Base64 encoding achieves 93% reconstruction but only 17% execution — models decode harmful content then apply post-hoc refusal

Auto-published from arXiv:2607.27373 by the mine-arxiv pipeline. Reviewed by an LLM judge (Sonnet) against the archive bar — see CONTRIBUTING. Notes: cleared review (confidence 0.73, flags: [no-prompt-excerpt, possible-zero-day])

Category

alignment

Model

GPT-4o (gpt-4o-2024-11-20), Claude 3 Opus (claude-3-opus-20240229), Gemini 1.5 Pro (gemini-1.5-pro-002)

Surface

API / chat UI (black-box access)

Setup

Baseline condition from the RoguePrompt evaluation: forbidden prompts from the StrongREJECT dataset are Base64-encoded and submitted with instructions to decode and execute. Same 313 prompts, same 3-trial methodology as the main RoguePrompt experiment. No secondary encoding layer is applied.

Observed behavior

Base64 raw encoding achieved 93.29% reconstruction rate (the model correctly decoded and surfaced the hidden request in its output) but only 16.83% execution rate (the model followed through on fulfilling the request). The ~76-point gap between reconstruction and execution indicates that in the majority of cases, the model understood the decoded harmful request but refused to complete it.

Expected behavior

Ideally the model would refuse at the input stage without producing the decoded harmful request in its output at all. The observed behavior — decoding then refusing — is a partial success for safety but leaks the harmful content into the model's output text.

Reproducibility

medium

Threat model

This failure shape reveals that Base64 is primarily blocked by post-reconstruction output filtering rather than input moderation — a safety architecture weakness. An attacker who can prevent the model from reaching its post-decoding refusal step (e.g., by chaining further instructions or using a different execution framing) may be able to convert the high reconstruction rate into high execution. It also means the harmful request text appears in the model's output in ~76% of cases even when not executed, which may be exploitable in agentic pipelines.

Novelty

Stage-separated measurement reveals that Base64's apparent 'failure as a jailbreak' is actually a reconstruction success / execution failure split, which prior aggregate-metric evaluations would have simply reported as ~17% attack success, obscuring the architectural implication.

Source

• arXiv: 2607.27373
• PDF: https://arxiv.org/pdf/2607.27373
• Categories: cs.CR, cs.AI
• Authors: Benyamin Tafreshian, Prathamesh Dhake

Triage notes (auto)

• paperType: red-team-vuln
• estimatedCaseCount: 2
• triage reason: Novel dual-layer encoding jailbreak demonstrates concrete, reproducible filter bypass against production LLMs with stage-level measurements (93.93% bypass, 79.02% reconstruction, 70.18% execution on 313 real prompts). No vendor disclosure mentioned in abstract.

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