diff --git a/backend/docs/GUARDRAILS.md b/backend/docs/GUARDRAILS.md
index 81fc4be4..86f0e0f2 100644
--- a/backend/docs/GUARDRAILS.md
+++ b/backend/docs/GUARDRAILS.md
@@ -112,6 +112,34 @@ guardrails:
 3. Ask the agent: "Use bash to run echo hello"
 4. The agent sees: `Guardrail denied: tool 'bash' was blocked (oap.tool_not_allowed)`
 
+### Option 1.5: Built-in SensitiveDataProvider (Strict Secret Blocking)
+
+For secret-leak prevention, DeerFlow also ships `SensitiveDataProvider`, which
+blocks tool calls targeting sensitive file patterns (for example `.env`,
+`.env.*`, `*.pem`, `*.key`, `id_rsa*`, `secrets.*`, `credentials.*`).
+
+This provider is strict: it blocks matching access even when the user
+explicitly asks the agent to reveal those files.
+
+**config.yaml:**
+```yaml
+guardrails:
+  enabled: true
+  fail_closed: true
+  provider:
+    use: deerflow.guardrails.builtin:SensitiveDataProvider
+    config:
+      protected_tools: ["read_file", "write_file", "str_replace", "ls", "glob", "grep", "bash"]
+      deny_basenames: [".env"]
+      deny_globs: [".env.*", "*.pem", "*.key", "id_rsa*", "secrets.*", "credentials.*"]
+      block_skills_env: true
+```
+
+**Behavior summary:**
+- `read_file / ls / glob / grep / bash` attempting sensitive path access are denied
+- `write_file / str_replace` touching sensitive targets are denied
+- Denials are logged as structured audit events (tool/reason/thread/timestamp)
+
 ### Option 2: OAP Passport Provider (Policy-Based)
 
 For policy enforcement based on the [Open Agent Passport (OAP)](https://github.com/aporthq/aport-spec) open standard. An OAP passport is a JSON document that declares an agent's identity, capabilities, and operational limits. Any provider that reads an OAP passport and returns OAP-compliant decisions works with DeerFlow.
diff --git a/backend/packages/harness/deerflow/agents/middlewares/sensitive_output_redaction_middleware.py b/backend/packages/harness/deerflow/agents/middlewares/sensitive_output_redaction_middleware.py
new file mode 100644
index 00000000..88069669
--- /dev/null
+++ b/backend/packages/harness/deerflow/agents/middlewares/sensitive_output_redaction_middleware.py
@@ -0,0 +1,64 @@
+"""Redact sensitive values from tool outputs before they re-enter the model context."""
+
+from __future__ import annotations
+
+import re
+from collections.abc import Awaitable, Callable
+from typing import override
+
+from langchain.agents import AgentState
+from langchain.agents.middleware import AgentMiddleware
+from langchain_core.messages import ToolMessage
+from langgraph.prebuilt.tool_node import ToolCallRequest
+from langgraph.types import Command
+
+_SIMPLE_REDACTION = "[REDACTED]"
+
+_PATTERNS = [
+    re.compile(r"(?im)\b([A-Z][A-Z0-9_]{1,64})\s*=\s*([^\s\"'`]{6,})"),
+    re.compile(r"(?i)\b(bearer\s+)[A-Za-z0-9._\-+/=]{8,}"),
+    re.compile(r"(?i)\b(sk-[A-Za-z0-9]{12,})\b"),
+    re.compile(r"(?i)\b(eyJ[A-Za-z0-9_\-]{10,}\.[A-Za-z0-9_\-]{10,}\.[A-Za-z0-9_\-]{10,})\b"),
+    re.compile(r"(?is)-----BEGIN [A-Z ]*PRIVATE KEY-----.*?-----END [A-Z ]*PRIVATE KEY-----"),
+]
+
+
+def _redact_text(text: str) -> str:
+    value = text
+    # Preserve var name for KEY=VALUE style output.
+    value = _PATTERNS[0].sub(lambda m: f"{m.group(1)}={_SIMPLE_REDACTION}", value)
+    value = _PATTERNS[1].sub(lambda m: f"{m.group(1)}{_SIMPLE_REDACTION}", value)
+    for pattern in _PATTERNS[2:]:
+        value = pattern.sub(_SIMPLE_REDACTION, value)
+    return value
+
+
+class SensitiveOutputRedactionMiddleware(AgentMiddleware[AgentState]):
+    """Redact secrets from tool outputs."""
+
+    def _redact_tool_result(self, result: ToolMessage | Command) -> ToolMessage | Command:
+        if not isinstance(result, ToolMessage):
+            return result
+        if isinstance(result.content, str):
+            redacted = _redact_text(result.content)
+            if redacted != result.content:
+                return ToolMessage(content=redacted, tool_call_id=result.tool_call_id, name=result.name, status=result.status)
+            return result
+        return result
+
+    @override
+    def wrap_tool_call(
+        self,
+        request: ToolCallRequest,
+        handler: Callable[[ToolCallRequest], ToolMessage | Command],
+    ) -> ToolMessage | Command:
+        return self._redact_tool_result(handler(request))
+
+    @override
+    async def awrap_tool_call(
+        self,
+        request: ToolCallRequest,
+        handler: Callable[[ToolCallRequest], Awaitable[ToolMessage | Command]],
+    ) -> ToolMessage | Command:
+        return self._redact_tool_result(await handler(request))
+
diff --git a/backend/packages/harness/deerflow/agents/middlewares/tool_error_handling_middleware.py b/backend/packages/harness/deerflow/agents/middlewares/tool_error_handling_middleware.py
index 096e6dac..66992204 100644
--- a/backend/packages/harness/deerflow/agents/middlewares/tool_error_handling_middleware.py
+++ b/backend/packages/harness/deerflow/agents/middlewares/tool_error_handling_middleware.py
@@ -127,8 +127,10 @@ def _build_runtime_middlewares(
         middlewares.append(GuardrailMiddleware(provider, fail_closed=guardrails_config.fail_closed, passport=guardrails_config.passport))
 
