diff --git a/claude_code_log/html/assistant_formatters.py b/claude_code_log/html/assistant_formatters.py
index 3128a50c..02295cf9 100644
--- a/claude_code_log/html/assistant_formatters.py
+++ b/claude_code_log/html/assistant_formatters.py
@@ -108,7 +108,10 @@ def format_image_content(image: ImageContent) -> str:
     src = export_image(image, mode="embedded")
     if src is None:
         return "[Image]"
-    return f'<img src="{src}" alt="image" class="uploaded-image" />'
+    # Escape the src: export_image allowlists the media type and
+    # validates the base64, but the data: URL still must not be able to
+    # break out of the attribute (issue #277).
+    return f'<img src="{escape_html(src)}" alt="image" class="uploaded-image" />'
 
 
 def format_unknown_content(content: UnknownMessage) -> str:
diff --git a/claude_code_log/html/renderer.py b/claude_code_log/html/renderer.py
index 92900896..3c8c4d55 100644
--- a/claude_code_log/html/renderer.py
+++ b/claude_code_log/html/renderer.py
@@ -473,7 +473,10 @@ def _format_image(self, image: ImageContent) -> str:
         )
         if src is None:
             return "[Image]"
-        return f'<img src="{src}" alt="image" class="uploaded-image" />'
+        # Escape the src: export_image allowlists the media type and
+        # validates the base64, but the data: URL (embedded mode) still
+        # must not be able to break out of the attribute (issue #277).
+        return f'<img src="{escape_html(src)}" alt="image" class="uploaded-image" />'
 
     # -------------------------------------------------------------------------
     # System Content Formatters
diff --git a/claude_code_log/image_export.py b/claude_code_log/image_export.py
index 1113ac9a..089e2d58 100644
--- a/claude_code_log/image_export.py
+++ b/claude_code_log/image_export.py
@@ -13,6 +13,47 @@
     from .models import ImageContent
 
 
+# Image media types we are willing to emit into a data: URL or write to
+# disk. Deliberately excludes ``image/svg+xml`` — SVG can carry inline
+# ``<script>`` and event handlers, so a data:image/svg+xml URL is a
+# scriptable XSS vector when the generated page is opened under
+# ``file://``. Mirrors the allowlist already enforced on the
+# tool-result image path in ``html/tool_formatters.py`` (issue #277).
+_ALLOWED_IMAGE_MEDIA_TYPES = frozenset(
+    {
+        "image/png",
+        "image/jpeg",
+        "image/gif",
+        "image/webp",
+    }
+)
+
+
+def _is_safe_image_source(media_type: str, data: str) -> bool:
+    """Whether an image's media type and base64 data are safe to emit.
+
+    Guards the embedded (data: URL) and referenced (write-to-disk)
+    paths against two problems:
+
+    - a non-allowlisted / scriptable media type (notably SVG), and
+    - malformed base64 (which could smuggle a raw ``"`` /  ``>`` past a
+      naive interpolation, or corrupt the written file).
+
+    Returns ``True`` only when the media type is allowlisted *and* the
+    data is strictly-valid base64. Callers should fall back to a
+    placeholder (``None``) otherwise. Note this does not, on its own,
+    make the result safe to drop into HTML unescaped — the HTML sink
+    must still ``escape_html`` the final ``src`` (issue #277).
+    """
+    if media_type not in _ALLOWED_IMAGE_MEDIA_TYPES:
+        return False
+    try:
+        base64.b64decode(data, validate=True)
+    except (binascii.Error, ValueError):
+        return False
+    return True
+
+
 def export_image(
     image: "ImageContent",
     mode: str,
@@ -40,12 +81,23 @@ def export_image(
         return None
 
     if mode == "embedded":
+        # Reject scriptable / non-allowlisted media types and malformed
+        # base64 before building the data: URL. Even with these guards
+        # the HTML sink must still escape the returned src (issue #277);
+        # returning None here degrades to a placeholder.
+        if not _is_safe_image_source(image.source.media_type, image.source.data):
+            return None
         return f"data:{image.source.media_type};base64,{image.source.data}"
 
     if mode == "referenced":
         if output_dir is None:
             return None
 
