Identifying and Handling Bad Characters

Identifying and Handling Bad Characters

Bad character identification prevents shellcode corruption during transmission or processing. Certain characters undergo transformation or cause string termination, breaking exploits. Common bad characters include null bytes (\x00), carriage returns (\x0d), and line feeds (\x0a). Application-specific processing might forbid additional characters. Systematic testing identifies all bad characters before shellcode generation.

Comprehensive bad character testing sends all possible byte values (\x00-\xff) in payloads. Monitor memory to identify transformed or truncated characters. Start with known bad characters like null bytes, progressively testing character ranges. Automated scripts comparing sent versus received bytes accelerate identification. Document all bad characters as shellcode generation requires complete lists.

Shellcode encoding bypasses bad character restrictions while maintaining functionality. Metasploit's msfvenom supports multiple encoders avoiding specified characters. Simple encoders like x86/shikata_ga_nai provide basic evasion, while complex encoders handle extensive bad character lists. Multiple encoding iterations might be necessary for highly restrictive character sets. Test encoded payloads to ensure successful transmission.

Manual bad character handling develops deeper understanding than relying solely on encoders. XOR encoding with carefully chosen keys avoids bad characters. Custom encoders tailored to specific restrictions provide size-optimized solutions. Understanding encoding principles enables troubleshooting when automated tools fail. This knowledge proves valuable for space-constrained exploits requiring minimal overhead.