diff --git a/backend/app/services/report/render.py b/backend/app/services/report/render.py
index 624a500..c397cd5 100644
--- a/backend/app/services/report/render.py
+++ b/backend/app/services/report/render.py
@@ -12,6 +12,7 @@
 from datetime import datetime
 
 import jinja2
+import jinja2.sandbox
 
 from app.services.report.markdown import prepare_docx_context
 from app.services.report.report_data import ReportContext
@@ -63,7 +64,10 @@ def render_html_report(
     Image data is already embedded as base64 in the context (FileReport.data_base64),
     so templates handle image rendering client-side.
     """
-    env = jinja2.Environment(autoescape=True)
+    # Use a SandboxedEnvironment so that templates (which originate from an
+    # external, admin-seeded source via CUSTOM_DATA_URL) cannot execute
+    # arbitrary Python via Jinja2 SSTI user generates a report.
+    env = jinja2.sandbox.SandboxedEnvironment(autoescape=True)
     env.filters["tojson"] = _tojson_filter
 
     template = env.from_string(template_content.decode("utf-8"))
@@ -90,7 +94,10 @@ def render_docx_report(
 
     tpl = DocxTemplate(io.BytesIO(template_content))
     docx_context = prepare_docx_context(asdict(context), tpl, image_data)
-    tpl.render(docx_context)
+    # Render with a SandboxedEnvironment to prevent Jinja2 SSTI/RCE from
+    # externally-sourced (admin-seeded) templates. docxtpl manages XML escaping
+    # itself, so autoescape stays off (the default).
+    tpl.render(docx_context, jinja_env=jinja2.sandbox.SandboxedEnvironment())
 
     output = io.BytesIO()
     tpl.save(output)
diff --git a/backend/app/services/report/report.py b/backend/app/services/report/report.py
index 09505b5..4811e95 100644
--- a/backend/app/services/report/report.py
+++ b/backend/app/services/report/report.py
@@ -7,9 +7,11 @@
 from datetime import datetime
 
 from fastapi import HTTPException, status
+from jinja2.exceptions import SecurityError, TemplateError
 from sqlalchemy import select
 from sqlalchemy.orm import Session
 
+from app.core.logging import app_logger
 from app.enums.enums import ReportTemplateFormat
 from app.models.report_template import ReportTemplate
 from app.models.user import User
@@ -54,33 +56,57 @@ def generate_report_service(
         sort_order=request.sort_order,
     )
 
-    if template.format == ReportTemplateFormat.HTML:
-        output = render_html_report(template.template_content, context)
-        result = GeneratedReport(
-            content=output.encode("utf-8"),
-            media_type="text/html",
-            filename=template.filename,
+    try:
+        if template.format == ReportTemplateFormat.HTML:
+            output = render_html_report(template.template_content, context)
+            result = GeneratedReport(
+                content=output.encode("utf-8"),
+                media_type="text/html",
+                filename=template.filename,
+            )
+            del context, output
+            release_memory()
+            return result
+        elif template.format == ReportTemplateFormat.DOCX:
+            # Collect image data for DOCX embedding (markdown → InlineImage)
+            image_data = collect_report_images(session, assessment_id)
+            output = render_docx_report(template.template_content, context, image_data)
+
+            del context, image_data
+            release_memory()
+
+            return GeneratedReport(
+                content=output,
+                media_type="application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+                filename=template.filename,
+            )
+        else:
+            raise HTTPException(
+                status_code=status.HTTP_400_BAD_REQUEST,
+                detail=f"Unsupported format: {template.format}",
+            )
+    except SecurityError as e:
+        # The template tried to access a sandbox-disallowed attribute — a
+        # blocked SSTI attempt. Log as a security event (template id/filename
+        # and the non-sensitive sandbox message) but keep the client response
+        # generic so no payload/traceback is leaked.
+        app_logger.error(
+            f"Rejected report template id={template.id} "
+            f"filename={template.filename!r}: blocked unsafe expression: {e}"
         )
-        del context, output
-        release_memory()
-        return result
-    elif template.format == ReportTemplateFormat.DOCX:
-        # Collect image data for DOCX embedding (markdown → InlineImage)
-        image_data = collect_report_images(session, assessment_id)
-        output = render_docx_report(template.template_content, context, image_data)
-
-        del context, image_data
-        release_memory()
-
-        return GeneratedReport(
-            content=output,
-            media_type="application/vnd.openxmlformats-officedocument.wordprocessingml.document",
-            filename=template.filename,
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_CONTENT,
+            detail="The selected report template was rejected: it contains disallowed expressions.",
+        )
+    except TemplateError as e:
+        # Malformed template (syntax / undefined / runtime error).
+        app_logger.error(
+            f"Failed to render report template id={template.id} "
+            f"filename={template.filename!r}: {e}"
         )
-    else:
         raise HTTPException(
-            status_code=status.HTTP_400_BAD_REQUEST,
-            detail=f"Unsupported format: {template.format}",
+            status_code=status.HTTP_422_UNPROCESSABLE_CONTENT,
+            detail="The selected report template could not be rendered. Please check the template.",
         )
 
