gimphash

gimphash is a proposed method to calculate an imphash equivalent for Go binaries. It's name stands for Go-import-hash.

Golang binaries contain their dependencies as part of the executable. These dependencies include both standard library packages and third party dependencies and can be used, analogous to a classical imphash, to identify a Golang project.

The dependencies can be listed using the pclntab that is part of each Golang binary (also see this blog post by Mandiant). The pclntab contains a number of interesting elements for reverse engineering; for the gimphash we will use the function names that are contained there.

Calculation

1. Locate the pclntab within a Golang binary
2. Enumerate golang functions using the functab within the pclntab and iterate over their names:
   1. Ignore function names starting with go. or type. (compile artefacts, runtime internals)
   2. If a function name contains vendor/, discard that substring and everything before it (e.g. transform vendor/golang.org/x/text to golang.org/x/text)
   3. Ignore function names containing internal/
   4. Ignore function names that start with one of the following standard library packages:
      • runtime
      • sync
      • syscall
      • type
      • time
      • unicode
      • reflect
      • strconv
   5. Ignore function names that are not public or where their receivers are not public. In order to do so:
      1. Find the last / in the function name. If no / is found, use the start instead. Starting from that position, find the next ..
      2. Extract the base function name as everything after that .. If no . was found, use everything after the / index calculated in the previous step.
      3. Ignore the function if the first alphanumeric character in the base function name is a lower case character.
      4. If another . exists within the base function name, ignore the function if the first alphanumeric character after that . is a lower case character.
   6. Store the resulting name, if it was not ignored so far, in an ordered list
3. Calculate the SHA-256 hash over the concatenated names (no delimiter)

Proof of Concept Implementations

This repository contains proof-of-concept code in the following languages:

• C
• Go

The release section contains prebuilt binaries for Windows and Linux.

Usage Examples

Run the Gimphash calculator on a single file

./c_gimphash_linux /mnt/malware-repo/Godoh/godoh-windows64.exe
8200e76e42c4e9cf2bb308d76c017cbdcde5cbbf95e99e02b14d05e7b21505f3 /mnt/mal/Godoh/godoh-windows64.exe

Run the Gimphash calculator on a malware repository

find /mnt/malware-repo/ -type f -exec ./go_gimphash_linux {} \; 2>/dev/null
...

Clustering Example