A reverse-engineering software for Nim-compiled binaries

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 package deal and performance names, and making use of structs to strings

Introducing Nimfilt: A reverse-engineering tool for Nim-compiled binaries

The Nim programming language has develop into more and more engaging to malware builders because of its sturdy compiler and its capacity 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 similar to 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 out there, similar to Winim, that make interplay with the working system painless. All these capabilities enable straightforward integration of Nim into growth 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 enjoying the Nim recreation, and the impetus for growing Nimfilt, is the Mustang Panda APT group. ESET Analysis recorded Mustang Panda using Nim in its toolset for the primary time in a marketing campaign towards a governmental group in Slovakia in August 2023. The malicious DLL detected – and used as a part of the group’s basic 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 may be run as a Python script each on the command line (with a subset of its performance) and in Hex-Rays’ IDA program, will probably be introduced 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. Through the initialization of Nimfilt, the plugin makes use of fundamental 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 comprises each of the next strings:
  • The binary comprises any of the next well-known Nim perform names:
    • NimMain
    • NimMainInner
    • NimMainModule
  • The binary comprises at the least 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 related checks to find out whether or not an ELF or PE file has been compiled with Nim. Collectively, these checks are way more sturdy than the method taken by different instruments, similar to Detect It Easy, which presently 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 about to true, the plugin runs instantly. In any other case, Nimfilt may be run as normal from IDA’s plugins menu, as proven in Determine 1.

Figure 1. Initializing and running the Nimfilt plugin in IDA
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 way of every perform identify within the binary, checking whether or not the identify is a Nim package deal or perform identify. Found names are renamed to their demangled kinds.

Apparently, these names can leak details about the developer’s surroundings, in a lot the identical means as PDB paths. That is because of the Nim compiler including the file path to the identify throughout mangling – Nimfilt reveals the trail upon demangling.

For instance, perform names from third-party packages are saved as absolute paths throughout the mangling course of. Determine 2 exhibits a perform identify that’s saved as an absolute path revealing the model and checksum of the nimSHA2 package deal used, together with the developer’s set up path for nimble – Nim’s default package deal supervisor.

python nimfilt.py GET_UINT32_BE__6758Z85sersZ85serOnameZOnimbleZpkgs50Znim837265504548O49O494554555453d57a4852c515056c5452eb5354b51fa5748f5253545748505752cc56fdZnim83726550_u68

C:/Customers/Person.identify/.nimble/pkgs2/nimSHA2-0.1.1-6765d9a04c328c64eb56b3fa90f45690294cc8fd/nimSHA2::GET_UINT32_BE u68

Determine 2. Demangling the identify of a perform from a third-party package deal

In distinction, Determine 3 exhibits the identify of a perform from a regular Nim package deal 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 perform from a regular Nim package deal

Nonetheless, names aren’t at all times mangled in the identical means. Determine 4 exhibits that the identical perform identify above from the nimSHA2 package deal is saved on Linux as a relative path.

Determine 4. Demangling the identify of a perform from a third-party package deal on Linux

Bundle initialization features are mangled in a totally completely different means: the package deal identify is saved as a file path (together with the file extension) positioned earlier than the perform identify and an escaping scheme is used to symbolize sure characters like ahead slashes, hyphens, and dots. Upon demangling, Nimfilt cleans up the package deal identify by eradicating the .nim file extension, as proven in Determine 5.


Figure 5 – Demangling name of an initialization function from third-party package

Determine 5. Demangling the identify of an initialization perform from a third-party package deal

Determine 6 exhibits how names of initialization features from native packages are saved as absolute paths.

python nimfilt.py atmCatcatstoolsatsNimatsnimminus2dot0dot0atslibatssystemdotnim_Init000


Determine 6. Demangling the identify of an initialization perform from a local package deal

In IDA, Nimfilt’s identify demangling course of is adopted by the creation of directories within the Features window to arrange features in response to their package deal identify or path, as proven in Determine 7.

Figure 7. The IDA Functions window before (left) and after (right) Nimfilt organizes function names by package or path
Determine 7. The IDA Features window earlier than (left) and after (proper) Nimfilt organizes perform names by package deal 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 quite 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. Be aware that in disassembled kind, a Nim-compiled binary makes use of the prefix _TM as part of the short-term identify of some variables; these are sometimes Nim strings.

Figure 8. A Nim string before (left) and after (right) running Nimfilt
Determine 8. A Nim string earlier than (left) and after (proper) operating Nimfilt

Nimfilt iterates by way of every handle within the .rdata or .rodata section, and in every other read-only knowledge section, searching for Nim strings. Structs are utilized to any found strings; the struct comprises a size discipline and a pointer to the payload consisting of the characters within the string.


On its solution to being compiled as an executable, Nim supply code is often translated to C or C++; nevertheless, this course of doesn’t totally take away all traces of Nim. By taking a journey by way of the Nim compiler supply code, now we have unraveled a number of the paths taken within the compilation course of and have been thus capable of construct Nimfilt as a Python software, and IDA plugin, to help on this untangling.

Briefly, whether or not or not you’re new to Nim, turning to Nimfilt will make your reverse engineering work with Nim-compiled binaries nearly immediately simpler and extra centered. In no way, nevertheless, is Nimfilt’s growth 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 package deal 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.