AgenticMail: Unauthenticated inbound mail triggers bypassPermissions resume of the operator's Claude Code session (bridge-wake)
- When
- Where
- Global (internet)
- Category
- cyber_advisory · npm
## Summary Two inbound-mail handlers act on a privileged effect without verifying that the sender is the operator, while a sibling handler in the same repo does. The higher-impact one: any external email routed to the bridge inbox causes the dispatcher to resume the operator's Claude Code session with `permissionMode: 'bypassPermissions'`, embedding the attacker-controlled `from`/`subject`/`preview` verbatim into the prompt the resumed agent reads — an indirect prompt injection into a fully-privileged agent (Bash/Write/Edit/WebFetch + the agenticmail MCP toolbelt) running as the operator's OAuth identity. The sibling operator-query email-reply hook gates the same untrusted-From provenance with `isOperatorReplySender(replyFrom, config.operatorEmail)` (fail-closed); the bridge-wake path — a strictly higher-privilege effect — has no equivalent. ## Affected code (current HEAD, commit b95f52e) Untrusted provenance: external inbound email enters at `packages/api/src/routes/inbound.ts:41` (POST /mail/inbound); the `x-inbound-secret` authenticates only the relay->API hop, not the external sender, so `from`/`subject`/`preview` are attacker-controlled. Privileged sink (bridge-wake, bypassPermissions): - `packages/claudecode/src/dispatcher.ts:2040` `handleBridgeMail` extracts `subject`/`from`/`preview` (`:2045-2049`) and calls `planBridgeWake({ session, mail: { ..., from, preview } })` (`:2052`) with NO sender check — routing keys only on session freshness (skip-live / escalate / resume). - `planBridgeWake` -> `packages/core/src/host-bridge.ts:141` `composeBridgeWakePrompt` embeds the untrusted `from`/`subject`/`preview` (preview sliced to 600 chars at `:144`) verbatim into the prompt. - `packages/claudecode/src/bridge-wake.ts:103` `resumeBridgeSession` runs the prompt via the Claude Code SDK with `permissionMode: 'bypassPermissions'` against the operator's last session (resume + same mcpServers). Guarded sibling (same class, authenticated): `packages/api/src/routes/inbound.ts:102` rejects an operator-query email reply unless `isOperatorReplySender(replyFrom, config.operatorEmail)` (def `packages/core/src/phone/realtime-tools.ts:999`, fail-closed when no operatorEmail), with a v0.9.53 security-review comment (`inbound.ts:93-100`) stating inbound mail provenance is untrusted and an emailed answer is only honored when its From matches the configured operator. The Telegram sibling likewise gates on `operatorChatId`. The bridge-wake path ignores this exact lesson. Secondary instance (same root cause): `packages/core/src/gateway/manager.ts:261` `tryProcessApprovalReply` releases a held outbound email on an "approve" reply matched only by `In-Reply-To` / `notification_message_id`, with no sender check — again unlike the `isOperatorReplySender` sibling. ## Impact In a configured headless-bridge deployment (operator uses the CLI so a host-session is saved; session fresh <24h per `host-sessions.ts:127`; external mail routed to the bridge inbox via relay sub-addressing or a Cloudflare webhook), an external sender achieves indirect prompt injection into a `bypassPermissions` operator session -> arbitrary OS command execution, filesystem read/write, and exfiltration under the operator's OAuth identity. The auth gap (no sender check on the bridge path) is structural and unconditional; the impact realization is config-conditional and depends on the resumed model following injected instructions. ## Proof of concept (static / request-difference; dynamic on operator's OWN setup only) Static: the from/subject/preview extracted at dispatcher.ts:2045-2049 flow into composeBridgeWakePrompt (host-bridge.ts:141) and resumeBridgeSession (bridge-wake.ts:103) with no interposed sender check, while the sibling inbound.ts:102 has one — the same untrusted-From provenance is authenticated on one privileged email path and not on the higher-privileged one. Dynamic (own instance only): with a configured bridge + fresh host-session, send mail from a non-operator address into the bridge inbox whose subject/preview contains a benign instruction writing a fresh CSPRNG marker; observe the resumed bypassPermissions session act on it. Use only your own instance; do not target third-party deployments. ## Suggested fix Mirror the guarded sibling: before any `bypassPermissions` resume (dispatcher.ts handleBridgeMail, before planBridgeWake), require trusted provenance — an internal sub-agent wake OR `isOperatorReplySender(from, config.operatorEmail)`; otherwise deliver the mail normally but do NOT resume. Reuse the existing exported `isOperatorReplySender` from @agenticmail/core so the two privileged email paths share one authentication helper. Defense-in-depth: in composeBridgeWakePrompt, wrap the untrusted fields in explicit untrusted-data delimiters and drop `bypassPermissions` for mail-triggered resumes whose provenance is not the operator. Apply the same sender gate to `tryProcessApprovalReply` (manager.ts:261). ## Affected versions Present on current HEAD (core 0.9.42 / claudecode 0.2.38, commit b95f52e). No fix retrofits the sender check onto bridge-wake. ## Severity (honest, both ways) HIGH, plausibly CRITICAL in a configured headless-bridge deployment. Ceiling ~9.0 (unauthenticated external sender -> operator-privileged code execution). Floor ~7.0: the auth gap is unconditional, but full impact requires (1) a fresh <24h host-session, (2) external mail routed to the bridge inbox, (3) the resumed model obeying injected instructions (non-deterministic). Not a deterministic RCE primitive. CWE-306 (missing authentication for the privileged action) + CWE-77/CWE-94 (injected instructions realized as command execution). Novelty: the two existing agenticmail advisories (CVE-2026-50287 MCP missing-auth; CVE-2026-47255 storage SQL) do not cover this sink. Please rate per your deployment assumptions.
Sources
- GitHub Advisory Database ↗ · first seen 2026-06-18 17:21 UTC
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