Yara Undefined values


Author: David Zimmer
Date: 02.25.20 - 10:14am


In the yara pe module, anything that is not explicitly set is initialized with the value UNDEFINED. This causes that section of rule (and or etc) to evaluate to a false in the condition statement, however there is no explicit way to test for the undefined value in itself.

So what if you want to test to see if the rich_signature exists or not?

The only way I have found to do this is to wrap it into its own distinct rule, which then gets properly cast to a boolean that you can then work with. 

private rule richHeaderExists 
{
    condition: pe.rich_signature.offset 
}

rule noRichHeader
{
    condition: not richHeaderExists 
}
If you are up for modifying the yara source, you can also add a new function such as the following which I added to pe.c 

//declare_function("isdef", "s", "i", isdef); 
//ex: pe.dbg(pe.isdef("rich_signature.offset"))
//    pe.dbg(pe.isdef("number_of_signatures"))
define_function(isdef) 
{
	YR_OBJECT* module = module();
	char* txt = string_argument(1);
	if (yr_object_has_undefined_value(module, txt)){
		return_integer(0);
	}else{
		return_integer(1);
	}
}




Comments: (2)

On 03.27.20 - 11:02pm Dave wrote:
from yara dev plusvic:

Actually, as part of the fix for issue #1180, Im changing the behavior for comparison operators. When some operand is UNDEFINED, the result is false instead of UNDEFINED, so not not (pe.section_index(".symtab") >= 0) will work in YARA 4.0.

For arithmetic and bitwise operations the result is still UNDEFINED if some operand is UNDEFINED

On 11.08.21 - 5:52pm Dave wrote:
This has been addressed formally in issue Added unary operation defined #1529.

More details in the discussion in Is it possible to use UNDEFINED value in "pe" module of YARA? #835.

Solution was a new keyword "defined" example:

condition: defined pe.entry_point

This is not yet in the 4.1.3 release.

 
Leave Comment:
Name:
Email: (not shown)
Message: (Required)
Math Question: 2 + 65 = ? followed by the letter: R 



About Me
More Blogs
Main Site
Posts: (year)
2026 (2)
     VBDec AI Automation
     Misc Updates
2025 (12)
     Close IDA decompiler windows
     IDA Xrefs From/To
     IDA Jscript 3 Beta
     IDA JScript Updates
     IdaCompare StringDiff
     Sensationalization
     iPhone to PC Dictation
     pediff and chatgpt
     Py Magic Numbers
     upside down pyramid
     IDA Compare Updates
     Bindiff Niceity
2024 (1)
     MAP Update
2023 (6)
     Yara Workbench Automation
     VS linker versions
     IDA decompiler comments
     PublicBytes struct
     DispCallFunc
     VB6 Class Instances
2022 (7)
     VB6 Implements
     VBDec Bug
     vbdec remote scripting
     VB6 Stubs BS
     VB6 TypeInfo
     VB6 VTable Layout
     Yara isPCode rule
2021 (4)
     VB6 Hijacking
     rtcTypeName
     VB6 Gosub
     VB App object
2020 (8)
     AutoIT versions
     IDA JScript 2
     Using VB6 Obj files from C
     Vb6 PCode Internals
     Vb6 Runtime ForLoop Disasm
     VB6 Pcode - For Loops
     Yara Corrupt Imports
     Yara Undefined values
2019 (12)
     Yara WorkBench
     SafeArrayGetVartype
     vbdec dbg updates
     vb6 PCode NOP
     vb6 API and call backs
     how pcode works Pt1
     PrintFile
     ImpAdCallNonVirt
     Reversing PCode Args
     VB6 PCode Disassembly
     VB6 PCode Debugger
     UConnect Disable Cell Modem
2017 (5)
     IDA python over IPC
     dns wildcard blocking
     64bit IDA Plugins
     anterior lines
     misc news/updates
2016 (4)
     KANAL Mod
     Decoders again
     CDO.Message Breakpoints
     SysAnalyzer Updates
2015 (5)
     SysAnalyzer and Site Updates
     crazy decoder
     ida js w/dbg
     flash patching 2
     JS Graphing
2014 (5)
     Delphi IDA Plugin
     scdbg IDA integration
     API Hash Database
     Winmerge plugin
     IDACompare Updates
2013 (9)
     Guest Post @ hexblog
     TCP Stream Reassembly
     SysAnalyzer Updates
     Apilogger Video
     Shellcode2Exe trainer
     scdbg updates
     IDA Javascript w/IDE
     Rop Analysis II
     scdbg vrs ROP
2012 (13)
     flash patching
     x64 Hooks
     micro hook
     jmp api+5 *2
     SysAnalyzer Updates
     InjDll runtime config
     C# Asm/Dsm Library
     Shellcode Hook Detection
     Updates II
     findDll
     Java Hacking
     Windows 8
     Win7 x64
2011 (19)
     Graphing ideas
     .Net Hacking
     Old iDefense Releases
     BootLoaders
     hll shellcode
     ActionScript Tips
     -patch fu
     scdbg ordinal lookup
     scdbg -api mode
     Peb Module Lists
     scdbg vrs Process Injection
     GetProcAddress Scanner
     scdbg fopen mode
     scdbg findsc mode
     scdbg MemMonitor
     demo shellcodes
     scdbg download
     api hashs redux
     Api hash gen
2010 (11)
     Retro XSS Chat Codes
     Exe as DLL
     Olly Plugins
     Debugging Explorer
     Attach to hidden process
     JS Refactoring
     Asm and Shellcode in CSharp
     Fancy Return Address
     PDF Stream Dumper
     Malcode Call API by Hash
     WinDbg Cheat Sheet
2009 (1)
     GPG Automation