BIOS Boots What? Finding Evil in Boot Code at Scale!

The second issue is that reverse engineering all boot records is
impractical. Given the job of determining if a single system is
infected with a bootkit, a malware analyst could acquire a disk image
and then reverse engineer the boot bytes to determine if anything
malicious is present in the boot chain. However, this process takes
time and even an army of skilled reverse engineers wouldn’t scale to
the size of modern enterprise networks. To put this in context, the
compromised enterprise network referenced in our ROCKBOOT blog post had approximately
10,000 hosts. Assuming a minimum
of two boot records per host, a Master Boot Record (MBR) and a Volume
Boot Record (VBR), that is an average of 20,000 boot records to analyze! An
initial reaction is probably, “Why not just hash the boot records and
only analyze the unique ones?” One would assume that corporate
networks are mostly homogeneous, particularly with respect to boot
code, yet this is not the case. Using the same network as an example,
the 20,000 boot records reduced to only 6,000 unique records based on
MD5 hash. Table 1 demonstrates this using data we’ve collected across
our engagements for various enterprise sizes.

Enterprise Size (# hosts)

Avg # Unique Boot Records (md5)

100-1000

428

1000-10000

4,738

10000+

8,717

Table 1 – Unique boot records by MD5 hash

Now, the next thought might be, “Rather than hashing the entire
record, why not implement a custom hashing technique where only
subsections of the boot code are hashed, thus avoiding the dynamic
data portions?” We tried this as well. For example, in the case of
Master Boot Records, we used the bytes at the following two offsets to
calculate a hash:

md5( offset[0:218] + offset[224:440] )

In one network this resulted in approximately 185,000 systems
reducing to around 90 unique MBR hashes. However, this technique had
drawbacks. Most notably, it required accounting for numerous special
cases for applications such as Altiris, SafeBoot, and PGPGuard. This
required small adjustments to the algorithm for each environment,
which in turn required reverse engineering many records to find the
appropriate offsets to hash.

Ultimately, we concluded that to solve the problem we needed a
solution that provided the following:

  • A reliable collection of
    boot records from systems
  • A behavioral analysis of boot
    records, not just static analysis
  • The ability to analyze
    tens of thousands of boot records in a timely manner

The remainder of this post describes how we solved each of these challenges.

Collect the Bytes

Malicious drivers insert themselves into the disk driver stack so
they can intercept disk I/O as it traverses the stack. They do this to
hide their presence (the real bytes) on disk. To address this attack
vector, we developed a custom kernel driver (henceforth, our “Raw
Read” driver) capable of targeting various altitudes in the disk
driver stack. Using the Raw Read driver, we identify the lowest level
of the stack and read the bytes from that level (Figure 1).



Figure 1: Malicious driver inserts itself
as a filter driver in the stack, raw read driver reads bytes from
lowest level

This allows us to bypass the rest of the driver stack, as well as
any user space hooks. (It is important to note, however, that if the
lowest driver on the I/O stack has an inline code hook an attacker can
still intercept the read requests.) Additionally, we can compare the
bytes read from the lowest level of the driver stack to those read
from user space. Introducing our first indicator of a compromised
boot system: the bytes retrieved from user space don’t match those
retrieved from the lowest level of the disk driver stack.

Analyze the Bytes

As previously mentioned, reverse engineering and static analysis are
impractical when dealing with hundreds of thousands of boot records.
Automated dynamic analysis is a more practical approach, specifically
through emulating the execution of a boot record. In more technical
terms, we are emulating the real mode instructions of a boot record.

The emulation engine that we chose is the Unicorn project. Unicorn
is based on the QEMU emulator and supports 16-bit real mode emulation.
As boot samples are collected from endpoint machines, they are sent to
the emulation engine where high-level functionality is captured during
emulation. This functionality includes events such as memory access,
disk reads and writes, and other interrupts that execute during emulation.

The Execution Hash

Folding down (aka stacking) duplicate samples is critical to reduce
the time needed on follow-up analysis by a human analyst. An
interesting quality of the boot samples gathered at scale is that
while samples are often functionally identical, the data they use
(e.g. strings or offsets) is often very different. This makes it quite
difficult to generate a hash to identify duplicates, as demonstrated
in Table 1. So how can we solve this problem with emulation? Enter the
“execution hash”. The idea is simple: during emulation, hash the
mnemonic of every assembly instruction that executes (e.g.,
“md5(‘and’ + ‘mov’ + ‘shl’ + ‘or’)”). Figure 2 illustrates this
concept of hashing the assembly instruction as it executes to
ultimately arrive at the “execution hash”



Figure 2: Execution hash

Using this method, the 650,000 unique boot samples we’ve collected
to date can be grouped into a little more than 300 unique execution
hashes. This reduced data set makes it far more manageable to identify
samples for follow-up analysis. Introducing our second indicator of
a compromised boot system: an execution hash that is only found on a
few systems in an enterprise!

Behavioral Analysis

Like all malware, suspicious activity executed by bootkits can vary
widely. To avoid the pitfall of writing detection signatures for
individual malware samples, we focused on identifying behavior that
deviates from normal OS bootstrapping. To enable this analysis, the
series of instructions that execute during emulation are fed into an
analytic engine. Let’s look in more detail at an example of malicious
functionality exhibited by several bootkits that we discovered by
analyzing the results of emulation.

