[Submitted on 21 Oct 2026]
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Abstract:Coding agents that run long enough must compact their context, summarizing the conversation, discarding the original, and carrying the summary forward as memory. We show that in popular open-source coding agents this step is an exploitable trust boundary. Harnesses that flatten the typed conversation into a single plain string, marking each message with a text label such as [User]:, destroy message provenance in code, before the summarizer runs. An attacker who controls anything the agent reads can forge a user turn that survives compaction as a genuine instruction. Across five flattening harnesses (pi, opencode, Cline, Aider, OpenHands) the forged approval launders into the agent's memory 87-100% of the time, up to a ~908,000-token context window; across four role-typed harnesses (Codex, Gemini CLI, Qwen Code, Continue) the identical attack succeeds 0% of the time. We follow the attack to executed consequences. A laundered standing approval drives real tool-running agents to execute attacker-controlled
curl | sh in a sandbox, and end to end in the live pi TUI at ~50% against a 0% paired clean arm. The same channel reliably steers an agent's plan away from a file an attacker designates off-limits, which can blind a code review to a planted backdoor. Because the provenance is destroyed deterministically in harness code, no prompt-level defense can restore it; we measure a code-level provenance check that catches 77% of laundering while demoting 2% of legitimate summary content. The defect also has a straightforward fix at the serializer, escaping the role labels before flattening; the role-typed harnesses avoid it entirely by keeping messages role-typed.
Submission history #
From: Arshan Dabirsiaghi [
[view email](https://arxiv.org/show-email/770d37e9/2610.29869)]
**[v1]** Wed, 21 Oct 2026 15:24:23 UTC (123 KB)
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