Description
1 Overview
The Heartbleed bug (CVE-2014-0160) is a severe implementation flaw in the OpenSSL library, which enables attackers to steal data from the memory of the victim server. The contents of the stolen data depend on
what is there in the memory of the server. It could potentially contain private keys, TLS session keys, user
names, passwords, credit cards, etc. The vulnerability is in the implementation of the Heartbeat protocol,
which is used by SSL/TLS to keep the connection alive.
The objective of this lab is for students to understand how serious this vulnerability is, how the attack
works, and how to fix the problem. The affected OpenSSL version range is from 1.0.1 to 1.0.1f. The
version in our Ubuntu VM is 1.0.1.
2 Lab Environment
In this lab, we need to set up two VMs: one called attacker machine and the other called victim server.
We use the pre-built SEEDUbuntu12.04 VM. The VMs need to use the NAT-Network adapter for the
network setting. This can be done by going to the VM settings, picking Network, and clicking the Adaptor
tag to switch the adapter to NAT-Network. Make sure both VMs are on the same NAT-Network.
The website used in this attack can be any HTTPS website that uses SSL/TLS. However, since it is
illegal to attack a real website, we have set up a website in our VM, and conduct the attack on our own
VM. We use an open-source social network application called ELGG, and host it in the following URL:
https://www.heartbleedlabelgg.com.
We need to modify the /etc/hosts file on the attacker machine to map the server name to the IP address of the server VM. Search the following line in /etc/hosts, and replace the IP address 127.0.0.1
with the actual IP address of the server VM that hosts the ELGG application.
127.0.0.1 www.heartbleedlabelgg.com
3 Lab Tasks
Before working on the lab tasks, you need to understand how the heartbeat protocol works. The heartbeat
protocol consists of two message types: HeartbeatRequest packet and HeartbeatResponse packet. Client
sends a HeartbeatRequest packet to the server. When the server receives it, it sends back a copy of the
received message in the HeartbeatResponse packet. The goal is to keep the connection alive. The protocol
is illustrated in Figure 1.
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I want to know whether my
connection is still alive…
Server, are u still there? if
so, return “ABC”(3
characters) to me.
TLS/SSL
User wants the message
“ABC”(3 characters)
“ABC” (3 characters)
Both of us are still connected. We
should keep the connection alive…
TLS/SSL
TLS/SSL
TLS/SSL
TLS/SSL
Figure 1: Overview of the Heartbeat Protocol
3.1 Task 1: Launch the Heartbleed Attack.
In this task, students will launch the Heartbleed attack on our social network site and see what kind of
damages can be achieved. The actual damage of the Heartbleed attack depends on what kind of information
is stored in the server memory. If there has not been much activity on the server, you will not be able to
steal useful data. Therefore, we need to interact with the web server as legitimate users. Let us do it as the
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administrator, and do the followings:
• Visit https://www.heartbleedlabelgg.com from your browser.
• Login as the site administrator. (User Name:admin; Password:seedelgg)
• Add Boby as friend. (Go to More -> Members and click Boby -> Add Friend)
• Send Boby a private message.
After you have done enough interaction as legitimate users, you can launch the attack and see what
information you can get out of the victim server. Writing the program to launch the Heartbleed attack from
scratch is not easy, because it requires the low-level knowledge of the Heartbeat protocol. Fortunately, other
people have already written the attack code. Therefore, we will use the existing code to gain first-hand
experience in the Heartbleed attack. The code that we use is called attack.py, which was originally
written by Jared Stafford. We made some small changes to the code for educational purposes. You can
download the code from the lab’s web site, change its permission so the file is executable. You can then run
the attack code as follows:
$ ./attack.py www.heartbleedlabelgg.com
You may need to run the attack code multiple times to get useful data. Try and see whether you can get
the following information from the target server.
• User name and password.
• User’s activity (what the user has done).
• The exact content of the private message.
For each piece of secret that you steal from the Heartbleed attack, you need to show the screen-dump as
the proof and explain how you did the attack, and what your observations are.
3.2 Task 2: Find the Cause of the Heartbleed Vulnerability
In this task, students will compare the outcome of the benign packet and the malicious packet sent by the
attacker code to find out the fundamental cause of the Heartbleed vulnerability.
The Heartbleed attack is based on the Heartbeat request. This request just sends some data to the server,
and the server will copy the data to its response packet, so all the data are echoed back. In the normal case,
suppose that the request includes 3 bytes of data ”ABC”, so the length field has a value 3. The server will
place the data in the memory, and copy 3 bytes from the beginning of the data to its response packet. In the
attack scenario, the request may contain 3 bytes of data, but the length field may say 1003. When the server
constructs its response packet, it copies from the starting of the data (i.e. “ABC”), but it copies 1003 bytes,
instead of 3 bytes. These extra 1000 types obviously do not come from the request packet; they come from
the server’s private memory, and they may contain other user’s information, secret keys, password, etc.
In this task, we will play with the length field of the request. First, let’s understand how the Heartbeat
response packet is built from Figure 2. When the Heartbeat request packet comes, the server will parse the
packet to get the payload and the Payload length value (which is highlighted in Figure 2). Here, the
payload is only a 3-byte string “ABC” and the Payload length value is exactly 3. The server program
will blindly take this length value from the request packet. It then builds the response packet by pointing to
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Type(request)
Payload[Payload_length]
Padding[padding_length]
Payload_length
Type(response)
Payload[Payload_length]
Padding[padding_length]
Payload_length
“ABC”
Memory
=
Request packet Response packet
Figure 2: The Benign Heartbeat Communication
Type(request)
Payload[Payload_length]
Padding[padding_length]
Payload_length+1000
Type(response)
Payload[Payload_length+
1000]
Padding[padding_length]
Payload_length+1000
Memory
=
Extra data
“ABC”
Request packet Response packet Figure 3: The Heartbleed Attack Communication
the memory storing “ABC” and copy Payload length bytes to the response payload. In this way, the
response packet would contain a 3-byte string “ABC”.
