diff --git a/hermes_cli/config.py b/hermes_cli/config.py
index 4fca9906d17..c292ab9a05e 100644
--- a/hermes_cli/config.py
+++ b/hermes_cli/config.py
@@ -74,6 +74,82 @@ def _warn_config_parse_failure(config_path: Path, exc: Exception) -> None:
 
 _IS_WINDOWS = platform.system() == "Windows"
 _ENV_VAR_NAME_RE = re.compile(r"^[A-Za-z_][A-Za-z0-9_]*$")
+
+# Env var names that influence how the next subprocess executes —
+# never writable through ``save_env_value``. Anything that controls
+# the loader, interpreter, shell, or replacement editor counts:
+#
+# * ``LD_PRELOAD`` / ``LD_LIBRARY_PATH`` / ``LD_AUDIT`` — Linux dynamic
+#   loader. ``DYLD_*`` — macOS equivalent. Planting a path here means
+#   the next ``subprocess.run([...])`` Hermes makes loads attacker code
+#   before main().
+# * ``PYTHONPATH`` / ``PYTHONHOME`` / ``PYTHONSTARTUP`` /
+#   ``PYTHONUSERBASE`` — Python interpreter init. Hermes itself starts
+#   from one of these on every restart.
+# * ``NODE_OPTIONS`` / ``NODE_PATH`` — Node interpreter; affects npm,
+#   ``hermes update``, the TUI build.
+# * ``PATH`` — too broad to allow. The dashboard never needs to rewrite
+#   the operator's PATH; if a tool can't be found, the fix is to add an
+#   absolute path in the integration config, not to mutate PATH globally.
+# * ``GIT_SSH_COMMAND`` / ``GIT_EXEC_PATH`` — git rewrites that fire
+#   on every plugin install / ``hermes update``.
+# * ``BROWSER`` / ``EDITOR`` / ``VISUAL`` / ``PAGER`` — commands the
+#   shell or CLI invokes implicitly. Wrong values here = RCE on next
+#   ``$EDITOR``.
+# * ``SHELL`` — what subprocess uses with ``shell=True`` (we try to
+#   avoid that, but defense in depth).
+# * ``HERMES_HOME`` / ``HERMES_PROFILE`` / ``HERMES_CONFIG`` /
+#   ``HERMES_ENV`` — Hermes runtime location flags. Writing these into
+#   ``.env`` would relocate state in ways the user did not request from
+#   the dashboard. ``config.yaml`` is the supported surface for these.
+#
+# IMPORTANT: ``HERMES_*`` overall is NOT blocked. Many legitimate
+# integration credentials follow that prefix (HERMES_GEMINI_CLIENT_ID,
+# HERMES_LANGFUSE_PUBLIC_KEY, HERMES_SPOTIFY_CLIENT_ID, ...). The
+# denylist is name-by-name on purpose so the gate stays narrow and
+# doesn't accidentally break provider setup wizards.
+#
+# This is enforced on *write* only — values already in ``.env`` (set
+# by the operator out-of-band, or pre-existing) keep working. The
+# point is that the dashboard's writable surface cannot escalate by
+# planting them.
+_ENV_VAR_NAME_DENYLIST: frozenset[str] = frozenset({
+    # Loader / linker
+    "LD_PRELOAD", "LD_LIBRARY_PATH", "LD_AUDIT", "LD_DEBUG",
+    "DYLD_INSERT_LIBRARIES", "DYLD_LIBRARY_PATH", "DYLD_FRAMEWORK_PATH",
+    "DYLD_FALLBACK_LIBRARY_PATH", "DYLD_FALLBACK_FRAMEWORK_PATH",
+    # Python
+    "PYTHONPATH", "PYTHONHOME", "PYTHONSTARTUP", "PYTHONUSERBASE",
+    "PYTHONEXECUTABLE", "PYTHONNOUSERSITE",
+    # Node
+    "NODE_OPTIONS", "NODE_PATH",
+    # General
+    "PATH", "SHELL", "BROWSER", "EDITOR", "VISUAL", "PAGER",
+    # Git
+    "GIT_SSH_COMMAND", "GIT_EXEC_PATH", "GIT_SHELL",
+    # Hermes runtime location — never via dashboard env writer.
+    # NOT a HERMES_* blanket: integration credentials (HERMES_GEMINI_*,
+    # HERMES_LANGFUSE_*, HERMES_SPOTIFY_*, ...) ARE allowed.
+    "HERMES_HOME", "HERMES_PROFILE", "HERMES_CONFIG", "HERMES_ENV",
+})
+
+
+def _reject_denylisted_env_var(key: str) -> None:
+    """Raise if ``key`` is in :data:`_ENV_VAR_NAME_DENYLIST`.
