diff --git a/CHANGELOG.md b/CHANGELOG.md
index ad7f49cd..0f4566a4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,8 @@
 
 ### Added
 
+- **Opt-in CSP `frame-src` allowlist so an extension can embed a self-hosted web app in an iframe.** A new `HERMES_WEBUI_CSP_FRAME_EXTRA` env var widens what the WebUI page may embed in an `<iframe>` (default is same-origin only), mirroring the existing `HERMES_WEBUI_CSP_CONNECT_EXTRA` knob — space-separated `http(s)` origins with optional wildcard subdomain and port, validated and ignored if malformed. This unblocks an "external app tab" extension (pin Grafana / Vaultwarden / your own dashboard as a tab). It only governs what *this* page can embed; `frame-ancestors` stays `'none'`, so it does not let anyone embed the WebUI itself. Default-off; with the var unset, behavior is unchanged. Documented in `docs/EXTENSIONS.md`.
+
 - **`/moa` now works in the WebUI, routing the turn through your configured Mixture-of-Agents preset.** Typing `/moa <prompt>` runs that turn through the agent's MoA config (the same preset/usage you'd get from the CLI) instead of a plain single-model turn; `/moa` with no prompt shows the available usage. The MoA config is resolved **server-side** from your profile config — the browser only signals that an override is wanted, it can't supply the config — and it's applied per-turn (not persisted). It fails closed on gateway-backed sessions. Thanks @rodboev. (#5070, #5057)
 
 - **Three new opt-in appearance skins: GitHub, Codex, and Terracotta.** All are CSS-only, namespaced under `[data-skin]`, and selectable from Settings → Appearance — they change nothing unless you pick them. GitHub uses a restrained graphite + Primer-blue palette; Codex a minimal editor look with a muted sage accent; Terracotta a warm clay accent on a soft neutral background. Thanks @gottipx (GitHub #4634, Codex #4636, Terracotta #4635 — renamed from the originally-proposed name to a descriptive material name).
diff --git a/api/helpers.py b/api/helpers.py
index ed5b7755..1388e2b7 100644
--- a/api/helpers.py
+++ b/api/helpers.py
@@ -60,6 +60,13 @@ _CSP_CONNECT_BASE = (
 _CSP_EXTRA_CONNECT_RE = _re.compile(
     r"^(?:https?|wss?)://(?:\*\.)?[A-Za-z0-9._~-]+(?::(?P<port>\d{1,5}|\*))?$"
 )
+# Validator for an opt-in frame-src allowlist entry (HERMES_WEBUI_CSP_FRAME_EXTRA).
+# Only http(s) origins (optional wildcard subdomain + optional port) are accepted —
+# the same shape as the connect-extra validator minus the ws/wss schemes, since an
+# iframe src is always http(s).
+_CSP_EXTRA_FRAME_RE = _re.compile(
+    r"^https?://(?:\*\.)?[A-Za-z0-9._~-]+(?::(?P<port>\d{1,5}|\*))?$"
+)
 _CSP_HEADER_NAME = 'Content-Security-Policy'
 _CSP_SHARED_POLICY_TEMPLATE = (
     "default-src 'self' https://*.cloudflareaccess.com; "
@@ -72,9 +79,16 @@ _CSP_SHARED_POLICY_TEMPLATE = (
     "font-src 'self' data: https://fonts.gstatic.com; "
     "media-src 'self' data: blob:; "
     "connect-src {connect_src}; "
+    "frame-src {frame_src}; "
     "manifest-src 'self' https://*.cloudflareaccess.com; "
     "base-uri 'self'; form-action 'self'"
 )
+# Base frame-src: same-origin only by default (so the existing same-origin
+# dashboard/extension iframes keep working). An operator can widen it, opt-in,
+# via HERMES_WEBUI_CSP_FRAME_EXTRA — e.g. to embed a self-hosted dashboard in an
+# extension tab. This governs what THIS page may embed; it does NOT affect
+# frame-ancestors (who may embed the WebUI), which stays 'none'.
+_CSP_FRAME_BASE = "'self'"
 
 
 def _valid_csp_extra_connect_source(source: str) -> bool:
@@ -101,21 +115,58 @@ def _csp_extra_connect_src() -> str:
     return " " + " ".join(sources)
 