Several malicious bootkits we discovered hooked the interrupt vector
table (IVT) and the BIOS Data Area (BDA) to intercept system
interrupts and data during the boot process. This can provide an
attacker the ability to intercept disk reads and also alter the
maximum memory reported by the system. By hooking these structures,
bootkits can attempt to hide themselves on disk or even in memory.

These hooks can be identified by memory writes to the memory ranges
reserved for the IVT and BDA during the boot process. The IVT
structure is located at the memory range 0000:0000h to 0000:03FCh and
the BDA is located at 0040:0000h. The malware can hook the interrupt
13h handler to inspect and modify disk writes that occur during the
boot process. Additionally, bootkit malware has been observed
modifying the memory size reported by the BIOS Data Area in order to
potentially hide itself in memory.

This leads us to our final category of indicators of a compromised
boot system: detection of suspicious behaviors such as IVT hooking,
decoding and executing data from disk, suspicious screen output from
the boot code, and modifying files or data on disk.

Do it at Scale

Dynamic analysis gives us a drastic improvement when determining the
behavior of boot records, but it comes at a cost. Unlike static
analysis or hashing, it is orders of magnitude slower. In our cloud
analysis environment, the average time to emulate a single record is
4.83 seconds. Using the compromised enterprise network that contained
ROCKBOOT as an example (approximately 20,000 boot records), it would
take more than 26 hours to dynamically analyze (emulate) the records
serially! In order to provide timely results to our analysts we needed
to easily scale our analysis throughput relative to the amount of
incoming data from our endpoint technologies. To further complicate
the problem, boot record analysis tends to happen in batches, for
example, when our endpoint technology is first deployed to a new enterprise.

With the advent of serverless cloud computing, we had the
opportunity to create an emulation analysis service that scales to
meet this demand – all while remaining cost effective. One of the
advantages of serverless computing versus traditional cloud instances
is that there are no compute costs during inactive periods; the only
cost incurred is storage. Even when our cloud solution receives tens
of thousands of records at the start of a new customer engagement, it
can rapidly scale to meet demand and maintain near real-time detection
of malicious bytes.

The cloud infrastructure we selected for our application is Amazon
Web Services (AWS). Figure 3 provides an overview of the architecture.



Figure 3: Boot record analysis workflow

Our design currently utilizes:

  • API Gateway to provide a RESTful interface.
  • Lambda functions to do validation, emulation, analysis, as
    well as storage and retrieval of results.
  • DynamoDB to track progress of processed boot records through
    the system.
  • S3 to store boot records and emulation reports.

The architecture we created exposes a RESTful API that provides a
handful of endpoints. At a high level the workflow is:

  1. Endpoint agents in
    customer networks automatically collect boot records using FireEye’s
    custom developed Raw Read kernel driver (see “Collect the bytes”
    described earlier) and return the records to FireEye’s Incident
    Response (IR) server.
  2. The IR server submits batches of boot
    records to the AWS-hosted REST interface, and polls the interface
    for batched results.
  3. The IR server provides a UI for
    analysts to view the aggregated results across the enterprise, as
    well as automated notifications when malicious boot records are
    found.

The REST API endpoints are exposed via AWS’s API Gateway, which then
proxies the incoming requests to a “submission” Lambda. The submission
Lambda validates the incoming data, stores the record (aka boot code)
to S3, and then fans out the incoming requests to “analysis” Lambdas.

The analysis Lambda is where boot record emulation occurs. Because
Lambdas are started on demand, this model allows for an incredibly
high level of parallelization. AWS provides various settings to
control the maximum concurrency for a Lambda function, as well as
memory/CPU allocations and more. Once the analysis is complete, a
report is generated for the boot record and the report is stored in
S3. The reports include the results of emulation and other metadata
extracted from the boot record (e.g., ASCII strings).

As described earlier, the IR server periodically polls the AWS REST
endpoint until processing is complete, at which time the report is downloaded.

Find More Evil in Big Data

Our workflow for identifying malicious boot records is only
effective when we know what malicious indicators to look for, or what
execution hashes to blacklist. But what if a new malicious boot record
(with a unique hash) evades our existing signatures?

For this problem, we leverage our in-house big data platform engine
that we integrated into FireEye
Helix
following the acquisition of X15 Software. By loading the
results of hundreds of thousands of emulations into the engine X15,
our analysts can hunt through the results at scale and identify
anomalous behaviors such as unique screen prints, unusual initial jump
offsets, or patterns in disk reads or writes.

This analysis at scale helps us identify new and interesting samples
to reverse engineer, and ultimately helps us identify new detection
signatures that feed back into our analytic engine.

Conclusion

Within weeks of going live we detected previously unknown
compromised systems in multiple customer environments. We’ve
identified everything from ROCKBOOT and HDRoot!
bootkits to the admittedly humorous JackTheRipper,
a bootkit that spreads itself via floppy disk (no joke). Our system
has collected and processed nearly 650,000 unique records to date and
continues to find the evil needles (suspicious and malicious boot
records) in very large haystacks.

In summary, by combining advanced endpoint boot record extraction
with scalable serverless computing and an automated emulation engine,
we can rapidly analyze thousands of records in search of evil. FireEye
is now using this solution in both our Managed
Defense
and Incident
Response
offerings.

Acknowledgements

Dimiter Andonov, Jamin Becker, Fred House, and Seth Summersett
contributed to this blog post.



*** This is a Security Bloggers Network syndicated blog from Threat Research authored by Threat Research Blog. Read the original post at: http://www.fireeye.com/blog/threat-research/2018/08/bios-boots-what-finding-evil-in-boot-code-at-scale.html