     from deerflow.agents.middlewares.sandbox_audit_middleware import SandboxAuditMiddleware
+    from deerflow.agents.middlewares.sensitive_output_redaction_middleware import SensitiveOutputRedactionMiddleware
 
     middlewares.append(SandboxAuditMiddleware())
+    middlewares.append(SensitiveOutputRedactionMiddleware())
     middlewares.append(ToolErrorHandlingMiddleware())
     return middlewares
 
diff --git a/backend/packages/harness/deerflow/guardrails/__init__.py b/backend/packages/harness/deerflow/guardrails/__init__.py
index 3c23cd09..de92d28f 100644
--- a/backend/packages/harness/deerflow/guardrails/__init__.py
+++ b/backend/packages/harness/deerflow/guardrails/__init__.py
@@ -1,11 +1,12 @@
 """Pre-tool-call authorization middleware."""
 
-from deerflow.guardrails.builtin import AllowlistProvider
+from deerflow.guardrails.builtin import AllowlistProvider, SensitiveDataProvider
 from deerflow.guardrails.middleware import GuardrailMiddleware
 from deerflow.guardrails.provider import GuardrailDecision, GuardrailProvider, GuardrailReason, GuardrailRequest
 
 __all__ = [
     "AllowlistProvider",
+    "SensitiveDataProvider",
     "GuardrailDecision",
     "GuardrailMiddleware",
     "GuardrailProvider",
diff --git a/backend/packages/harness/deerflow/guardrails/builtin.py b/backend/packages/harness/deerflow/guardrails/builtin.py
index 53ce9f8d..6fceb4d4 100644
--- a/backend/packages/harness/deerflow/guardrails/builtin.py
+++ b/backend/packages/harness/deerflow/guardrails/builtin.py
@@ -1,7 +1,20 @@
 """Built-in guardrail providers that ship with DeerFlow."""
 
