PyTorch is one of the most popular and widely-used machine learning toolkits out there.
(We’re not going to be drawn on where it sits on the artifical intelligence leaderboard – as with many widely-used open source tools in a competitive field, the answer seems to depend on whom you ask, and which toolkit they happen to use themselves.)
Originally developed and released as an open-source project by Facebook, now Meta, the software was handed over to the Linux Foundation in late 2022, which now runs it under the aegis of the PyTorch Foundation.
Unfortunately, the project was compromised by means of a supply-chain attack during the holiday season at the end of 2022, between Christmas Day [2022-12-25] and the day before New Year’s Eve [2022-12-30].
The attackers malevolently created a Python package called
torchtriton on PyPI, the popular Python Package Index repository.
torchtriton was chosen so it would match the name of a package in the PyTorch system itself, leading to a dangerous situation explained by the PyTorch team (our emphasis) as follows:
[A] malicious dependency package (
torchtriton) […] was uploaded to the Python Package Index (PyPI) code repository with the same package name as the one we ship on the PyTorch nightly package index. Since the PyPI index takes precedence, this malicious package was being installed instead of the version from our official repository. This design enables somebody to register a package by the same name as one that exists in a third party index, and
pipwill install their version by default.
pip, by the way, used to be known as
pyinstall, and is apparently a recursive joke that’s short for
pip installs packages. Despite its original name, it’s not for installing Python itself – it’s the standard way for Python users to manage software libraries and applications that are written in Python, such as PyTorch and many other popular tools.
Pwned by a supply-chain trick
Anyone unfortunate enough to install the pwned version of PyTorch during the danger period almost certainly ended up with data-stealing malware implanted on their computer.
According to PyTorch’s own short but useful analysis of the malware, the attackers stole some, most or all of the following significant data from infected systems:
- System information, including hostname, username, known users on the system, and the content of all system environment variables. Environment variables are a way of providing memory-only input data that programs can access when they start up, often including data that’s not supposed to be saved to disk, such as cryptographic keys and authentication tokens giving access to cloud-based services. The list of known users is extracted from
/etc/passwd, which, fortunately, doesn’t actually contain any passwords or password hashes.
- Your local Git configuration. This is stolen from
$HOME/.gitconfig, and typically contains useful information about the personal setup of anyone using the popular Git source code management system.
- Your SSH keys. These are stolen from the directory
$HOME/.ssh. SSH keys typically include the private keys used for connecting securely via SSH (secure shell) or using SCP (secure copy) to other servers on your own networks or in the cloud. Lots of developers keep at least some of their private keys unencrypted, so that scripts and software tools they use can automatically connect to remote systems without pausing to ask for a password or a hardware security key every time.
- The first 1000 other files in the your home directory smaller that 100 kilobytes in size. The PyTorch malware description doesn’t say how the “first 1000 file list” is computed. The content and ordering of file listings depends on whether the list is sorted alphabetically; whether subdirectories are visited before, during or after processing the files in any directory; whether hidden files are included; and whether any randomness is used in the code that walks its way through the directories. You should probably assume that any files below the size threshold could be the ones that end up stolen.
At this point, we’ll mention the good news: only those who fetched the so-called “nightly”, or experimental, version of the software were at risk. (The name “nightly” comes from the fact that it’s the very latest build, typically created automatically at the end of each working day.)
Most PyTorch users will probably stick to the so-called “stable” version, which was not affected by this attack.
Also, from PyTorch’s report, it seems that the Triton malware executable file specifically targeted 64-bit Linux environments.
We’re therefore assuming that this malicious program would only run on Windows computers if the Windows Subsystem for Linux (WSL) were installed.
Don’t forget, though that the people most likely to install regular “nightlies” include developers of PyTorch itself or of applications that use it – perhaps including your own in-house developers, who might have private-key-based access to corporate build, test and production servers.
DNS data stealing
Intriguingly, the Triton malware doesn’t exfiltrate its data (the militaristic jargon term that the cybersecurity industry likes to use instead of steal or copy illegally) using HTTP, HTTPS, SSH, or any other high-level protocol.
Instead, it compresses, scrambles and text-encodes the data it wants to steal into a sequence of what look like “server names” that belong to a domain name controlled by the criminals.
By making a sequence of DNS lookups containing carefully constructed data that could be series of legal server names but isn’t, the crooks can sneak out stolen data without relying on traditional protocols usually used for uploading files and other data.
This is the same sort of trick that was used by Log4Shell hackers at the end of 2021, who leaked encryption keys by doing DNS lookups for “servers” with “names” that just happened to be the value of your secret AWS access key, plundered from an in-memory environment variable.
So what looked like an innocent, if pointless, DNS lookup for a “server” such as
S3CR3TPA55W0RD.DODGY.EXAMPLE would quietly leak your access key under the guise of a simple lookup that directed to the official DNS server listed for the
LIVE LOG4SHELL DEMO EXPLAINING DATA EXFILTRATION VIA DNS
If you can’t read the text clearly here, try using Full Screen mode, or watch directly on YouTube.
Click on the cog in the video player to speed up playback or to turn on subtitles.
If the crooks own the domain
DODGY.EXAMPLE, they get to tell the world which DNS server to connect to when doing those lookups.
More importantly, even networks that strictly filter TCP-based network connections using HTTP, SSH and other high-level data sharing protocols…
…sometimes don’t filter UDP-based network connections used for DNS lookups at all.
The only downside for the crooks is that DNS requests have a rather limited size.
