Openclaw cli: OpenClaw: ACP prompt-size checks missing in local stdio bridge could reduce responsiveness with very large inputs (CVE-2026-27576)

The OpenClaw CLI's ACP bridge lacked prompt-size validation, allowing very large prompt text blocks to be accepted and concatenated into oversized payloads before forwarding to chat.send. This vulnerability affects local ACP clients communicating over local stdio, potentially causing reduced responsiveness and increased model usage costs. The affected versions are up to and including 2026.2.17, with a patch planned for 2026.2.18. The vulnerability does not enable privilege escalation or remote attacks. The fix involves enforcing a 2 MiB prompt-text limit, counting newline separator bytes during size checks, validating final message size before sending, and cleaning up session state when oversized prompts are rejected.

Local ACP sessions may experience reduced responsiveness when very large prompts are submitted. Additionally, oversized prompts can lead to unexpectedly high model usage and associated costs. There is no risk of privilege escalation or direct remote exploitation in the default ACP model.

A patch is available and planned for release in version 2026.2.18. It enforces a 2 MiB prompt-text size limit before concatenation, includes size checks accounting for newline separators, validates message size before forwarding to chat.send, and cleans up session state when oversized prompts are rejected. Users should upgrade to version 2026.2.18 once released to remediate this issue.