+from __future__ import annotations
+
+import fnmatch
+import json
+import logging
+import re
+import shlex
+from datetime import UTC, datetime
+from pathlib import PurePosixPath
+from typing import Any
+
 from deerflow.guardrails.provider import GuardrailDecision, GuardrailReason, GuardrailRequest
 
+logger = logging.getLogger(__name__)
+
 
 class AllowlistProvider:
     """Simple allowlist/denylist provider. No external dependencies."""
@@ -21,3 +34,138 @@ class AllowlistProvider:
 
     async def aevaluate(self, request: GuardrailRequest) -> GuardrailDecision:
         return self.evaluate(request)
+
+
+class SensitiveDataProvider:
+    """Block tool calls that may access sensitive files such as .env and keys."""
+
+    name = "sensitive-data"
+
+    _DEFAULT_PROTECTED_TOOLS = {
+        "read_file",
+        "write_file",
+        "str_replace",
+        "ls",
+        "glob",
+        "grep",
+        "bash",
+    }
+    _DEFAULT_DENY_BASENAMES = {".env"}
+    _DEFAULT_DENY_GLOBS = {
+        ".env.*",
+        "*.pem",
+        "*.key",
+        "id_rsa*",
+        "secrets.*",
+        "credentials.*",
+    }
+
+    def __init__(
+        self,
+        *,
+        protected_tools: list[str] | None = None,
+        deny_basenames: list[str] | None = None,
+        deny_globs: list[str] | None = None,
+        block_skills_env: bool = True,
+        **_: Any,
+    ):
+        self._protected_tools = {t.lower() for t in (protected_tools or list(self._DEFAULT_PROTECTED_TOOLS))}
+        self._deny_basenames = {n.lower() for n in (deny_basenames or list(self._DEFAULT_DENY_BASENAMES))}
+        self._deny_globs = {p.lower() for p in (deny_globs or list(self._DEFAULT_DENY_GLOBS))}
+        self._block_skills_env = block_skills_env
+
+    def _normalize_candidate(self, raw: str | None) -> str:
+        if not raw:
+            return ""
+        return str(raw).strip().strip("\"'")
+
+    def _looks_sensitive_path(self, raw_path: str) -> bool:
+        value = self._normalize_candidate(raw_path)
+        if not value:
+            return False
+        lowered = value.lower()
+        if self._block_skills_env and "/mnt/skills/" in lowered:
+            basename = PurePosixPath(lowered).name
+            if basename == ".env" or basename.startswith(".env."):
+                return True
+        basename = PurePosixPath(lowered).name
+        if basename in self._deny_basenames:
+            return True
+        return any(fnmatch.fnmatch(basename, pat) for pat in self._deny_globs)
+
+    def _extract_bash_candidates(self, command: str) -> list[str]:
+        candidates: list[str] = []
+        if not command:
+            return candidates
+        try:
+            tokens = shlex.split(command)
+        except ValueError:
+            tokens = command.split()
+        for token in tokens:
+            t = token.strip()
+            if not t:
+                continue
+            # Path-like tokens
+            if "/" in t or t.startswith("."):
+                candidates.append(t)
+            # file.env style arguments may not contain slash
+            if t.lower().startswith(".env"):
+                candidates.append(t)
+        return candidates
+
+    def _collect_candidates(self, request: GuardrailRequest) -> list[str]:
+        args = request.tool_input if isinstance(request.tool_input, dict) else {}
+        tool = request.tool_name
+        candidates: list[str] = []
+        if tool in {"read_file", "write_file", "str_replace", "ls"}:
+            path = args.get("path")
+            if isinstance(path, str):
+                candidates.append(path)
+        elif tool in {"glob", "grep"}:
+            path = args.get("path")
+            if isinstance(path, str):
+                candidates.append(path)
+            glob_pat = args.get("glob")
+            if isinstance(glob_pat, str):
+                candidates.append(glob_pat)
+        elif tool == "bash":
+            command = str(args.get("command") or "")
+            candidates.extend(self._extract_bash_candidates(command))
+            # Fast-path for common secret exposure commands
+            if re.search(r"\b(printenv|env)\b", command, flags=re.IGNORECASE):
+                candidates.append(".env")
+        return candidates
+
+    def _audit(self, request: GuardrailRequest, decision: GuardrailDecision) -> None:
+        if decision.allow:
+            return
+        code = decision.reasons[0].code if decision.reasons else "oap.blocked_pattern"
+        rec = {
+            "timestamp": datetime.now(UTC).isoformat(),
+            "provider": self.name,
+            "tool_name": request.tool_name,
+            "reason_code": code,
+            "thread_id": request.thread_id,
+            "agent_id": request.agent_id,
+        }
+        logger.warning("[SensitiveDataGuardrail] %s", json.dumps(rec, ensure_ascii=False))
+
+    def evaluate(self, request: GuardrailRequest) -> GuardrailDecision:
+        tool = (request.tool_name or "").lower()
+        if tool not in self._protected_tools:
+            return GuardrailDecision(allow=True, reasons=[GuardrailReason(code="oap.allowed")])
+
+        candidates = self._collect_candidates(request)
+        if any(self._looks_sensitive_path(c) for c in candidates):
+            decision = GuardrailDecision(
+                allow=False,
+                reasons=[GuardrailReason(code="oap.blocked_pattern", message="sensitive path access is blocked by policy")],
+                policy_id="sensitive-data.v1",
+            )
+            self._audit(request, decision)
+            return decision
+
+        return GuardrailDecision(allow=True, reasons=[GuardrailReason(code="oap.allowed")])
+
+    async def aevaluate(self, request: GuardrailRequest) -> GuardrailDecision:
+        return self.evaluate(request)
diff --git a/backend/tests/test_guardrail_middleware.py b/backend/tests/test_guardrail_middleware.py
index 5c021ba4..f204c56a 100644
--- a/backend/tests/test_guardrail_middleware.py
+++ b/backend/tests/test_guardrail_middleware.py
@@ -8,7 +8,7 @@ from unittest.mock import MagicMock
 import pytest
 from langgraph.errors import GraphBubbleUp
 
