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analyzing-webshells

Analyzes suspected webshells (PHP, ASPX/ASP, JSP) by detecting dynamic-execution sinks, input-to-sink data flow, and common encoder/obfuscation chains, then decoding obvious layers to reveal the command interface. Activates for requests to analyze a webshell, identify a PHP/ASPX/JSP backdoor, or deobfuscate server-side malicious scripts.

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Files included (4)

  • SKILL.md2.8 KB
  • LICENSE340 B
  • references/api-reference.md1.4 KB
  • scripts/analyst.py3.4 KB

SKILL.md (original)

Review the instructions the agent will receive before you install.

Analyzing Webshells

When to Use

  • You found a suspicious server-side script (.php, .aspx, .jsp, .asp) and need to confirm it is a webshell and understand its command interface.
  • You are triaging a web-server compromise and want to classify dropped scripts.

Do not use a live web server to execute the script to "test" it — that runs attacker code. Analyze the source statically.

Prerequisites

  • The suspect script file(s), read inertly.

Safety & Handling

  • Read the file statically; never deploy or request it on a live server. Defang any URLs.

Workflow

Step 1: Detect execution sinks and input flow

python scripts/analyst.py scan shell.php

Flags dynamic-execution sinks (eval, assert, system, exec, passthru, Runtime.exec, ASPX eval()/Process.Start) and whether request input ($_GET/$_POST/Request) reaches them.

Step 2: Identify obfuscation/encoder chains

Detects base64_decode, gzinflate, str_rot13, eval(chr(...)), hex \x strings, and FromBase64String chains commonly wrapping the payload.

Step 3: Decode obvious layers

Apply base64/rot13/gzinflate where present to reveal the inner command interface (best-effort, non-executing).

Step 4: Classify and report

Note the language, the password/parameter gate (if any), and capabilities (file manager, command exec, SQL, upload); defang IOCs.

Validation

  • A flagged file pairs an execution sink with attacker-controlled input.
  • Encoder chains are identified and, where safe, decoded statically.
  • Capability classification is backed by concrete code evidence.

Pitfalls

  • Legitimate admin tools that also use eval/system — corroborate with input flow.
  • Multi-stage shells that fetch the real logic remotely.
  • Deeply nested or runtime-built obfuscation that static decode only partly unwinds.

References

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