+
+    Centralised so both the regular and "secure" env writers share the
+    same gate, and so the message is consistent for callers.
+    """
+    if key in _ENV_VAR_NAME_DENYLIST:
+        raise ValueError(
+            f"Environment variable {key!r} is on the writer denylist. "
+            "Names that influence subprocess execution (LD_PRELOAD, "
+            "PYTHONPATH, PATH, EDITOR, ...) or Hermes runtime location "
+            "(HERMES_HOME, HERMES_PROFILE, ...) cannot be persisted via "
+            "the env writer. If you really need this, edit "
+            "~/.hermes/.env directly."
+        )
+
 _LAST_EXPANDED_CONFIG_BY_PATH: Dict[str, Any] = {}
 # (path, mtime_ns, size) -> cached expanded config dict.
 # load_config() returns a deepcopy of the cached value when the file
@@ -4874,6 +4950,7 @@ def save_env_value(key: str, value: str):
         return
     if not _ENV_VAR_NAME_RE.match(key):
         raise ValueError(f"Invalid environment variable name: {key!r}")
+    _reject_denylisted_env_var(key)
     value = value.replace("\n", "").replace("\r", "")
     # API keys / tokens must be ASCII — strip non-ASCII with a warning.
     value = _check_non_ascii_credential(key, value)
diff --git a/hermes_cli/web_server.py b/hermes_cli/web_server.py
index f7a73f17e48..d8d7996b868 100644
--- a/hermes_cli/web_server.py
+++ b/hermes_cli/web_server.py
@@ -1223,6 +1223,12 @@ async def set_env_var(body: EnvVarUpdate):
     try:
         save_env_value(body.key, body.value)
         return {"ok": True, "key": body.key}
+    except ValueError as exc:
+        # save_env_value raises ValueError for invalid names and for keys
+        # on the denylist (LD_PRELOAD, PATH, PYTHONPATH, …). Surface the
+        # message to the SPA so the user understands why the write was
+        # refused instead of seeing an opaque 500.
+        raise HTTPException(status_code=400, detail=str(exc)) from exc
     except Exception:
         _log.exception("PUT /api/env failed")
         raise HTTPException(status_code=500, detail="Internal server error")
@@ -4543,6 +4549,17 @@ async def serve_plugin_asset(plugin_name: str, file_path: str):
 
     Only serves files from the plugin's ``dashboard/`` subdirectory.
     Path traversal is blocked by checking ``resolve().is_relative_to()``.
+
+    Restricted to a browser-fetchable suffix allowlist (JS/CSS/JSON/HTML/
+    SVG/PNG/JPG/WOFF). The dashboard loads plugin JS via ``<script src>``
+    and CSS via ``<link href>``, neither of which can attach a custom
+    auth header — so this route stays unauthenticated to keep the SPA
+    working. But user-installed plugins ship a ``plugin_api.py``
+    backend module that the browser never fetches; it's only imported
+    by :func:`_mount_plugin_api_routes` at startup. Without a suffix
+    allowlist, anyone on the loopback port can curl the ``.py`` source
+    of a private third-party plugin. Reject everything outside the
+    browser-asset set.
     """
     plugins = _get_dashboard_plugins()
     plugin = next((p for p in plugins if p["name"] == plugin_name), None)
@@ -4557,7 +4574,11 @@ async def serve_plugin_asset(plugin_name: str, file_path: str):
     if not target.exists() or not target.is_file():
         raise HTTPException(status_code=404, detail="File not found")
 
-    # Guess content type
+    # Browser-asset suffix allowlist. Everything outside this set is
+    # rejected with 404 so we don't leak ``.py`` backend sources, README
+    # files, ``.env.example`` templates, etc. — none of which the SPA
+    # actually fetches. Add to this set deliberately when a new asset
+    # type comes up; do NOT change the default fallback.
