Validate downloaded theme assets against extension and MIME allowlists by mikachan · Pull Request #852 · WordPress/create-block-theme

mikachan · 2026-06-16T10:57:57Z

Add a per-sink extension allowlist and post-download MIME validation to the four places the plugin calls download_url() and writes the response to disk: media downloads from template/pattern content (add_media_to_local), font downloads from user global styles (copy_font_assets_to_theme), and the zip-export equivalents of both (add_media_to_zip, add_activated_fonts_to_zip). Also rejects multi-extension polyglot URLs (evil.php.jpg).

To test, ensure the following tests pass:

  npm run test:unit:php:base -- --filter "Test_Create_Block_Theme_(Media|Fonts|Zip)"

…ction

Copilot

Pull request overview

This PR hardens Create Block Theme’s asset-downloading code paths by adding pre-download URL extension allowlists and post-download content-type validation for media and font assets, including the zip-export flows, to reduce the risk of downloading/extracting unsafe payloads.

Changes:

Add URL allowlist validators to reject disallowed and multi-extension polyglot URLs before calling download_url().
Add post-download validation: MIME allowlist for media and magic-byte validation for font files.
Add PHPUnit coverage for the new validators and for “skip without downloading” behavior; update contributor test commands in docs.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
includes/create-theme/theme-media.php	Adds media URL allowlist + post-download MIME allowlist; wires checks into local media download flow.
includes/create-theme/theme-fonts.php	Adds font URL allowlist + magic-byte validation; wires checks into font asset copying.
includes/create-theme/theme-zip.php	Wires the same validations into zip export for media and activated fonts.
tests/test-theme-media.php	Adds tests for media URL/file validation and for short-circuiting before HTTP.
tests/test-theme-fonts.php	Adds tests for font URL/file validation and for short-circuiting before HTTP.
tests/test-theme-zip.php	Adds zip-specific test ensuring disallowed media URLs don’t trigger HTTP.
tests/data/evil.php.txt	Adds malicious PHP fixture used by validator tests.
AGENTS.md	Updates documented npm commands for running PHP unit tests.

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Two recent Copilot suggestion commits accidentally removed structural code while making single-line changes: - c045da7 ("Pass $font_src to download_url") dropped the `} else {` and the `is_wp_error( $tmp_file )` guard. - 617f6ec ("Stream downloaded media into zip") dropped the closing brace of the `foreach` in add_media_to_zip(). Both broke PHP parse on every CI matrix. Restoring the missing statements so the file is valid again.

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Two more Copilot suggestion commits dropped surrounding context lines: - ad31fe4 ("Strip query string from filename before renaming downloaded media") removed the closing brace of the foreach loop in add_media_to_local(), causing fatal lint errors. - 860f1bc ("Strip query string from font URL before deriving filename in zip") removed the `$font_family_dir_name = sanitize_title(...)` assignment, leaving an undefined variable used immediately below.

`$font_filename = basename( $font_face['src'] )` threw TypeError on PHP 8+ when src was already an array (valid per Theme JSON spec) and was redefined inside the inner src loop anyway. The adjacent `$font_dir = wp_get_font_dir()` was likewise redundant — also redefined inside the inner loop. Removing both.

wp_check_filetype_and_ext() was inconsistent with the URL allowlist: SVG isn't in WP Core's default mime registry at all, and WP maps wmv to video/x-ms-wmv / avi to video/avi while the post-check allowlist had only video/x-msvideo. Legitimate SVG/WMV/AVI URLs passed preflight and then silently failed post-download. Switching media to magic-byte verification (matching the font sink) removes the dependency on WP Core's mime registry and libmagic versions. Recognised formats: JPEG, PNG, GIF, WebP, SVG, MP4/M4V/MOV/ 3GP/3G2 (via ftyp), WebM, OGV, WMV (ASF), AVI (RIFF), MPEG.

scruffian · 2026-06-16T16:07:26Z

I found 3 issues:

The temp file is now deleted before ZipArchive has read it.
URL rewriting still uses the raw URL basename, so query-string slashes can produce broken or dangerous-looking asset references even though the download sink now uses the parsed path. For example http://example.com/cat.jpg?/evil.php now becomes cat.jpg for validation/download, but exported template markup points at /assets/images/evil.php.
is_allowed_media_file() does not verify that the downloaded bytes match the URL extension. e.g. a .jpg URL with SVG/MP4/WebM bytes passes and gets saved as .jpg.
'
I added unit tests to demonstrate the issues (but not fixes)

