diff --git a/apps/api/src/services/host_repair_agent.py b/apps/api/src/services/host_repair_agent.py
index 1b307b09..bb5f1c06 100644
--- a/apps/api/src/services/host_repair_agent.py
+++ b/apps/api/src/services/host_repair_agent.py
@@ -56,6 +56,8 @@ _SUPPORTED_SCHEMES = {"openclaw", "ansible", "ssh"}
 # Prevents: pipe (|), redirect (>, <), command substitution ($(), ${}, ``), logic ops (;, &)
 _SHELL_METACHAR_RE = re.compile(r'[;&|<>`\n]|(\$\{|\$\()')
 _MAX_COMMAND_LEN = 512
+# 2026-04-07 Claude Code: S2 — SSH username 防禦性驗證 (小寫英數+底線+連字號)
+_SSH_USER_RE = re.compile(r'^[a-z][a-z0-9_-]{0,31}$')
 
 # Ansible 控制節點設定 — 從 env/ConfigMap 讀取
 # 2026-04-06 Claude Code: Sprint 3 T3
@@ -100,6 +102,21 @@ def validate_ssh_target_host(host: str) -> None:
         )
 
 
+def validate_ssh_user(user: str) -> None:
+    """
+    驗證 SSH username 僅含安全字元，防止 user@host 拼接產生非預期行為。
+    2026-04-07 Claude Code: Re-Review S2 — 防禦性工程
+
+    Raises:
+        ValueError: username 格式不合法
+    """
+    if not _SSH_USER_RE.match(user):
+        raise ValueError(
+            f"Security Block: SSH user '{user}' contains invalid characters. "
+            f"Expected: lowercase alphanumeric, underscore, hyphen (1-32 chars)"
+        )
+
+
 def parse_uri_command(command: str) -> SshCommandURI:
     """
     解析 SSH_COMMAND URI scheme。
@@ -179,6 +196,13 @@ class HostRepairAgent:
             cls._instance._in_process_locks = {}
         return cls._instance
 
+    @classmethod
+    def _reset_for_test(cls) -> None:
+        """測試專用：重置 singleton 狀態，避免跨測試污染。
+        2026-04-07 Claude Code: Re-Review S3
+        """
+        cls._instance = None
+
     def __init__(self) -> None:
         # in-process 鎖表 — key: lock_key → asyncio.Lock
         # 2026-04-06 Claude Code: Sprint 3 T4
@@ -271,12 +295,58 @@ class HostRepairAgent:
             logger.error("ssh_command_audit_failed", uri=uri, error=str(e))
             # Do not re-raise — audit failure must not affect repair result
 
+    async def _execute_and_observe(
+        self,
+        command: str,
+        uri: SshCommandURI,
+        execute_fn,
+    ) -> HostRepairResult:
+        """
+        執行修復 + AuditLog + Langfuse trace 的公用邏輯。
+        2026-04-07 Claude Code: Re-Review S1 — 消除 repair_by_uri 中 3 處重複
+        """
+        import time as _time
+        _start = _time.monotonic()
+        result = await execute_fn()
+        duration_ms = int((_time.monotonic() - _start) * 1000)
+
+        # AuditLog (fire and forget)
+        try:
+            await self._write_audit_log(
+                uri=command,
+                success=result.success,
+                output=result.output,
+                error=result.error or None,
+                duration_ms=duration_ms,
+            )
+        except Exception:
+            pass
+
+        # Langfuse trace (fire and forget)
+        try:
+            from src.services.langfuse_client import get_langfuse
+            lf = get_langfuse()
+            if lf:
+                trace = lf.trace(name="ssh_command_repair")
+                trace.span(
+                    name=f"{uri.scheme}_execute",
+                    input={"uri": command},
+                    output={"success": result.success, "output": result.output[:500] if result.output else ""},
+                    metadata={"duration_ms": duration_ms, "scheme": uri.scheme},
+                )
+                lf.flush()
+        except Exception as lf_err:
+            logger.debug("langfuse_trace_skipped", error=str(lf_err))
+
+        return result
+
     async def repair_by_uri(self, command: str, approved: bool = False) -> HostRepairResult:
         """
         根據 URI scheme 路由至對應的執行路徑。
         2026-04-06 Claude Code: Sprint 3 T3
         2026-04-06 Claude Code: Sprint 3 T4 — Redis 冪等鎖防止重複修復
         2026-04-06 Claude Code: Sprint 3 T5 — AuditLog + Langfuse Trace
+        2026-04-07 Claude Code: Re-Review S1 — 抽取 _execute_and_observe 消除重複
         """
         try:
             uri = parse_uri_command(command)
@@ -334,30 +404,15 @@ class HostRepairAgent:
             except Exception:
                 redis_lock = None  # Redis 未連線，fail open
 
+            # Redis 無法取得 → fail open，直接執行
             if redis_lock is None:
-                import time as _time
-                _start = _time.monotonic()
-                result = await _execute()
-                duration_ms = int((_time.monotonic() - _start) * 1000)
-                try:
-                    await self._write_audit_log(uri=command, success=result.success, output=result.output, error=result.error or None, duration_ms=duration_ms)
-                except Exception:
-                    pass
-                return result
+                return await self._execute_and_observe(command, uri, _execute)
 
             try:
                 acquired = await redis_lock.acquire()
             except Exception:
                 # Redis 不可用，fail open
-                import time as _time
-                _start = _time.monotonic()
-                result = await _execute()
-                duration_ms = int((_time.monotonic() - _start) * 1000)
-                try:
-                    await self._write_audit_log(uri=command, success=result.success, output=result.output, error=result.error or None, duration_ms=duration_ms)
-                except Exception:
-                    pass
-                return result
+                return await self._execute_and_observe(command, uri, _execute)
 
             if not acquired:
                 # Redis 鎖已被其他 Pod 持有
@@ -369,44 +424,13 @@ class HostRepairAgent:
                 )
 
             try:
-                import time as _time
-                _start = _time.monotonic()
-                result = await _execute()
-                duration_ms = int((_time.monotonic() - _start) * 1000)
+                result = await self._execute_and_observe(command, uri, _execute)
             finally:
                 try:
                     await redis_lock.release()
                 except Exception:
                     pass
 