 
+def _valid_csp_extra_frame_source(source: str) -> bool:
+    match = _CSP_EXTRA_FRAME_RE.fullmatch(source)
+    if not match:
+        return False
+    port = match.group("port")
+    if not port or port == "*":
+        return True
+    try:
+        return 1 <= int(port) <= 65535
+    except ValueError:
+        return False
+
+
+def _csp_extra_frame_src() -> str:
+    raw = os.getenv("HERMES_WEBUI_CSP_FRAME_EXTRA", "").strip()
+    if not raw:
+        return ""
+    sources = raw.split()
+    if not sources or any(not _valid_csp_extra_frame_source(src) for src in sources):
+        logger.warning("Ignoring invalid HERMES_WEBUI_CSP_FRAME_EXTRA value")
+        return ""
+    return " " + " ".join(sources)
+
+
 def _csp_connect_src(extra_connect_src: str = "") -> str:
     return f"{_CSP_CONNECT_BASE} https://cdn.jsdelivr.net{extra_connect_src}"
 
 
-def _build_csp_enforced_policy(extra_connect_src: str | None = None) -> str:
+def _csp_frame_src(extra_frame_src: str = "") -> str:
+    return f"{_CSP_FRAME_BASE}{extra_frame_src}"
+
+
+def _build_csp_enforced_policy(
+    extra_connect_src: str | None = None,
+    extra_frame_src: str | None = None,
+) -> str:
     if extra_connect_src is None:
         extra_connect_src = _csp_extra_connect_src()
+    if extra_frame_src is None:
+        extra_frame_src = _csp_extra_frame_src()
     return _CSP_SHARED_POLICY_TEMPLATE.format(
-        connect_src=_csp_connect_src(extra_connect_src)
+        connect_src=_csp_connect_src(extra_connect_src),
+        frame_src=_csp_frame_src(extra_frame_src),
     )
 
 
-def _build_csp_report_only_policy(extra_connect_src: str | None = None) -> str:
+def _build_csp_report_only_policy(
+    extra_connect_src: str | None = None,
+    extra_frame_src: str | None = None,
+) -> str:
     return (
-        _build_csp_enforced_policy(extra_connect_src)
+        _build_csp_enforced_policy(extra_connect_src, extra_frame_src)
         + "; report-uri /api/csp-report; report-to csp-endpoint"
     )
 
@@ -123,11 +174,13 @@ def _build_csp_report_only_policy(extra_connect_src: str | None = None) -> str:
 def _security_headers(handler):
     """Add security headers to every response."""
     extra_connect_src = _csp_extra_connect_src()
+    extra_frame_src = _csp_extra_frame_src()
     handler._csp_extra_connect_src = extra_connect_src
+    handler._csp_extra_frame_src = extra_frame_src
     handler.send_header('X-Content-Type-Options', 'nosniff')
     handler.send_header('X-Frame-Options', 'DENY')
     handler.send_header('Referrer-Policy', 'same-origin')
-    handler.send_header(_CSP_HEADER_NAME, _build_csp_enforced_policy(extra_connect_src))
+    handler.send_header(_CSP_HEADER_NAME, _build_csp_enforced_policy(extra_connect_src, extra_frame_src))
     handler.send_header(
         'Permissions-Policy',
         'camera=(), microphone=(self), geolocation=(), clipboard-write=(self)'
diff --git a/docs/EXTENSIONS.md b/docs/EXTENSIONS.md
index 51e5b5e7..01daef3c 100644
--- a/docs/EXTENSIONS.md
+++ b/docs/EXTENSIONS.md
@@ -251,6 +251,37 @@ paths are never returned by the status endpoint. If `health_path` is omitted,
 diagnostics use `/health`; if `health_path` is present but invalid, the sidecar is
 skipped rather than probed.
 
