API Reference: Analyzing Webshells

Overview

scripts/analyst.py scans a server-side script (PHP/ASPX/JSP) for dynamic-execution sinks, request-input-to-sink data flow, and encoder chains, and best-effort decodes base64/raw-deflate layers that wrap webshell logic. It reads the file statically, executes nothing, and defangs URLs.

Dependencies

Package	Version	Purpose
(stdlib)	—	base64, zlib, regex, JSON, file IO

Core functions

`scan(path)`

Returns language, execution sinks, input sources, an input-to-sink flow flag, encoder chains, decoded layers, defanged URLs, and a webshell likelihood flag.

`_decode_layers(text)`

Decodes base64 (and raw-deflate) blobs and keeps those whose plaintext contains execution sinks.

Detection logic

Sinks: PHP eval/system/passthru/preg_replace /e; ASPX eval/Process.Start; JSP Runtime.exec/ProcessBuilder.
Input flow: a sink plus $_GET/$_POST/Request/getParameter raises confidence.
Encoders: base64_decode, gzinflate, str_rot13, FromBase64String, \x hex.

Usage

python scripts/analyst.py scan shell.php

Sources

MITRE ATT&CK T1505.003 Web Shell — https://attack.mitre.org/techniques/T1505/003/
Server-side execution sink and encoder-chain patterns from public webshell research
PHP/ASP/JSP language execution APIs from their official documentation

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