     suffix = target.suffix.lower()
     content_types = {
         ".js": "application/javascript",
@@ -4568,10 +4589,22 @@ async def serve_plugin_asset(plugin_name: str, file_path: str):
         ".svg": "image/svg+xml",
         ".png": "image/png",
         ".jpg": "image/jpeg",
+        ".jpeg": "image/jpeg",
+        ".gif": "image/gif",
+        ".webp": "image/webp",
+        ".ico": "image/x-icon",
         ".woff2": "font/woff2",
         ".woff": "font/woff",
+        ".ttf": "font/ttf",
+        ".otf": "font/otf",
+        ".map": "application/json",
     }
-    media_type = content_types.get(suffix, "application/octet-stream")
+    if suffix not in content_types:
+        raise HTTPException(
+            status_code=404,
+            detail="File not found",
+        )
+    media_type = content_types[suffix]
     return FileResponse(
         target,
         media_type=media_type,
diff --git a/tests/hermes_cli/test_config.py b/tests/hermes_cli/test_config.py
index 0c6b9c16115..d86017f2211 100644
--- a/tests/hermes_cli/test_config.py
+++ b/tests/hermes_cli/test_config.py
@@ -4,6 +4,7 @@ import os
 from pathlib import Path
 from unittest.mock import patch, MagicMock
 
+import pytest
 import yaml
 
 from hermes_cli.config import (
@@ -775,3 +776,120 @@ class TestUserMessagePreviewConfig:
         preview = DEFAULT_CONFIG["display"]["user_message_preview"]
         assert preview["first_lines"] == 2
         assert preview["last_lines"] == 2
+
+
+class TestEnvWriteDenylist:
+    """``save_env_value`` refuses to persist env-var names that
+    influence how subprocesses execute — ``LD_PRELOAD``, ``PYTHONPATH``,
+    ``PATH``, ``EDITOR``, etc. — or any ``HERMES_*`` runtime flag.
+
+    The dashboard exposes ``PUT /api/env`` to any authed caller (and
+    the session token lives in the SPA's HTML where any future plugin
+    XSS or local process could exfiltrate it). Without this gate, an
+    attacker who steals the token could plant
+    ``LD_PRELOAD=/tmp/evil.so`` in ``.env`` and own the next Hermes
+    process on next startup via the dotenv → ``os.environ`` chain in
+    ``hermes_cli/env_loader.py``.
+
+    Regression test for the dashboard pentest finding filed alongside
+    the ``web-pentest`` skill (PR #32265 / issue #32267).
+    """
+
+    @pytest.fixture(autouse=True)
+    def _hermes_home(self, tmp_path, monkeypatch):
+        monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+        ensure_hermes_home()
+
+    @pytest.mark.parametrize(
+        "denied_key",
+        [
+            "LD_PRELOAD",
+            "LD_LIBRARY_PATH",
+            "LD_AUDIT",
+            "DYLD_INSERT_LIBRARIES",
+            "DYLD_LIBRARY_PATH",
+            "PYTHONPATH",
+            "PYTHONHOME",
+            "PYTHONSTARTUP",
+            "NODE_OPTIONS",
+            "NODE_PATH",
+            "PATH",
+            "SHELL",
+            "EDITOR",
+            "VISUAL",
+            "PAGER",
+            "BROWSER",
+            "GIT_SSH_COMMAND",
+            "GIT_EXEC_PATH",
+            "HERMES_HOME",
+            "HERMES_PROFILE",
+            "HERMES_CONFIG",
+            "HERMES_ENV",
+        ],
+    )
+    def test_denylisted_keys_rejected(self, denied_key):
+        """Each denylisted name raises ``ValueError`` and never reaches
+        the on-disk ``.env`` file."""
+        with pytest.raises(ValueError, match="denylist"):
+            save_env_value(denied_key, "anything")
+
+        # And nothing landed on disk either.
+        env = load_env()
+        assert denied_key not in env
+
+    @pytest.mark.parametrize(
+        "allowed_key",
+        [
+            "HERMES_GEMINI_CLIENT_ID",
+            "HERMES_LANGFUSE_PUBLIC_KEY",
+            "HERMES_SPOTIFY_CLIENT_ID",
+            "HERMES_QWEN_BASE_URL",
+            "HERMES_MAX_ITERATIONS",
+        ],
+    )
+    def test_hermes_integration_keys_still_writable(self, allowed_key):
+        """``HERMES_*`` overall is NOT blocked — only the four runtime
+        location names (HOME/PROFILE/CONFIG/ENV) are. Integration
+        credentials following the ``HERMES_*`` convention must keep
+        working or we'd regress every provider setup wizard that
+        currently writes one of these (auth.py, Spotify, Langfuse, …)."""
+        save_env_value(allowed_key, "test-value-123")
+        env = load_env()
+        assert env[allowed_key] == "test-value-123"
+
+    def test_legitimate_provider_key_still_works(self):
+        """The denylist must not regress on real provider key writes."""
+        save_env_value("OPENROUTER_API_KEY", "sk-or-test-1234")
+        env = load_env()
+        assert env["OPENROUTER_API_KEY"] == "sk-or-test-1234"
+
+    def test_arbitrary_user_key_still_works(self):
+        """Plugin / user-defined env vars (anything outside the
+        denylist and outside ``HERMES_*``) keep working. The denylist
+        is narrow on purpose."""
+        save_env_value("MY_PLUGIN_TOKEN", "plugin-secret-123")
+        env = load_env()
+        assert env["MY_PLUGIN_TOKEN"] == "plugin-secret-123"
+
+    def test_save_env_value_secure_inherits_denylist(self):
+        """The ``_secure`` variant goes through ``save_env_value`` so
+        it inherits the gate — verify, don't assume."""
