diff --git a/api/server.py b/api/server.py
index a03dd0e..547bbb1 100644
--- a/api/server.py
+++ b/api/server.py
@@ -26,6 +26,24 @@ API_KEY = os.environ.get("LLM_LIB_API_KEY", "change-me")
 log = logging.getLogger("llm-library")
 logging.basicConfig(level=logging.INFO, format="%(asctime)s %(levelname)s %(message)s")
 
+# ─── Security helpers ──────────────────────────────────
+import re
+SAFE_ID = re.compile(r'^[a-zA-Z0-9_.\-]+$')
+
+def safe_paper_id(raw: str) -> str:
+    """Validate paper ID, reject path traversal attempts."""
+    if not raw or len(raw) > 128 or not SAFE_ID.match(raw) or '..' in raw:
+        raise HTTPException(status_code=400, detail="Invalid paper ID")
+    return raw
+
+def safe_pdf_path(base: Path, paper_id: str) -> Path:
+    """Build safe PDF path, ensuring it stays within base directory."""
+    pid = safe_paper_id(paper_id)
+    fp = (base / f"{pid}.pdf").resolve()
+    if not str(fp).startswith(str(base.resolve())):
+        raise HTTPException(status_code=400, detail="Invalid path")
+    return fp
+
 # ─── App ───────────────────────────────────────────────
 app = FastAPI(
     title="LLM 论文图书馆",
@@ -280,7 +298,7 @@ def delete_paper(
 @app.get("/papers/arxiv/{arxiv_id}")
 def serve_arxiv_pdf(arxiv_id: str):
     """从本地缓存提供 arXiv PDF（无 .pdf 后缀路由防 IDM 拦截）"""
-    pdf_path = PAPERS_DIR / "arxiv" / f"{arxiv_id}.pdf"
+    pdf_path = safe_pdf_path(PAPERS_DIR / "arxiv", arxiv_id)
     if not pdf_path.exists():
         raise HTTPException(status_code=404, detail=f"PDF not in local cache: {arxiv_id}")
     return FileResponse(
@@ -448,8 +466,8 @@ def translate_status(arxiv_id: str):
 @app.post("/api/download/{arxiv_id}")
 def download_single_pdf(arxiv_id: str, _=Depends(verify_api_key)):
     """按需下载单篇 arXiv PDF"""
-    import subprocess, sys
-    pdf_path = PAPERS_DIR / "arxiv" / f"{arxiv_id}.pdf"
+    safe_paper_id(arxiv_id)
+    pdf_path = safe_pdf_path(PAPERS_DIR / "arxiv", arxiv_id)
     if pdf_path.exists():
         return {"ok": True, "arxiv_id": arxiv_id, "status": "cached"}
 
@@ -470,13 +488,16 @@ TRANSLATED_DIR = PAPERS_DIR / "translated"
 @app.get("/api/translated/{arxiv_id}")
 def check_translation(arxiv_id: str):
     """Check if translation exists for a paper"""
-    fn = f"{arxiv_id}.pdf"
-    return {"arxiv_id": arxiv_id, "exists": (TRANSLATED_DIR / fn).exists()}
+    try:
+        fp = safe_pdf_path(TRANSLATED_DIR, arxiv_id)
+        return {"arxiv_id": arxiv_id, "exists": fp.exists()}
+    except HTTPException:
+        return {"arxiv_id": arxiv_id, "exists": False}
 
 @app.get("/papers/translated/{arxiv_id}.pdf")
 def serve_translated(arxiv_id: str):
     """Serve translated PDF from cache"""
-    fp = TRANSLATED_DIR / f"{arxiv_id}.pdf"
+    fp = safe_pdf_path(TRANSLATED_DIR, arxiv_id)
     if not fp.exists():
         raise HTTPException(status_code=404, detail="Translation not found")
     return FileResponse(fp, media_type="application/pdf",
@@ -497,25 +518,26 @@ def translation_status():
 async def trigger_translation(paper_id: str, _=Depends(verify_api_key)):
     """Trigger pdf2zh translation for a paper (DeepSeek V4 Flash).
     paper_id can be arxiv ID (e.g. 1706.03762) or a HF filename."""
+    safe_paper_id(paper_id)  # validate
     # Find the PDF
     pdf_path = None
     # Try arxiv
-    candidate = PAPERS_DIR / "arxiv" / f"{paper_id}.pdf"
+    candidate = safe_pdf_path(PAPERS_DIR / "arxiv", paper_id)
     if candidate.exists():
         pdf_path = candidate
     else:
         # Try HF papers directory
-        hf_dir = PAPERS_DIR / "hf"
+        hf_dir = (PAPERS_DIR / "hf").resolve()
         if hf_dir.exists():
             for f in hf_dir.glob("*.pdf"):
-                if paper_id in f.stem:
+                if paper_id in f.stem and str(f.resolve()).startswith(str(hf_dir)):
                     pdf_path = f
                     break
     
     if not pdf_path:
         raise HTTPException(status_code=404, detail=f"PDF not found for {paper_id}")
     
-    out_path = TRANSLATED_DIR / f"{paper_id}.pdf"
+    out_path = safe_pdf_path(TRANSLATED_DIR, paper_id)
     if out_path.exists():
         return {"paper_id": paper_id, "status": "already_translated"}