-            # AuditLog (fire and forget)
-            try:
-                await self._write_audit_log(
-                    uri=command,
-                    success=result.success,
-                    output=result.output,
-                    error=result.error or None,
-                    duration_ms=duration_ms,
-                )
-            except Exception:
-                pass
-
-            # Langfuse trace (fire and forget)
-            try:
-                from src.services.langfuse_client import get_langfuse
-                lf = get_langfuse()
-                if lf:
-                    trace = lf.trace(name="ssh_command_repair")
-                    trace.span(
-                        name=f"{uri.scheme}_execute",
-                        input={"uri": command},
-                        output={"success": result.success, "output": result.output[:500] if result.output else ""},
-                        metadata={"duration_ms": duration_ms, "scheme": uri.scheme},
-                    )
-                    lf.flush()
-            except Exception as lf_err:
-                logger.debug("langfuse_trace_skipped", error=str(lf_err))
-
             return result
 
     async def _execute_openclaw(self, layer: str, component: str) -> HostRepairResult:
@@ -509,7 +533,10 @@ class HostRepairAgent:
         )
 
     async def _ssh_execute(self, host: str, user: str, key_path: str, command: str) -> str:
-        """執行 SSH 命令，回傳 stdout。key_path 由呼叫方傳入，不反查。"""
+        """執行 SSH 命令，回傳 stdout。key_path 由呼叫方傳入，不反查。
+        2026-04-07 Claude Code: Re-Review S2 — 加 user 防禦性驗證
+        """
+        validate_ssh_user(user)
         import time
         deadline = time.monotonic() + SSH_TIMEOUT
         proc = await asyncio.wait_for(
diff --git a/apps/api/tests/test_host_repair_agent.py b/apps/api/tests/test_host_repair_agent.py
index bc42cbbb..74548f2d 100644
--- a/apps/api/tests/test_host_repair_agent.py
+++ b/apps/api/tests/test_host_repair_agent.py
@@ -437,3 +437,74 @@ class TestSSHTargetWhitelist:
         result = await agent.repair_by_uri("ssh://wooo@192.168.0.999/ls", approved=True)
         assert result.success is False
         assert "not in allowed whitelist" in result.error
+
+
+# =============================================================================
+# S2 Tests: SSH username validation
+# 2026-04-07 Claude Code: Re-Review S2
+# =============================================================================
+
+class TestSSHUserValidation:
+    """2026-04-07 Claude Code: Re-Review S2 Tests"""
+
+    def test_valid_user_wooo(self):
+        from src.services.host_repair_agent import validate_ssh_user
+        validate_ssh_user("wooo")  # must not raise
+
+    def test_valid_user_ollama(self):
+        from src.services.host_repair_agent import validate_ssh_user
+        validate_ssh_user("ollama")  # must not raise
+
+    def test_valid_user_with_underscore(self):
+        from src.services.host_repair_agent import validate_ssh_user
+        validate_ssh_user("deploy_user")  # must not raise
+
+    def test_invalid_user_with_space(self):
+        from src.services.host_repair_agent import validate_ssh_user
+        with pytest.raises(ValueError, match="invalid characters"):
+            validate_ssh_user("bad user")
+
+    def test_invalid_user_with_at(self):
+        from src.services.host_repair_agent import validate_ssh_user
+        with pytest.raises(ValueError, match="invalid characters"):
+            validate_ssh_user("user@host")
+
+    def test_invalid_user_root(self):
+        """root 是合法格式但這裡不做權限限制，只驗格式"""
+        from src.services.host_repair_agent import validate_ssh_user
+        validate_ssh_user("root")  # format-valid, authorization is separate
+
+    def test_invalid_user_empty(self):
+        from src.services.host_repair_agent import validate_ssh_user
+        with pytest.raises(ValueError, match="invalid characters"):
+            validate_ssh_user("")
+
+    def test_invalid_user_starts_with_digit(self):
+        from src.services.host_repair_agent import validate_ssh_user
+        with pytest.raises(ValueError, match="invalid characters"):
+            validate_ssh_user("1baduser")
+
+
+# =============================================================================
+# S3 Tests: Singleton reset for test isolation
+# 2026-04-07 Claude Code: Re-Review S3
+# =============================================================================
+
+class TestSingletonReset:
+    """2026-04-07 Claude Code: Re-Review S3 Tests"""
+
+    def test_reset_creates_new_instance(self):
+        from src.services.host_repair_agent import HostRepairAgent
+        agent1 = HostRepairAgent()
+        HostRepairAgent._reset_for_test()
+        agent2 = HostRepairAgent()
+        assert agent1 is not agent2, "_reset_for_test should create a fresh instance"
+
+    def test_reset_clears_locks(self):
+        from src.services.host_repair_agent import HostRepairAgent
+        agent = HostRepairAgent()
+        agent._get_in_process_lock("test_key")
+        assert len(agent._in_process_locks) > 0
+        HostRepairAgent._reset_for_test()
+        new_agent = HostRepairAgent()
+        assert len(new_agent._in_process_locks) == 0, "Reset should give fresh lock dict"