori MacVittie

Lori MacVittie is responsible for education and evangelism of application services available across F5’s entire product suite. Her role includes authorship of technical materials and participation in a number of community-based forums and industry standards organizations, among other efforts. MacVittie has extensive programming experience as an application architect, as well as network and systems development and administration expertise. Prior to joining F5, MacVittie was an award-winning Senior Technology Editor at Network Computing Magazine, where she conducted product research and evaluation focused on integration with application and network architectures, and authored articles on a variety of topics aimed at IT professionals. Her most recent area of focus included SOA-related products and architectures. She holds a B.S. in Information and Computing Science from the University of Wisconsin at Green Bay, and an M.S. in Computer Science from Nova Southeastern University.

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