diff --git a/backend/app/main.py b/backend/app/main.py
index d88dc1f..f262faa 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -13,6 +13,7 @@
 from datetime import datetime, timezone
 from pathlib import Path
 from typing import List
+from urllib.parse import quote
 
 import aiosqlite
 import httpx
@@ -67,6 +68,36 @@
 from .scanners.semgrep import run_semgrep
 from .utils.fs import ensure_dir, safe_rmtree, unzip_to_dir
 
+GIT_REF_RE = re.compile(r"^[A-Za-z0-9._/-]+$")
+
+
+def validate_git_ref(ref: str) -> str:
+    if not ref:
+        raise HTTPException(status_code=400, detail="Invalid Git reference")
+
+    if len(ref) > 255:
+        raise HTTPException(status_code=400, detail="Invalid Git reference")
+
+    if not GIT_REF_RE.fullmatch(ref):
+        raise HTTPException(status_code=400, detail="Invalid Git reference")
+
+    forbidden = (
+        "..",
+        "./",
+        "\\",
+        "//",
+    )
+
+    if (
+        any(token in ref for token in forbidden)
+        or ref.startswith("/")
+        or ref.endswith("/")
+    ):
+        raise HTTPException(status_code=400, detail="Invalid Git reference")
+
+    return ref
+
+
 _MAX_UPLOAD_MB_RAW = os.environ.get("MAX_UPLOAD_MB")
 RANKER = load_ranker()
 
@@ -327,7 +358,8 @@ def github_zip_url(repo_url: str, ref: str = "main") -> str:
             status_code=400, detail="Only GitHub repo URLs are supported right now."
         )
     owner, repo = m.group(1), m.group(2)
-    return f"https://github.com/{owner}/{repo}/archive/refs/heads/{ref}.zip"
+    encoded_ref = quote(ref, safe="/")
+    return f"https://github.com/{owner}/{repo}/archive/refs/heads/{encoded_ref}.zip"
 
 
 ALLOWED_REDIRECT_HOSTS = {
@@ -660,7 +692,8 @@ async def scan_url(
     archive_path = job_dir / "repo.zip"
     repo_dir = job_dir / "repo"
     ensure_dir(repo_dir)
-    zip_url = github_zip_url(repo_url, ref=ref)
+    validated_ref = validate_git_ref(ref)
+    zip_url = github_zip_url(repo_url, ref=validated_ref)
 
     try:
         await download_to_path(zip_url, archive_path)
diff --git a/backend/tests/test_scan_url.py b/backend/tests/test_scan_url.py
index ff9c3c6..2a474b2 100644
--- a/backend/tests/test_scan_url.py
+++ b/backend/tests/test_scan_url.py
@@ -1,10 +1,57 @@
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import httpx
+import pytest
 from fastapi.testclient import TestClient
 
 from app.main import app
 
+
+@pytest.mark.parametrize(
+    "ref",
+    [
+        "main",
+        "feature/login",
+        "release-1.0",
+    ],
+)
+def test_scan_url_valid_refs(ref):
+    res = client.post(
+        "/scan-url",
+        data={
+            "repo_url": "not-a-url",
+            "ref": ref,
+            "project_name": "test_project",
+        },
+    )
+
+    # Validation should reach repo URL parsing, proving the ref was accepted.
+    assert res.status_code == 400
+    assert "Only GitHub repo URLs are supported right now." in res.json()["detail"]
+
+
+@pytest.mark.parametrize(
+    "ref",
+    [
+        "../main",
+        "./main",
+        r"main\hack",
+    ],
+)
+def test_scan_url_invalid_refs(ref):
+    res = client.post(
+        "/scan-url",
+        data={
+            "repo_url": "https://github.com/owner/repo",
+            "ref": ref,
+            "project_name": "test_project",
+        },
+    )
+
+    assert res.status_code == 400
+    assert res.json()["detail"] == "Invalid Git reference"
+
+
 client = TestClient(app)