Can I decompile an existing installer?

About

Currently NSIS installers cannot be fully decompiled. The installer itself doesn't provide any method to extract files or the script without installation. It is the developer's choice whether the source code and/or the files for the installer are available to the public or not.

Extraction Tools

There are, however, external tools that allow this:

• Since version 4.42[may 2006] 7-zip supports decompressing NSIS installers.
  Since version 9.34 [Jun 2014] 7-zip is also able to extract the compiled scriptcode.
• The decompression plug-in InstallExplorer InstExpl.wcx is also available for TotalCommander. Beside the files it'll create the file 'script.bin' compiled scriptcode.For use without the TotalCommander the Universal Extractor is a good option.
  

Well since [Feb 2014] InstExpl suffers from problems naming file names and dirs correctly that were created with NSIS 3. That's because the implementation GetNSISString() was slightly changed so names like $INSTDIR, $PROGRAMFILE ... inside strings are not expanded correctly.

Decompilers

• 7-zip Since version 9.34 [Jun 2014] till 15.06. [Jun 2015] it'll extract *beside the files of the setup* the compiled script code to a file named [NSIS].nsi
• NullsoftDecompiler or NSIDis
  NSIDis is a open source Python script that'll help you to nearly fully recover your NSIS-installation scripts. Its state is currently alpha - and so not very user friendly and stable.
• NSIS Reversing Suite NRS is a set of Python librairies used to unpack and analysis NSIS installer's data. It also has an IDA plugin used to disassembly the NSIS Script of an installer.
• NsisDecompiler

Protection against Decompilers

As a general note to software developers, you should use a plugin like DCryptDll if you need to protect certain files in your installer.

.. or if ya in the mood for compiling the NSIS have a look into nsis-3.xx-src\Source\exehead\fileform.h. Mixing up the order of the enum with all the EW_* a little bit as recommend in the Comment. It will mess up decompilers output that expect these tokes to be in the standard order.

Or shift or enlarge the .reloc section in the PE-header by 0x400. <-I saw that trick @ some old 'conduit'-adwareinstaller. Inserting so fill bytes between the EOF-exe at the start of the script might also do the trick to stop 7-zip and maybe some Antiviruses.

Technical Details

NSIS instruction encoding was never designed to be stable across versions and any information listed here should be taken with a grain of salt.

Detection

• 2.0rc1 [r3306] _LANG_INVALIDCRC string changed
• 2.35 [r5459] _LANG_INVALIDCRC string changed
• 2.45 PE.OptionalHeader.MajorImageVersion == 6
• 2.47 [r6659] & v3.0b1 [r6506] String: InitiateShutdownA/W
• A Unicode stub imports CreateFileW and other wide API functions. Theses stubs support EW_FPUTWS and EW_FGETWS.
• 2.47 [r6658] & 3.0b3 [r6657] SetFileSecurityA/W
• 2.51 [r6719] & 3.0b3 [r6706] String: CLBCATQ
• 3.0rc1 [r6721] StringTable: $TEMP\$1u_.exe changed to $TEMP\Un_$1.exe.
• 3.02 [r6839] ShellExecuteA/W has been replaced by ShellExecuteExA/W.
• 3.05 [r7084] No longer links to LoadBitmapA/W
• 3.06 [r7154] Links to IIDFromString (or SHCLSIDFromString) and String: SHGetKnownFolderPath

EW_ASSIGNVAR

• 3.01 [r6810] Empty maxlen string is treated as parameter not present.

EW_CREATEDIR

• 2.51 [r6701] & 3.0b3 [r6657] parm2 = CreateRestrictedDirectory

EW_CREATESHORTCUT

• 3.0b0 [r6452] 0x8000 in parm4 will disable SetWorkingDirectory
• 3.0b3 [r6638] parm4 packing changed to support larger icon index.

EW_FPUTWS

• 3.0b3 [r6626] parm3 = TryWriteBOM

EW_INTCMP

• 3.03 [r6929] parm5 is no longer a BOOL, it is a flags DWORD: bit 0x01 is set for unsigned operations and bit 0x8000 is set for 64-bit operations.

EW_INTFMT

• 3.03 [r6931] parm3 is != 0 for 64-bit operations

EW_INTOP

• 2.0b4 [r2387] (parm3 >= 7) operators changed because the old 7 (~ bneg) is implemented using ^
• 3.03 [r6926] Added >>> (SHR) operator (parm3 == 13)

EW_LOADANDSETIMAGE

• 3.05 Renamed EW_SETBRANDINGIMAGE. parm2 now contains flags describing parm0 and parm1 and LR_*.
• 3.06 [7179] parm0 is now the output var index, the other parameters shifted one place.

EW_SETFLAG

• 3.02 [r6841] alter_reg_view can be KEY_WOW64_32KEY in 64-bit stubs and KEY_WOW64_64KEY in 32-bit stubs.

EW_SHELLEXEC

• 3.02 [r6839] Parameter count changed. parm4 is SHELLEXECUTEINFO.fMask and SEE_MASK_NOCLOSEPROCESS is set for ExecShellWait.

EW_DELREG

• 3.02 [r6871] ent.offsets[4] can also contain (shifted) KEY_WOW64_xxKEY bits.
• 3.06 [r7166] parm4: 0x04 = ONLYIFNOVALUES

EW_WRITEREG

• 3.02 [r6829] ent.offsets[5] is REG_MULTI_SZ for WriteRegMultiStr.

EW_GETOSINFO

• 3.06 [r7154] parm0 == 0: TOK_GETKNOWNFOLDERPATH