-from deerflow.guardrails.builtin import AllowlistProvider
+from deerflow.guardrails.builtin import AllowlistProvider, SensitiveDataProvider
 from deerflow.guardrails.middleware import GuardrailMiddleware
 from deerflow.guardrails.provider import GuardrailDecision, GuardrailReason, GuardrailRequest
 
@@ -105,6 +105,46 @@ class TestAllowlistProvider:
         assert decision.allow is False
 
 
+class TestSensitiveDataProvider:
+    def test_denies_reading_env_file(self):
+        provider = SensitiveDataProvider()
+        req = GuardrailRequest(tool_name="read_file", tool_input={"path": "/tmp/.env"})
+        decision = provider.evaluate(req)
+        assert decision.allow is False
+        assert decision.reasons[0].code == "oap.blocked_pattern"
+
+    def test_allows_normal_source_file(self):
+        provider = SensitiveDataProvider()
+        req = GuardrailRequest(tool_name="read_file", tool_input={"path": "/workspace/app/main.py"})
+        decision = provider.evaluate(req)
+        assert decision.allow is True
+
+    def test_denies_skills_env(self):
+        provider = SensitiveDataProvider()
+        req = GuardrailRequest(tool_name="read_file", tool_input={"path": "/mnt/skills/public/foo/.env.local"})
+        decision = provider.evaluate(req)
+        assert decision.allow is False
+
+    def test_denies_glob_or_grep_targeting_env(self):
+        provider = SensitiveDataProvider()
+        req = GuardrailRequest(tool_name="grep", tool_input={"path": "/workspace", "glob": "**/.env.*"})
+        decision = provider.evaluate(req)
+        assert decision.allow is False
+
+    def test_denies_bash_cat_env(self):
+        provider = SensitiveDataProvider()
+        req = GuardrailRequest(tool_name="bash", tool_input={"command": "cat /workspace/.env"})
+        decision = provider.evaluate(req)
+        assert decision.allow is False
+
+    def test_denies_case_variant_and_key_material(self):
+        provider = SensitiveDataProvider()
+        req1 = GuardrailRequest(tool_name="read_file", tool_input={"path": "/workspace/.ENV"})
+        req2 = GuardrailRequest(tool_name="read_file", tool_input={"path": "/workspace/id_rsa.pub"})
+        assert provider.evaluate(req1).allow is False
+        assert provider.evaluate(req2).allow is False
+
+
 # --- GuardrailMiddleware tests ---
 