+## Embedding an external web app in an iframe
+
+By default the WebUI's Content-Security-Policy only allows it to embed
+**same-origin** content in an `<iframe>` (the `frame-src` directive falls back to
+`'self'`). An extension that wants to pin an external self-hosted web app — a
+Grafana board, Vaultwarden, a personal dashboard — as a tab therefore needs the
+operator to widen `frame-src`, opt-in, via an environment variable:
+
+```bash
+# space-separated http(s) origins; optional *. subdomain wildcard and port.
+export HERMES_WEBUI_CSP_FRAME_EXTRA="https://grafana.example.com https://*.dash.example.com:8443"
+```
+
+Rules and guarantees:
+
+- Only `http(s)` origins are accepted (an iframe `src` is always http(s)).
+  Entries may include a `*.` subdomain wildcard and a port or `*` port; a path,
+  a `ws://`/`wss://` scheme, an invalid port, or any attempt to inject another
+  directive is rejected and the whole value is ignored (with a logged warning).
+- This mirrors the existing `HERMES_WEBUI_CSP_CONNECT_EXTRA` knob (which widens
+  `connect-src` for `fetch`/WebSocket); the two are independent.
+- It only governs what the WebUI page may **embed**. It does **not** touch
+  `frame-ancestors`, which stays `'none'` — so widening `frame-src` never lets
+  another site embed the WebUI itself.
+- Default-off: with the variable unset, the policy is unchanged (same-origin
+  iframes only).
+
+An "external app tab" extension should document the exact origin(s) it needs so
+the operator can set this knob deliberately, rather than assuming a wide-open
+policy.
+
 ## Static file serving
 
 When `HERMES_WEBUI_EXTENSION_DIR` points at an existing directory, files under
diff --git a/server.py b/server.py
index 44dd9a2d..a2bd2011 100644
--- a/server.py
+++ b/server.py
@@ -292,12 +292,13 @@ class Handler(BaseHTTPRequestHandler):
     _CSP_REPORT_TO = '{"group":"csp-endpoint","max_age":10886400,"endpoints":[{"url":"/api/csp-report"}]}'
 
     @classmethod
-    def csp_report_only_policy(cls, extra_connect_src=None) -> str:
-        return _build_csp_report_only_policy(extra_connect_src)
+    def csp_report_only_policy(cls, extra_connect_src=None, extra_frame_src=None) -> str:
+        return _build_csp_report_only_policy(extra_connect_src, extra_frame_src)
 
     def end_headers(self) -> None:
         extra_connect_src = getattr(self, "_csp_extra_connect_src", None)
-        self.send_header("Content-Security-Policy-Report-Only", self.csp_report_only_policy(extra_connect_src))
+        extra_frame_src = getattr(self, "_csp_extra_frame_src", None)
+        self.send_header("Content-Security-Policy-Report-Only", self.csp_report_only_policy(extra_connect_src, extra_frame_src))
         self.send_header("Report-To", self._CSP_REPORT_TO)
         super().end_headers()
 