Individual server names are limited to 64 alphanumeric characters each, and many networks limit individual DNS packets, including all enclosed requests, headers and metadata, to just 512 bytes each.
We’re guessing that’s why the malware in this case started out by going after your private keys, then restricted itself to at most 1000 files, each smaller than 100,000 bytes.
That way, the crooks get to thieve plenty of private data, notably including server access keys, without generating an unmanageably large number of DNS lookups.
An unusually large number of DNS lookups might get noticed for routine operational reasons, even in the absence of any scrutiny applied specifically for cybersecurity purposes.
How the malware works
Decompiling the compiled
triton executable shows that it compresses, obfuscates and text-encodes the data it steals in order to convert it into a format that can be embedded directly into DNS lookups.
Note that we said above that your stolen data merely gets obfuscated above, rather than encrypted, because the process is roughly as follows:
- Compress the data using the
deflate()algorithm. Deflate is defined in RFC 1951, and is commonly used in software including gzip and PKZIP, as well as to save bandwidth in HTTP downloads.
- Encrypt the data using AES-256-GCM, but with a hard-coded key and initialisation vector. We described this process merely as obfuscation, not as proper encryption, given that anyone with a copy of the leaked DNS requests can easily unscramble them by extracting the “secret” key material from the malware executable.
- Encode the data into alphanumeric characters, using Base62 encoding. This process is similar to Base64 or URL64 encoding, but uses only
0-9, with no punctuation characters appearing in the encoded output. This sidesteps the problem that only one punctuation symbol, the dash or hyphen, is allowed in DNS names.
- Split the data into DNS-sized chunks, and append the domain name
h4ck.cfdto each request. You won’t find that domain name string in the executable file. It appears as
&z-%`-(*instead, where each character is XORed with 0x4E to unscramble it when the program runs.
-- The domain suffix gets unscrambled as shown here: suffix = [[&z-%`-(*]] -- how it is stored in the executable for i = 1,suffix:len() do -- for each char in suffix: local inp = suffix:sub(i,i) -- get current scrambled char local enc = string.byte(inp) -- convert to ASCII number local dec = enc ~ 0x4E -- XOR it with 0x4E local out = string.char(dec) -- convert back to character print(inp,enc,'XOR(0x4E)->',dec,out) -- show what we've got end --Output: & 38 XOR(0x4E)-> 104 h z 122 XOR(0x4E)-> 52 4 - 45 XOR(0x4E)-> 99 c % 37 XOR(0x4E)-> 107 k ` 96 XOR(0x4E)-> 46 . - 45 XOR(0x4E)-> 99 c ( 40 XOR(0x4E)-> 102 f * 42 XOR(0x4E)-> 100 d
Assuming that the crooks beind the malware own the domain
h4ck.cfd (which was registered on 2022-12-21, presumably for use in this attack), then they also get to specify which DNS server to use to answer queries for this domain, and therefore to collect all the stolen data via DNS lookups alone.
Of course, their obfuscation-only exfiltration scheme means, in theory, that the stolen data is also open to surveillance, collection and decoding by almost anyone on your network path, thus greatly increasing the risk of your private keys falling into the hands of multiple threat actors.
What to do?
PyTorch has already taken action to shut down this attack, so if you haven’t been hit yet, you almost certainly won’t get hit now, because the malicious
torchtriton package on PyPI has been replaced with a deliberately “dud”, empty package of the same name.
This means that any person, or any software, that tried to install
torchtriton from PyPI after 2022-12-30T08:38:06Z, whether by accident or by design, would not receive the malware.
PyTorch has published a handy list of IoCs, or indicators of compromise, that you can search for across your network.
Remember, as we mentioned above, that even if almost all of your users stick to the “stable” version, which was not affected by this attack, you may have developers or enthusiasts who experiment with “nightlies”, even if they use the stable release as well.
According to PyTorch:
- The malware is installed with the filename
triton. By default, you would expect to find it in the subdirectory
triton/runtimein your Python site packages directory. Given that filenames alone are weak malware indicators, however, treat the presence of this file as evidence of danger; don’t treat its absence as an all-clear.
- The malware in this particular attack has the SHA256 sum
2385b29489cd9e35f92c072780f903ae2e517ed422eae67246ae50a5cc738a0e. Once again, the malware could easily be recompiled to produce a different checksum, so the absence of this file is not a sign of definite health, but you can treat its presence as a sign of infection.
- DNS lookups used for stealing data ended with the domain name
H4CK.CFD. If you have network logs that record DNS lookups by name, you can search for this text string as evidence that secret data leaked out.
- The malicious DNS replies apparently went to, and replies, if any, came from a DNS server called
WHEEZY.IO. At the moment, we can’t find any IP numbers associated with that service, and PyTorch hasn’t provided any IP data that would tie DNS taffic to this malware, so we’re not sure how much use this information is for threat hunting at the moment [2023-01-01T21:05:00Z].
Fortunately, we’re guessing that the majority of PyTorch users won’t have been affected by this, either because they don’t use nightly builds, or weren’t working over the vacation period, or both.
But if you are a PyTorch enthusiast who does tinker with nightly builds, and if you’ve been working over the holidays, then even if you can’t find any clear evidence that you were compromised…
…you might nevertheless want to consider generating new SSH keypairs as a precaution, and updating the public keys that you’ve uploaded to the various servers that you access via SSH.
If you suspect you were compromised, of course, then don’t put off those SSH key updates – if you haven’t done them already, do them right now!