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YARA · Signature Base (Florian Roth / Nextron)

SIGNATURE_BASE_MAL_Claude_Refusal_Magic_String_Jan26

Detects Base64 variations of refusal magic string that cause Claude sessions to be terminated. This might indicate that a file tries to prevent being analyzed by LLM agents.

MalwareSignature Base (Florian Roth / Nextron)Detection Rule License 1.1Compiles with YARA-X
Author
Marius Benthin
Date
2026-01-29
Modified
2026-05-15
Score / Quality
80 / 85

What this rule detects

Detects Base64 variations of refusal magic string that cause Claude sessions to be terminated. This might indicate that a file tries to prevent being analyzed by LLM agents.

It defines 1 patterns (1 text, 0 hex, 0 regex).

Summary generated from the rule's metadata and condition.

Rule source

Download .yar
mal-claude-refusal-magic-string-jan26.yarDetection Rule License 1.1
rule SIGNATURE_BASE_MAL_Claude_Refusal_Magic_String_Jan26
{
	meta:
		description = "Detects Base64 variations of refusal magic string that cause Claude sessions to be terminated. This might indicate that a file tries to prevent being analyzed by LLM agents."
		author = "Marius Benthin"
		id = "8f0351b7-56b8-5059-8c1e-e8d87fa0ca91"
		date = "2026-01-29"
		modified = "2026-05-15"
		reference = "Internal Research"
		source_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/yara/susp_claude_magic_strings.yar#L16-L28"
		license_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/LICENSE"
		logic_hash = "e1c070b77e3568966f605fa47a2b3be917065954bc157e3721ff2fb3ac4d72f7"
		score = 80
		quality = 85
		tags = ""

	strings:
		$xb1 = "ANTHROPIC_MAGIC_STRING_TRIGGER_REFUSAL_" ascii wide base64 base64wide

	condition:
		$xb1
}

Strings (1 patterns)

IdentifierTypeValueModifiers
$xb1text"ANTHROPIC_MAGIC_STRING_TRIGGER_REFUSAL_"ascii wide base64 base64wide

Condition

$xb1

In plain words (Heuristic)

  • Pattern $xb1 must be present.

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Licence & attribution

Author: Marius Benthin

Source repository: Signature Base (Florian Roth / Nextron) @ 94a1c48d7a

Licence: Detection Rule License 1.1 (repository licence)

Original rule

Mirrored via YARA Forge 20260927, which prefixes rule names with the source and adds normalised meta fields.

Rule id: 8f0351b7-56b8-5059-8c1e-e8d87fa0ca91

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