We can launch the HeartBleed attack like what is shown in Figure 3. We keep the same payload (3 bytes),
but set the Payload length field to 1003. The server will again blindly take this Payload length
value when building the response packet. This time, the server program will point to the string “ABC” and
copy 1003 bytes from the memory to the response packet as a payload. Besides the string ”ABC”, the extra
1000 bytes are copied into the response packet, which could be anything from the memory, such as secret
activity, logging information, password and so on.
Our attack code allows you to play with different Payload length values. By default, the value is
set to a quite large one (0x4000), but you can reduce the size using the command option “-l” (letter ell)
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or “–length” as shown in the following examples:
$./attack.py www.heartbleedlabelgg.com -l 0x015B
$./attack.py www.heartbleedlabelgg.com –length 83
Your task is to play with the attack program with different payload length values and answer the following questions:
• Question 2.1: As the length variable decreases, what kind of difference can you observe?
• Question 2.2: As the length variable decreases, there is a boundary value for the input length variable. At or below that boundary, the Heartbeat query will receive a response packet without attaching
any extra data (which means the request is benign). Please find that boundary length. You may need
to try many different length values until the web server sends back the reply without extra data. To
help you with this, when the number of returned bytes is smaller than the expected length, the program will print “Server processed malformed Heartbeat, but did not return
any extra data.”
3.3 Task 3: Countermeasure and Bug Fix
To fix the Heartbleed vulnerability, the best way is to update the OpenSSL library to the newest version.
This can be achieved using the following commands. It should be noted that once it is updated, it is hard to
go back to the vulnerable version. Therefore, make sure you have finished the previous tasks before doing
the update. You can also take a snapshot of your VM before the update.
#sudo apt-get update
#sudo apt-get upgrade
Task 3.1 Try your attack again after you have updated the OpenSSL library. Please describe your observations.
Task 3.2 The objective of this task is to figure out how to fix the Heartbleed bug in the source code.
The following C-style structure (not exactly the same as the source code) is the format of the Heartbeat
request/response packet.
struct {
HeartbeatMessageType type; // 1 byte: request or the response
uint16 payload_length; // 2 byte: the length of the payload
opaque payload[HeartbeatMessage.payload_length];
opaque padding[padding_length];
} HeartbeatMessage;
The first field (1 byte) of the packet is the type information, and the second field (2 bytes) is the payload
length, followed by the actual payload and paddings. The size of the payload should be the same as the value
in the payload length field, but in the attack scenario, payload length can be set to a different
value. The following code snippet shows how the server copies the data from the request packet to the
response packet.
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Listing 1: Process the Heartbeat request packet and generate the response packet
1 /* Allocate memory for the response, size is 1 byte
2 * message type, plus 2 bytes payload length, plus
3 * payload, plus padding
4 */
5
6 unsigned int payload;
7 unsigned int padding = 16; /* Use minimum padding */
8
9 // Read from type field first
10 hbtype = *p++; /* After this instruction, the pointer
11 * p will point to the payload_length field *.
12
13 // Read from the payload_length field
14 // from the request packet
15 n2s(p, payload); /* Function n2s(p, payload) reads 16 bits
16 * from pointer p and store the value
17 * in the INT variable “payload”. */
18
19
20 pl=p; // pl points to the beginning of the payload content
21
22 if (hbtype == TLS1_HB_REQUEST)
23 {
24 unsigned char *buffer, *bp;
25 int r;
26
27 /* Allocate memory for the response, size is 1 byte
28 * message type, plus 2 bytes payload length, plus
29 * payload, plus padding
30 */
31
32 buffer = OPENSSL_malloc(1 + 2 + payload + padding);
33 bp = buffer;
34
35 // Enter response type, length and copy payload
36 *bp++ = TLS1_HB_RESPONSE;
37 s2n(payload, bp);
38
39 // copy payload
40 memcpy(bp, pl, payload); /* pl is the pointer which
41 * points to the beginning
42 * of the payload content */
43
44 bp += payload;
45
46 // Random padding
47 RAND_pseudo_bytes(bp, padding);
48
49 // this function will copy the 3+payload+padding bytes
50 // from the buffer and put them into the heartbeat response
51 // packet to send back to the request client side.
52 OPENSSL_free(buffer);
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53 r = ssl3_write_bytes(s, TLS1_RT_HEARTBEAT, buffer,
54 3 + payload + padding);
55 }
Please point out the problem from the code in Listing 1 and provide a solution to fix the bug (i.e., what
modification is needed to fix the bug). You do not need to recompile the code; just describe how you can fix
the problem in your lab report.
Moreover, please comment on the following discussions by Alice, Bob, and Eva regarding the fundamental cause of the Heartbleed vulnerability: Alice thinks the fundamental cause is missing the boundary
checking during the buffer copy; Bob thinks the cause is missing the user input validation; Eva thinks that
we can just delete the length value from the packet to solve everything.
4 Submission
Students need to submit a detailed lab report to describe what they have done and what they have observed.
Report should include the evidences to support the observations. Evidences include packet traces, screen
dumps, etc.
References
[1] Heartbleed attack – Implementation: https://alexandreborgesbrazil.files.wordpress.com/
2014/04/hearbleed attack version a 1.pdf
[2] Heartbleed attack – Interesting explanation: http://xkcd.com/1354/