Three fixes for issues found via regression tests in 8da66ff: 1. is_allowed_media_file() now requires the downloaded bytes to match the URL extension specifically — a .jpg URL with SVG/MP4/anything-else bytes is rejected. Previously any allowed magic was accepted, so a browser MIME-sniffing the on-disk .jpg containing SVG could render it as SVG (with all the script-execution surface that brings). 2. make_relative_media_url() now derives the filename from the parsed URL path instead of raw basename(). A URL like cat.jpg?/evil.php previously produced `/assets/images/evil.php` in exported template markup because basename() treats query-string slashes as path separators. 3. add_media_to_zip() reads the downloaded bytes into memory and unlinks the tmp file BEFORE adding to the archive via addFromStringToTheme(). The previous `addFileToTheme() + unlink` sequence broke because ZipArchive defers reading files added via addFile() until close() — by which time the tmp file was gone.

mikachan · 2026-06-16T16:41:25Z

Thanks! These should all be fixed now.

add_media_to_zip() now reads the downloaded bytes into memory and unlinks the tmp file before adding via addFromStringToTheme().
make_relative_media_url() now uses basename(wp_parse_url($url, PHP_URL_PATH)) instead of raw basename($url). The example cat.jpg?/evil.php now produces /assets/images/cat.jpg as expected.
is_allowed_media_file() now switches on the URL extension and requires the bytes to match that specific format. A .jpg URL with SVG/MP4/anything-else bytes is rejected so we never persist a file whose extension lies about its content.

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Pull request overview

Copilot reviewed 8 out of 9 changed files in this pull request and generated 4 comments.

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

parse_url() omits the 'port' key when the URL has no explicit port (e.g. `http://localhost/foo`). The retry-on-localhost block in both add_media_to_zip() and add_media_to_local() read $parsed_url['port'] unconditionally, raising an undefined-index notice when the initial download failed for such a URL. Guard with isset() on both keys.

Previously any recognised font magic was accepted, so a .woff2 URL returning TTF/WOFF/OTF/EOT bytes would pass and be saved under the .woff2 filename. This defeated the extension+MIME allowlist intent and would produce broken assets in the exported theme/zip (browsers would fail to load a WOFF2-named file containing TTF content). The check now switches on the URL extension and validates the magic matches THAT specific format. Mirrors the same fix already applied to is_allowed_media_file().

scruffian · 2026-06-17T13:53:57Z

is_allowed_media_file has an mpeg branch, but is_allowed_media_url and the folder mapping only allow mpg. A template containing a .mpeg video would get rewritten to a local asset URL, then skipped before download, leaving the exported theme pointing at a missing file.

is_allowed_media_file() accepted both .mpg and .mpeg, but is_allowed_media_url() and get_media_folder_path_from_url() only listed .mpg. A template with a .mpeg video would get rewritten to a local asset URL, then skipped before download — leaving the exported theme pointing at a missing file. Add .mpeg to the URL allowlist and the folder mapping so all three lists agree (matching WP Core's wp_get_mime_types which maps mpeg|mpg|mpe to video/mpeg). Also drops a pre-existing duplicate 'ogv' entry in the folder mapping that was noticed while editing.

mikachan · 2026-06-17T15:20:19Z

is_allowed_media_file has an mpeg branch, but is_allowed_media_url and the folder mapping only allow mpg.

Good spot, thank you. This should be fixed in 375663c.

Same fix as add_media_to_zip: ZipArchive::addFile() defers reading the file until close() is called, so unlinking the download_url tmp file immediately after addFileToTheme() left an empty entry in the final archive when the zip was finalised. Read the bytes into memory, unlink, then addFromStringToTheme() — mirrors the pattern in add_media_to_zip(). Only the remote-download branch is affected; the local-copy branch points at a permanent file in the font library so its addFileToTheme() call stays.

AVIF is a WordPress-supported image format but was missing from the URL allowlist, folder mapping, and magic-byte check. The result: make_template_images_local() rewrote .avif URLs to local references, but the post-rewrite download was rejected — leaving exported themes pointing at missing assets. AVIF uses the ISO BMFF container (like MP4) with 'avif' or 'avis' as the major brand at offset 8. Added the brand-specific magic check alongside the URL/folder allowlist entries, plus tests for both AVIF still-image ('avif') and image-sequence ('avis') brands.