+        with pytest.raises(ValueError, match="denylist"):
+            save_env_value_secure("LD_PRELOAD", "/tmp/evil.so")
+
+    def test_pre_existing_value_in_env_file_is_left_alone(self, tmp_path):
+        """The gate is on *write*. If ``.env`` already contains
+        ``LD_PRELOAD`` (set out-of-band by the operator before this
+        change shipped, or hand-edited), we don't blow up — we just
+        refuse to add or update it via the API."""
+        env_path = tmp_path / ".env"
+        env_path.write_text("LD_PRELOAD=/something/legit.so\n")
+
+        # load_env returns it (the read path is intentionally permissive)
+        env = load_env()
+        assert env["LD_PRELOAD"] == "/something/legit.so"
+
+        # But the write path still refuses to update it
+        with pytest.raises(ValueError, match="denylist"):
+            save_env_value("LD_PRELOAD", "/tmp/evil.so")
+
diff --git a/tests/hermes_cli/test_web_server.py b/tests/hermes_cli/test_web_server.py
index d46e87c2862..13e8001e7d0 100644
--- a/tests/hermes_cli/test_web_server.py
+++ b/tests/hermes_cli/test_web_server.py
@@ -2375,3 +2375,78 @@ class TestPtyWebSocket:
             ):
                 pass
         assert exc.value.code == 4400
+
+
+class TestDashboardPluginStaticAssetAllowlist:
+    """``/dashboard-plugins/<name>/<path>`` is unauthenticated by design —
+    the SPA loads plugin JS via ``<script src>`` and CSS via
+    ``<link href>``, neither of which can attach a custom auth header.
+    Instead the route restricts file types to the browser-asset
+    allowlist (JS/CSS/JSON/images/fonts) so that user-installed
+    plugins shipping a ``plugin_api.py`` backend module don't leak
+    their Python source to anyone reachable on the loopback port.
+
+    Regression test for the dashboard pentest finding filed alongside
+    the ``web-pentest`` skill (PR #32265 / issue #32267).
+    """
+
+    @pytest.fixture(autouse=True)
+    def _setup_test_client(self, monkeypatch, _isolate_hermes_home):
+        try:
+            from starlette.testclient import TestClient
+        except ImportError:
+            pytest.skip("fastapi/starlette not installed")
+
+        from hermes_cli.web_server import app
+
+        self.client = TestClient(app)
+
+    def test_python_source_is_404(self):
+        """The example plugin's ``plugin_api.py`` must NOT be served as
+        a static asset, even though the file exists under the plugin's
+        dashboard directory. Suffix not in the allowlist → 404."""
+        resp = self.client.get("/dashboard-plugins/example/plugin_api.py")
+        assert resp.status_code == 404
+
+    def test_pycache_is_404(self):
+        """Same protection for compiled Python (``.pyc``) inside the
+        plugin's ``__pycache__/``. Real plugins ship these as a
+        side-effect of running tests / dashboard once."""
+        # __pycache__ files are only generated after the api file has
+        # been imported once. Use the path the example plugin actually
+        # generates during the dashboard test boot.
+        resp = self.client.get(
+            "/dashboard-plugins/example/__pycache__/plugin_api.cpython-311.pyc"
+        )
+        # 404 either way (file may not exist on this CI Python version);
+        # what matters is we never get a 200 with the bytes.
+        assert resp.status_code == 404
+
+    def test_manifest_json_still_served(self):
+        """JSON files remain browser-fetchable — manifests, localized
+        data, source maps, etc. all sit in this bucket."""
+        resp = self.client.get("/dashboard-plugins/example/manifest.json")
+        assert resp.status_code == 200
+        assert resp.headers["content-type"].startswith("application/json")
+        # And the body is actually the manifest, not the SPA fallback.
+        body = resp.json()
+        assert body.get("name") == "example"
+
+    def test_unknown_plugin_is_404(self):
+        """Existing behaviour preserved: nonexistent plugin name → 404."""
+        resp = self.client.get(
+            "/dashboard-plugins/_definitely_not_a_plugin_/manifest.json"
+        )
+        assert resp.status_code == 404
+
+    def test_path_traversal_still_blocked(self):
+        """The allowlist is on top of the existing ``.resolve()`` /
+        ``is_relative_to()`` check — a ``.js`` named file at an
+        out-of-base path is still rejected as traversal, not served."""
+        resp = self.client.get(
+            "/dashboard-plugins/example/..%2Fplugin_api.py"
+        )
+        # 403 traversal-blocked OR 404 (depending on URL decode order)
+        # — never 200.
+        assert resp.status_code in (403, 404)
+