Strengthening Polymorphic Prompt Assembling: Dynamic Separator Generation Against Emerging Prompt Injection Attacks

Computer Science > Cryptography and Security [Submitted on 28 May 2026] Strengthening Polymorphic Prompt Assembling: Dynamic Separator Generation Against Emerging Prompt Injection Attacks Nima Dorzhiev, Peng Liu Polymorphic Prompt Assembling (PPA) defends LLM agents against prompt injections by randomly selecting separator pairs from a fixed pool to isolate user input from system instructions. Although effective, static pool reuse exposes a blast-radius vulnerability: once a separator leaks, it can be exploited in future requests. We propose a dynamic per-request separator generation using domain-separated SHA-256 digests keyed on the timestamp, session identifier, and cryptographic nonce. Each assembled prompt receives a unique (BEGIN, END) canary pair, thereby limiting leakage exposure to a single request. We evaluated our extension against 16 injection payloads on Llama-3.3-70B-Instruct-Turbo, with cross-model validation on DeepSeek-V4-Flash model. Against the M1 obfuscation payload (leetspeak + urgency), the dynamic mode reduces the Attack Success Rate (ASR) from 0.88 to 0.38, yielding a statistically significant 2.3 x mitigation verified by non-overlapping 95% Wilson confidence intervals. Against format_breakout_salad, static separator leakage (leak_rate = 0.467) is eliminated entirely in the dynamic mode (0.000), confirming the blast-radius reduction in practice. The implementation requires no model fine-tuning, adds 2.7 microseconds prompt-assembly overhead per request, and is backward compatible with the existing PPA SDK. Comments: 5 pages, 3 tables, 1 figure Subjects: Cryptography and Security (cs.CR) Cite as: arXiv:2605.30534 [cs.CR]   (or arXiv:2605.30534v1 [cs.CR] for this version)   https://doi.org/10.48550/arXiv.2605.30534 Focus to learn more Submission history From: Nima Dorzhiev [view email] [v1] Thu, 28 May 2026 20:10:04 UTC (80 KB) Access Paper: HTML (experimental) view license Current browse context: cs.CR < prev   |   next > new | recent | 2026-05 Change to browse by: cs References & Citations NASA ADS Google Scholar Semantic Scholar Export BibTeX Citation Bookmark Bibliographic Tools Bibliographic and Citation Tools Bibliographic Explorer Toggle Bibliographic Explorer (What is the Explorer?) Connected Papers Toggle Connected Papers (What is Connected Papers?) Litmaps Toggle Litmaps (What is Litmaps?) scite.ai Toggle scite Smart Citations (What are Smart Citations?) Code, Data, Media Demos Related Papers About arXivLabs Which authors of this paper are endorsers? | Disable MathJax (What is MathJax?)