Previously a font source that failed the URL allowlist or post-download MIME check was skipped via continue, but the original $font_src stayed in $font_face['src']. copy_activated_fonts_to_theme() then merged that family into theme.json — leaving the user's activated font pointing at an uncopied (and possibly disallowed) source while the user custom settings were cleared. Both copy_font_assets_to_theme() and add_activated_fonts_to_zip() now build a fresh srcs list and only retain entries that successfully copied or were already file:./ asset paths. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Some AVIF files (typically those emitted by HEIF-derived tooling) set the major brand to mif1/miaf and only list avif/avis in the compatible brands. Parsing only the major brand at offset 8 would false-reject them. Scan the full FileTypeBox brand list (major brand plus compatible brands at offsets 16+) for any AVIF brand. HEIC files (no avif/avis anywhere) are still rejected. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Both copy_font_assets_to_theme() and add_activated_fonts_to_zip() treated a font src under the WP user-fonts directory as trusted — copying / zipping the bytes without ever inspecting them. Anything that ended up in that directory through a separate flow (a polyglot upload via another plugin, manual write, etc.) would be promoted to the exported theme as-is just because the URL extension was on the allowlist. Run the same is_allowed_font_file() check on the local source before copying / zipping. If the bytes don't match the URL extension's font format, drop the source from the returned families. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Plain str_replace on the whole template would substring-match a shorter validated URL inside a longer rejected URL when one is a prefix of the other (e.g. `photo.png` inside `photo.png.php`). That silently rewrote the rejected URL to a missing local asset and bypassed the validated-media guard. Walk the parsed block tree instead: WP_HTML_Tag_Processor handles img / video src+poster and inline `background-image:url(...)`, while direct array access handles block-comment JSON attrs. Replacements only touch values that exactly match a validated URL. The rewrites embed literal PHP open/close tags that must end up verbatim in the export, but both set_attribute() and wp_json_encode() (inside serialize_blocks) would escape `<` / `>` / quotes. Route the rewrites through opaque, URL-shaped placeholders so they survive both passes, then strtr() them back to the raw PHP at the end. The placeholder is a root-relative path because set_attribute prefixes schemeless values with `http://`. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scruffian

🚢
Thank you!

mikachan added 13 commits June 15, 2026 11:15

tests: add tiny.png and evil.php.txt fixtures for media validation

68d900f

docs(agents): update test:php commands to test:unit:php

66f625c

Add CBT_Theme_Media::is_allowed_media_url() extension allowlist

b714990

Add CBT_Theme_Media::is_allowed_media_file() MIME allowlist

8d06b1e

Apply media URL + file allowlist in add_media_to_local

a4591b6

Add CBT_Theme_Fonts::is_allowed_font_url() extension allowlist

70da5bc

Add CBT_Theme_Fonts::is_allowed_font_file() MIME allowlist

1a0c7cf

Apply font URL + file allowlist in copy_font_assets_to_theme

e73ad6a

Allow font/sfnt and application/vnd.ms-opentype for TTF/OTF MIME dete…

772b161

…ction

Reject multi-extension polyglots in URL allowlist

e949c52

Apply media+font allowlist to zip export sinks in theme-zip

3b2232e

Add positive end-to-end tests for media and font sinks

f48d0ed

Verify downloaded fonts by magic bytes instead of libmagic MIME

e6c1f1c

scruffian requested a review from Copilot June 16, 2026 11:31

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Comment thread includes/create-theme/theme-media.php Outdated

mikachan and others added 12 commits June 16, 2026 14:01

Pass $font_src string to download_url instead of $font_face src array

c045da7

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Stream downloaded media into zip and clean up tmp file

617f6ec

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Strip query string before deriving folder path from media URL

03dcbe1

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lint: align assignment operators in get_media_folder_path_from_url

63e7045

Strip query string from filename before renaming downloaded media

ad31fe4

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Strip query string from font URL before deriving filename

575eb1c

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Strip query string from font URL before deriving filename in zip

860f1bc

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Add regression tests for media asset validation

8da66ff

mikachan added 2 commits June 16, 2026 17:30

lint: suppress NoSilencedErrors warning on ZipArchive::close in test

56c0aa8

mikachan requested a review from Copilot June 17, 2026 12:29

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Comment thread tests/test-theme-zip.php

Comment thread includes/create-theme/theme-zip.php

Comment thread includes/create-theme/theme-media.php

Comment thread includes/create-theme/theme-fonts.php Outdated

mikachan and others added 3 commits June 17, 2026 13:39

Skip zip tests gracefully when ZipArchive extension is unavailable

aa017ad

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scruffian reviewed Jun 17, 2026

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Comment thread includes/create-theme/theme-zip.php Outdated

mikachan and others added 5 commits June 17, 2026 16:20

Tweak comment

3f974f5

Only rewrite media URLs after successful asset validation

84966da

scruffian reviewed Jun 19, 2026

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scruffian reviewed Jun 19, 2026

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Comment thread includes/create-theme/theme-zip.php

scruffian reviewed Jun 19, 2026

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mikachan and others added 2 commits June 19, 2026 14:17

scruffian approved these changes Jun 19, 2026

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mikachan merged commit da693bf into trunk Jun 19, 2026
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mikachan deleted the fix/validate-downloaded-theme-assets branch June 19, 2026 14:25

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mikachan commented Jun 16, 2026 •

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