 
diff --git a/backend/tests/services/report/test_render.py b/backend/tests/services/report/test_render.py
new file mode 100644
index 0000000..5f9bc8c
--- /dev/null
+++ b/backend/tests/services/report/test_render.py
@@ -0,0 +1,77 @@
+"""
+Unit tests for the report render layer.
+
+Focus: the Jinja2 SandboxedEnvironment must block SSTI/RCE payloads on both the
+HTML and DOCX paths while still rendering benign templates correctly.
+"""
+
+import io
+from datetime import datetime
+
+import pytest
+from docx import Document
+from jinja2.exceptions import SecurityError
+
+from app.services.report.render import render_docx_report, render_html_report
+from app.services.report.report_data import AssessmentInfo, ReportContext
+
+# Classic Jinja2 SSTI -> RCE payload (harmless `id` command).
+RCE_PAYLOAD = (
+    "{{ self.__init__.__globals__.__builtins__.__import__('os').popen('id').read() }}"
+)
+
+
+def _context() -> ReportContext:
+    """Minimal, DB-free ReportContext for rendering."""
+    return ReportContext(
+        assessment=AssessmentInfo(
+            id="1", name="Acme Engagement", description="", assessment_type="RedTeam"
+        ),
+        activities_grouped=[],
+        activities_flat=[],
+        statistics={},
+        generated_at=datetime(2026, 1, 1),
+        generated_by="tester",
+        template_filename="t",
+    )
+
+
+def _docx_bytes(paragraph_text: str) -> bytes:
+    doc = Document()
+    doc.add_paragraph(paragraph_text)
+    buf = io.BytesIO()
+    doc.save(buf)
+    return buf.getvalue()
+
+
+def test_html_sandbox_blocks_rce():
+    template = f"<pre>{RCE_PAYLOAD}</pre>".encode()
+    with pytest.raises(SecurityError):
+        render_html_report(template, _context())
+
+
+def test_docx_sandbox_blocks_rce():
+    template = _docx_bytes(RCE_PAYLOAD)
+    with pytest.raises(SecurityError):
+        render_docx_report(template, _context(), None)
+
+
+def test_html_benign_renders_and_tojson_works():
+    template = (
+        b"<h1>{{ assessment.name }}</h1>"
+        b"<script>var d = {{ statistics | tojson }};</script>"
+    )
+    out = render_html_report(template, _context())
+
+    assert "Acme Engagement" in out
+    # tojson emits raw JSON (not HTML-entity-encoded) so the <script> stays valid.
+    assert "var d = {};" in out
+
+
+def test_docx_benign_renders():
+    template = _docx_bytes("Assessment: {{ assessment.name }}")
+    result = render_docx_report(template, _context(), None)
+
+    rendered = Document(io.BytesIO(result))
+    text = "\n".join(p.text for p in rendered.paragraphs)
+    assert "Assessment: Acme Engagement" in text
diff --git a/backend/tests/services/report/test_report_errors.py b/backend/tests/services/report/test_report_errors.py
new file mode 100644
index 0000000..bfa2af4
--- /dev/null
+++ b/backend/tests/services/report/test_report_errors.py
@@ -0,0 +1,98 @@
+"""
+Tests for generate_report_service error handling.
+
+A sandbox-blocked (SSTI) or malformed template must surface as a clean HTTP 422
+with a generic message and be logged via app_logger — never an opaque 500 or a