 
diff --git a/backend/tests/test_sensitive_output_redaction_middleware.py b/backend/tests/test_sensitive_output_redaction_middleware.py
new file mode 100644
index 00000000..12eaa7d9
--- /dev/null
+++ b/backend/tests/test_sensitive_output_redaction_middleware.py
@@ -0,0 +1,62 @@
+from __future__ import annotations
+
+import asyncio
+from unittest.mock import MagicMock
+
+from langchain_core.messages import ToolMessage
+
+from deerflow.agents.middlewares.sensitive_output_redaction_middleware import SensitiveOutputRedactionMiddleware
+
+
+def _request(name: str = "bash", args: dict | None = None):
+    req = MagicMock()
+    req.tool_call = {"name": name, "args": args or {}, "id": "call_1"}
+    return req
+
+
+def test_redacts_key_value_and_bearer():
+    mw = SensitiveOutputRedactionMiddleware()
+    req = _request()
+    handler = MagicMock(
+        return_value=ToolMessage(
+            content="OPENAI_API_KEY=sk-abc123456789\nAuthorization: Bearer abcdefghijklmnop",
+            tool_call_id="call_1",
+            name="bash",
+            status="success",
+        )
+    )
+    result = mw.wrap_tool_call(req, handler)
+    assert "OPENAI_API_KEY=[REDACTED]" in result.content
+    assert "Bearer [REDACTED]" in result.content
+
+
+def test_redacts_private_key_block():
+    mw = SensitiveOutputRedactionMiddleware()
+    req = _request()
+    handler = MagicMock(
+        return_value=ToolMessage(
+            content="-----BEGIN PRIVATE KEY-----\nabc\n-----END PRIVATE KEY-----",
+            tool_call_id="call_1",
+            name="bash",
+            status="success",
+        )
+    )
+    result = mw.wrap_tool_call(req, handler)
+    assert result.content == "[REDACTED]"
+
+
+def test_async_path_redacts_jwt():
+    mw = SensitiveOutputRedactionMiddleware()
+    req = _request()
+
+    async def handler(_):
+        return ToolMessage(
+            content="eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.abcde12345.zyxwv98765",
+            tool_call_id="call_1",
+            name="bash",
+            status="success",
+        )
+
+    result = asyncio.run(mw.awrap_tool_call(req, handler))
+    assert result.content == "[REDACTED]"
+
diff --git a/config.example.yaml b/config.example.yaml
index be3cb7df..3512962f 100644
--- a/config.example.yaml
+++ b/config.example.yaml
@@ -832,3 +832,15 @@ checkpointer:
 #     use: my_package:MyGuardrailProvider
 #     config:
 #       key: value
+
+# --- Option 4: SensitiveDataProvider (strict secret-file blocking) ---
+# guardrails:
+#   enabled: true
+#   fail_closed: true
+#   provider:
+#     use: deerflow.guardrails.builtin:SensitiveDataProvider
+#     config:
+#       protected_tools: ["read_file", "write_file", "str_replace", "ls", "glob", "grep", "bash"]
+#       deny_basenames: [".env"]
+#       deny_globs: [".env.*", "*.pem", "*.key", "id_rsa*", "secrets.*", "credentials.*"]
+#       block_skills_env: true