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ESET Analysis
Accessible as each an IDA plugin and a Python script, Nimfilt helps to reverse engineer binaries compiled with the Nim programming language compiler by demangling bundle and performance names, and making use of structs to strings
23 Could 2024
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6 min. learn
The Nim programming language has turn into more and more enticing to malware builders resulting from its sturdy compiler and its means to work simply with different languages. Nim’s compiler can compile Nim to JavaScript, C, C++, and Goal-C, and cross-compile for main working methods resembling Home windows, Linux, macOS, Android, and iOS. Moreover, Nim helps importing features and symbols from the languages talked about above, and importing from dynamically linked libraries for Home windows and shared libraries for Linux. Nim wrapper modules are additionally obtainable, resembling Winim, that make interplay with the working system painless. All these capabilities enable simple integration of Nim into improvement pipelines utilizing these languages and enhance the event of recent instruments, each benign and malicious.
It’s no shock, then, that ESET Analysis has seen an ongoing use of malware developed in Nim within the wild. Way back to 2019, Sednit was noticed utilizing a malicious downloader written in Nim. One other infamous group taking part in the Nim sport, and the impetus for growing Nimfilt, is the Mustang Panda APT group. ESET Analysis recorded Mustang Panda utilizing Nim in its toolset for the primary time in a marketing campaign in opposition to a governmental group in Slovakia in August 2023. The malicious DLL detected – and used as a part of the group’s traditional trident Korplug loader – was written in Nim.
For researchers tasked with reverse engineering such binaries, Nimfilt is a strong software to hurry up evaluation. Whereas Nimfilt will be run as a Python script each on the command line (with a subset of its performance) and in Hex-Rays’ IDA program, it will likely be offered right here primarily as a Python plugin for IDA.
Initializing Nimfilt in IDA
When IDA is first opened, it hundreds and initializes any plugins within the IDA plugins listing. In the course of the initialization of Nimfilt, the plugin makes use of primary heuristics to find out whether or not the disassembled binary was compiled with the Nim compiler. If one of many following checks is handed, Nimfilt determines that this compiler was used:
The binary accommodates each of the next strings:
The binary accommodates any of the next well-known Nim operate names:
NimMain
NimMainInner
NimMainModule
The binary accommodates no less than two of the next error message strings:
@worth out of vary
@division by zero
@over- or underflow
@index out of bounds
YARA guidelines are offered together with Nimfilt that make comparable checks to find out whether or not an ELF or PE file has been compiled with Nim. Collectively, these checks are much more sturdy than the strategy taken by different instruments, resembling Detect It Straightforward, which at present solely checks the .rdata part of PE information for the string io.nim or deadly.nim.
As the ultimate initialization step, if Nimfilt’s AUTO_RUN flag is ready to true, the plugin runs instantly. In any other case, Nimfilt will be run as typical from IDA’s plugins menu, as proven in Determine 1.
Determine 1. Initializing and operating the Nimfilt plugin in IDA
Demangling with Nimfilt
Nim makes use of a customized identify mangling scheme that Nimfilt can decode. Throughout a run, Nimfilt iterates by every operate identify within the binary, checking whether or not the identify is a Nim bundle or operate identify. Found names are renamed to their demangled varieties.
Apparently, these names can leak details about the developer’s surroundings, in a lot the identical means as PDB paths. That is as a result of Nim compiler including the file path to the identify throughout mangling – Nimfilt reveals the trail upon demangling.
For instance, operate names from third-party packages are saved as absolute paths throughout the mangling course of. Determine 2 exhibits a operate identify that’s saved as an absolute path revealing the model and checksum of the nimSHA2 bundle used, together with the developer’s set up path for nimble – Nim’s default bundle supervisor.
python nimfilt.py GET_UINT32_BE__6758Z85sersZ85serOnameZOnimbleZpkgs50Znim837265504548O49O494554555453d57a4852c515056c5452eb5354b51fa5748f5253545748505752cc56fdZnim83726550_u68
C:/Customers/Consumer.identify/.nimble/pkgs2/nimSHA2-0.1.1-6765d9a04c328c64eb56b3fa90f45690294cc8fd/nimSHA2::GET_UINT32_BE u68
Determine 2. Demangling the identify of a operate from a third-party bundle
In distinction, Determine 3 exhibits the identify of a operate from a typical Nim bundle saved as a relative path (that’s, relative to the Nim set up path).
python nimfilt.py toHex__pureZstrutils_u2067
pure/strutils::toHex u2067
Determine 3. Demangling the identify of a operate from a typical Nim bundle
Nonetheless, names usually are not at all times mangled in the identical means. Determine 4 exhibits that the identical operate identify above from the nimSHA2 bundle is saved on Linux as a relative path.
Determine 4. Demangling the identify of a operate from a third-party bundle on Linux
Package deal initialization features are mangled in a very totally different means: the bundle identify is saved as a file path (together with the file extension) positioned earlier than the operate identify and an escaping scheme is used to characterize sure characters like ahead slashes, hyphens, and dots. Upon demangling, Nimfilt cleans up the bundle identify by eradicating the .nim file extension, as proven in Determine 5.
Determine 5. Demangling the identify of an initialization operate from a third-party bundle
Determine 6 exhibits how names of initialization features from native packages are saved as absolute paths.
python nimfilt.py atmCatcatstoolsatsNimatsnimminus2dot0dot0atslibatssystemdotnim_Init000
C:/instruments/Nim/nim-2.0.0/lib/system::Init000
Determine 6. Demangling the identify of an initialization operate from a local bundle
In IDA, Nimfilt’s identify demangling course of is adopted by the creation of directories within the Capabilities window to arrange features based on their bundle identify or path, as proven in Determine 7.
Determine 7. The IDA Capabilities window earlier than (left) and after (proper) Nimfilt organizes operate names by bundle or path
Making use of structs to Nim strings
The final motion carried out throughout a run of Nimfilt is making use of C-style structs to Nim strings. Simply as strings in another programming languages are objects fairly than null-terminated sequences of bytes, so are strings in Nim. Determine 8 exhibits how the string ABCDEF seems in IDA earlier than and after operating Nimfilt. Observe that in disassembled type, a Nim-compiled binary makes use of the prefix _TM as part of the momentary identify of some variables; these are sometimes Nim strings.
Determine 8. A Nim string earlier than (left) and after (proper) operating Nimfilt
Nimfilt iterates by every tackle within the .rdata or .rodata phase, and in every other read-only information phase, in search of Nim strings. Structs are utilized to any found strings; the struct accommodates a size area and a pointer to the payload consisting of the characters within the string.
Wrap-up
On its strategy to being compiled as an executable, Nim supply code is often translated to C or C++; nonetheless, this course of doesn’t solely take away all traces of Nim. By taking a journey by the Nim compiler supply code, we now have unraveled a few of the paths taken within the compilation course of and have been thus in a position to construct Nimfilt as a Python software, and IDA plugin, to help on this untangling.
Briefly, whether or not or not you might be new to Nim, turning to Nimfilt will make your reverse engineering work with Nim-compiled binaries virtually immediately simpler and extra centered. On no account, nonetheless, is Nimfilt’s improvement at a standstill; we’re engaged on extra options to deal with double mangling, and enhance the formatting of demangled names and the grouping of bundle names.
Nimfilt’s supply code and documentation can be found in a repository hosted on ESET’s GitHub group at https://github.com/eset/nimfilt.
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