+leaked payload.
+"""
+
+import logging
+
+import pytest
+from fastapi import HTTPException, status
+from sqlalchemy.orm import Session
+
+from app.core.config import settings
+from app.enums.enums import ReportTemplateFormat
+from app.models.assessment import Assessment
+from app.models.report_template import ReportTemplate
+from app.models.user import User
+from app.schemas.report import ReportGenerateRequest
+from app.services.report.report import generate_report_service
+
+RCE_PAYLOAD = (
+    "{{ self.__init__.__globals__.__builtins__.__import__('os').popen('id').read() }}"
+)
+
+
+@pytest.fixture
+def assessment(session: Session, test_admin_user: User) -> Assessment:
+    assessment = Assessment(
+        name="Test Assessment",
+        description="A test assessment",
+        assessment_type="RedTeam",
+        created_by=test_admin_user.id,
+    )
+    session.add(assessment)
+    session.commit()
+    return assessment
+
+
+def _add_template(session: Session, content: str) -> ReportTemplate:
+    template = ReportTemplate(
+        filename="report.html",
+        format=ReportTemplateFormat.HTML,
+        template_content=content.encode("utf-8"),
+    )
+    session.add(template)
+    session.commit()
+    return template
+
+
+def test_malicious_template_rejected_with_422_and_logged(
+    session: Session,
+    test_admin_user: User,
+    assessment: Assessment,
+    caplog,
+):
+    template = _add_template(session, f"<pre>{RCE_PAYLOAD}</pre>")
+    request = ReportGenerateRequest(template_id=template.id)
+
+    with caplog.at_level(logging.ERROR, logger=settings.APPLICATION_NAME):
+        with pytest.raises(HTTPException) as exc_info:
+            generate_report_service(assessment.id, request, test_admin_user, session)
+
+    assert exc_info.value.status_code == status.HTTP_422_UNPROCESSABLE_CONTENT
+    # Client message stays generic — no payload echoed back.
+    assert "disallowed expressions" in exc_info.value.detail
+    assert "os" not in exc_info.value.detail
+    # Security event is logged server-side with the template identity.
+    assert str(template.id) in caplog.text
+
+
+def test_malformed_template_rejected_with_422(
+    session: Session,
+    test_admin_user: User,
+    assessment: Assessment,
+):
+    template = _add_template(session, "<h1>{{ unclosed </h1>")
+    request = ReportGenerateRequest(template_id=template.id)
+
+    with pytest.raises(HTTPException) as exc_info:
+        generate_report_service(assessment.id, request, test_admin_user, session)
+
+    assert exc_info.value.status_code == status.HTTP_422_UNPROCESSABLE_CONTENT
+
+
+def test_benign_template_succeeds(
+    session: Session,
+    test_admin_user: User,
+    assessment: Assessment,
+):
+    template = _add_template(session, "<h1>{{ assessment.name }}</h1>")
+    request = ReportGenerateRequest(template_id=template.id)
+
+    result = generate_report_service(assessment.id, request, test_admin_user, session)
+
+    assert result.media_type == "text/html"
+    assert b"Test Assessment" in result.content