+        # Same allowlist/validation as embedded mode: don't write a
+        # scriptable or malformed image to disk and then reference it.
+        if not _is_safe_image_source(image.source.media_type, image.source.data):
+            return None
+
         try:
             # Create images subdirectory
             images_dir = output_dir / "images"
@@ -71,12 +123,17 @@ def export_image(
 
 
 def _get_extension(media_type: str) -> str:
-    """Get file extension from media type."""
+    """Get file extension from media type.
+
+    Only the allowlisted media types (``_ALLOWED_IMAGE_MEDIA_TYPES``)
+    reach this in referenced mode; ``image/svg+xml`` is intentionally
+    absent because SVG is rejected upstream as a scriptable XSS vector
+    (issue #277).
+    """
     ext_map = {
         "image/png": ".png",
         "image/jpeg": ".jpg",
         "image/gif": ".gif",
         "image/webp": ".webp",
-        "image/svg+xml": ".svg",
     }
     return ext_map.get(media_type, ".png")
diff --git a/test/test_image_export.py b/test/test_image_export.py
index 71482417..6771a31a 100644
--- a/test/test_image_export.py
+++ b/test/test_image_export.py
@@ -86,3 +86,81 @@ def test_unsupported_mode_returns_none(self, sample_image: ImageContent):
         """Unsupported mode returns None."""
         result = export_image(sample_image, mode="unknown_mode")
         assert result is None
+
+
+# Minimal valid PNG (1x1 transparent pixel), shared by the security tests.
+_VALID_PNG_B64 = (
+    "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR4nGNgYGBgAAAABQABpfZFQAAAAA"
+    "BJRU5ErkJggg=="
+)
+
+
+def _image(media_type: str, data: str = _VALID_PNG_B64) -> ImageContent:
+    return ImageContent(
+        type="image",
+        source=ImageSource(type="base64", media_type=media_type, data=data),
+    )
+
+
+class TestExportImageSecurity:
+    """Guards against the embedded/referenced-image XSS vector (issue #277).
+
+    ``media_type`` and ``data`` are unvalidated strings parsed from
+    transcript JSON, and can carry content that did not originate from
+    the user (tool/MCP-returned images, fetched web content). Without
+    an allowlist + base64 validation, a crafted media type could break
+    out of the ``<img src>`` attribute.
+    """
+
+    def test_embedded_rejects_attribute_breakout_media_type(self):
+        """A media type crafted to break out of the src attribute yields no data URL."""
+        hostile = _image('png"><script>alert(document.domain)</script>')
+        assert export_image(hostile, mode="embedded") is None
+
+    def test_embedded_rejects_svg(self):
+        """SVG is scriptable; embedded mode must not emit a data:image/svg+xml URL."""
+        svg = _image("image/svg+xml")
+        assert export_image(svg, mode="embedded") is None
+
+    def test_embedded_rejects_invalid_base64(self):
+        """Malformed base64 data is rejected rather than emitted verbatim."""
+        bad = _image("image/png", data='not"base64><script>')
+        assert export_image(bad, mode="embedded") is None
+
+    def test_embedded_allows_valid_allowlisted_types(self):
+        """The four allowlisted types with valid base64 still produce data URLs."""
+        for mt in ("image/png", "image/jpeg", "image/gif", "image/webp"):
+            result = export_image(_image(mt), mode="embedded")
+            assert result is not None, mt
+            assert result.startswith(f"data:{mt};base64,")
+
+    def test_referenced_rejects_svg(self, tmp_path: Path):
+        """Referenced mode must not write a scriptable SVG to disk and link it."""
+        svg = _image("image/svg+xml")
+        result = export_image(svg, mode="referenced", output_dir=tmp_path, counter=1)
+        assert result is None
+        assert not (tmp_path / "images").exists() or not list(
+            (tmp_path / "images").iterdir()
+        )
+
+    def test_referenced_rejects_invalid_base64(self, tmp_path: Path):
+        """Referenced mode rejects malformed base64 instead of writing garbage."""
+        bad = _image("image/png", data="!!!not-base64!!!")
+        result = export_image(bad, mode="referenced", output_dir=tmp_path, counter=1)
+        assert result is None
+
+    def test_html_sink_escapes_hostile_media_type(self):
+        """End-to-end: the HTML <img> sink never emits an unescaped breakout.
+
+        Even if a future change let a hostile media type through
+        export_image, the HTML formatter must escape the src so no
+        ``">`` can close the attribute/tag. Here the hostile type is
+        rejected upstream (rendering a placeholder), but we also assert
+        no live ``<script>`` or attribute-closing ``">`` leaks.
+        """
+        from claude_code_log.html import format_image_content
+
+        hostile = _image('png"><script>alert(1)</script>')
+        html = format_image_content(hostile)
+        assert "<script>" not in html
+        assert '"><script' not in html