diff --git a/tests/test_csp_frame_src_extra.py b/tests/test_csp_frame_src_extra.py
new file mode 100644
index 00000000..6b13ef1d
--- /dev/null
+++ b/tests/test_csp_frame_src_extra.py
@@ -0,0 +1,98 @@
+"""Regression coverage for the configurable CSP frame-src allowlist knob.
+
+`HERMES_WEBUI_CSP_FRAME_EXTRA` lets an operator widen what the WebUI page may
+embed in an <iframe> (e.g. a self-hosted dashboard pinned as an extension tab),
+opt-in and default-off. The base policy is same-origin only, and the knob never
+touches `frame-ancestors` (who may embed the WebUI), which stays 'none'.
+"""
+
+from __future__ import annotations
+
+
+def test_csp_frame_src_default_is_self_only(monkeypatch):
+    from server import Handler
+
+    monkeypatch.delenv("HERMES_WEBUI_CSP_FRAME_EXTRA", raising=False)
+
+    policy = Handler.csp_report_only_policy()
+    assert "frame-src 'self'; " in policy
+    # frame-ancestors must remain locked down regardless.
+    assert "frame-ancestors 'none'" in policy
+
+
+def test_csp_frame_src_includes_valid_extra_origins(monkeypatch):
+    from server import Handler
+
+    monkeypatch.setenv(
+        "HERMES_WEBUI_CSP_FRAME_EXTRA",
+        "https://grafana.example.com https://*.dash.example.com:8443",
+    )
+
+    policy = Handler.csp_report_only_policy()
+    assert (
+        "frame-src 'self' "
+        "https://grafana.example.com https://*.dash.example.com:8443; "
+    ) in policy
+
+
+def test_csp_frame_src_extra_in_enforced_policy(monkeypatch):
+    from api.helpers import _build_csp_enforced_policy
+
+    monkeypatch.setenv("HERMES_WEBUI_CSP_FRAME_EXTRA", "http://127.0.0.1:3000")
+    enforced = _build_csp_enforced_policy()
+    assert "frame-src 'self' http://127.0.0.1:3000;" in enforced
+
+
+def test_csp_frame_src_rejects_directive_injection(monkeypatch, caplog):
+    from server import Handler
+
+    monkeypatch.setenv(
+        "HERMES_WEBUI_CSP_FRAME_EXTRA",
+        "https://ok.example.com; script-src *",
+    )
+
+    policy = Handler.csp_report_only_policy()
+    assert "https://ok.example.com" not in policy
+    assert "script-src *" not in policy
+    assert "frame-src 'self'; " in policy  # falls back to safe default
+    assert "Ignoring invalid HERMES_WEBUI_CSP_FRAME_EXTRA" in caplog.text
+
+
+def test_csp_frame_src_rejects_paths(monkeypatch):
+    from server import Handler
+
+    monkeypatch.setenv("HERMES_WEBUI_CSP_FRAME_EXTRA", "https://app.example.com/embed")
+    policy = Handler.csp_report_only_policy()
+    assert "https://app.example.com/embed" not in policy
+
+
+def test_csp_frame_src_rejects_ws_scheme(monkeypatch):
+    """An iframe src is always http(s); ws/wss are not valid frame sources."""
+    from server import Handler
+
+    monkeypatch.setenv("HERMES_WEBUI_CSP_FRAME_EXTRA", "wss://socket.example.com")
+    policy = Handler.csp_report_only_policy()
+    assert "wss://socket.example.com" not in policy
+    assert "frame-src 'self'; " in policy
+
+
+def test_csp_frame_src_rejects_invalid_ports(monkeypatch):
+    from server import Handler
+
+    monkeypatch.setenv("HERMES_WEBUI_CSP_FRAME_EXTRA", "https://app.example.com:99999")
+    policy = Handler.csp_report_only_policy()
+    assert "https://app.example.com:99999" not in policy
+
+
+def test_csp_frame_src_does_not_affect_connect_src(monkeypatch):
+    """The frame knob and the connect knob are independent."""
+    from server import Handler
+
+    monkeypatch.setenv("HERMES_WEBUI_CSP_FRAME_EXTRA", "https://dash.example.com")
+    monkeypatch.delenv("HERMES_WEBUI_CSP_CONNECT_EXTRA", raising=False)
+    policy = Handler.csp_report_only_policy()
+    # frame-extra present in frame-src ...
+    assert "frame-src 'self' https://dash.example.com;" in policy
+    # ... and NOT leaked into connect-src (which ends at cdn.jsdelivr.net).
+    connect_seg = policy.split("connect-src", 1)[1].split(";", 1)[0]
+    assert "dash.example.com" not in connect_seg