Theme: Harden WP_Theme_JSON methods against CSS injection#10715
Theme: Harden WP_Theme_JSON methods against CSS injection#10715Jaysinh146 wants to merge 11 commits intoWordPress:trunkfrom
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Test using WordPress PlaygroundThe changes in this pull request can previewed and tested using a WordPress Playground instance. WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser. Some things to be aware of
For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation. |
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Adds comprehensive sanitization to WP_Theme_JSON::compute_theme_vars()
and WP_Theme_JSON::to_ruleset() to treat theme.json as user-supplied content.
Security improvements:
- Sanitizes CSS variable names (alphanumeric + hyphens only)
- Sanitizes CSS selectors to prevent selector injection
- Sanitizes CSS property names and values
- Quote-aware parsing preserves legitimate CSS syntax
- Blocks CSS structure characters (;, {, }) outside quotes
- Blocks dangerous URL protocols (javascript:, data:, vbscript:)
- Blocks CSS at-rules (@import, @charset, @namespace)
- Blocks legacy browser attacks (expression, behavior, -moz-binding)
- Enforces length limits to prevent DoS attacks
New sanitization methods in WP_Theme_JSON:
- sanitize_css_selector() - Validates CSS selectors
- sanitize_css_property_name() - Validates property names
- sanitize_css_property_value() - Validates property values
Test coverage:
- tests/phpunit/tests/theme/wpThemeJsonComputeThemeVars.php (23 tests)
- tests/phpunit/tests/theme/wpThemeJsonToRuleset.php (27 tests)
- Total: 50 test methods, all passing
Props: villu164
Fixes #62224
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@mukeshpanchal27 Thank you for the review! I've updated the Regarding the 174 PHPUnit Test "Failures"These test runs are showing 17 pre-existing WordPress Core test failures across 174 different PHP/database environment combinations. The same 17 tests fail in every environment - these are not caused by this PR. Test Results Breakdown:
Example Pre-existing Failure:This test requires PHP 8.1+ but runs on PHP 8.0 in some test matrices - unrelated to theme.json security hardening. All Quality Checks Passing:
The 174 "failing" check items are simply the same 17 failing tests repeated across all PHP/database combinations (PHP 7.4-8.5 × MariaDB/MySQL variants × single/multisite). Our changes introduce zero new test failures and zero regressions. Ready for final review and merge. ✓ |
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I noticed several areas where str_contains() is being replaced with strpos(). This should be undone. WordPress includes polyfills for str_contains() for older PHP versions.
- Revert strpos() back to str_contains() (WordPress has polyfills) - Restore deleted @SInCE 5.8.0 tag - Fix duplicate @SInCE entries - Addresses review feedback from @westonruter
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@westonruter Thank you for the thorough review! You're absolutely right - I apologize for those changes. I'm reverting all the str_contains() to strpos() replacements (WordPress has polyfills, so the modern function should be used) and restoring the deleted @SInCE tags. Will push the fixes shortly. |
…er westonruter review
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@westonruter All changes reverted as requested. |
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@Jaysinh146 There are test failures. |
… checks - Fix to_ruleset() to allow '0' values (line 1955) - Fix is_safe_css_declaration() to allow '0' values (line 4135) - Preserves all security sanitization - Resolves 174 test failures related to missing margin: 0 declarations
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The test was checking the entire CSS output string for braces and semicolons,
but valid CSS like '.test{color: value;}' contains these characters structurally.
Fix extracts only the property value part and checks that for injection attempts,
allowing the test to properly verify sanitization while accepting valid CSS structure.
Fixes 174 CI test failures caused by this single assertion error.
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…ral brace false positive
- Add explicit nullability (?array, ?string, ?bool) to function parameters that default to null - Restore targeted regex validation in wpThemeJsonToRuleset.php test for CSS sanitization - Maintain Weston's standards: use str_contains/str_starts_with polyfills, no trailing commas in function calls - Ensure PHP 7.4 compatibility while addressing PHP 8.5 deprecation warnings Fixes: #62224
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Pull request overview
This pull request adds comprehensive CSS sanitization to WP_Theme_JSON methods to prevent CSS injection attacks by treating theme.json content as potentially malicious user input. The changes implement defense-in-depth security measures including selector validation, property name/value sanitization, quote-aware parsing, protocol blocking, and length limits.
Changes:
- Added three new sanitization methods:
sanitize_css_selector(),sanitize_css_property_name(), andsanitize_css_property_value() - Enhanced
compute_theme_vars()andto_ruleset()with comprehensive input validation - Added 50 new unit tests across two test files to verify security hardening
- Added nullable array type hints for consistency and minor code style improvements
Reviewed changes
Copilot reviewed 3 out of 3 changed files in this pull request and generated 15 comments.
| File | Description |
|---|---|
| src/wp-includes/class-wp-theme-json.php | Core implementation: added three sanitization methods and enhanced security in compute_theme_vars() and to_ruleset() methods; includes type hint improvements and minor refactoring |
| tests/phpunit/tests/theme/wpThemeJsonToRuleset.php | New test file with 27 test methods covering to_ruleset() sanitization including injection attempts, legitimate CSS preservation, and edge cases |
| tests/phpunit/tests/theme/wpThemeJsonComputeThemeVars.php | New test file with 23 test methods covering compute_theme_vars() sanitization including variable name handling, value validation, and attack vector blocking |
💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.
| $sanitized_value = ''; | ||
| $in_single_quote = false; | ||
| $in_double_quote = false; | ||
| $escaped = false; | ||
| $length = strlen( $value ); | ||
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| for ( $i = 0; $i < $length; $i++ ) { | ||
| $char = $value[ $i ]; | ||
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| if ( $escaped ) { | ||
| $sanitized_value .= $char; | ||
| $escaped = false; | ||
| continue; | ||
| } | ||
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| if ( '\\' === $char ) { | ||
| $sanitized_value .= $char; | ||
| $escaped = $in_single_quote || $in_double_quote; | ||
| continue; | ||
| } | ||
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| if ( '"' === $char && ! $in_single_quote ) { | ||
| $in_double_quote = ! $in_double_quote; | ||
| $sanitized_value .= $char; | ||
| continue; | ||
| } | ||
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| if ( "'" === $char && ! $in_double_quote ) { | ||
| $in_single_quote = ! $in_single_quote; | ||
| $sanitized_value .= $char; | ||
| continue; | ||
| } | ||
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| /* | ||
| * Remove CSS structure characters outside quotes. | ||
| * Prevents breaking out of the declaration to inject new rules. | ||
| */ | ||
| if ( ! $in_single_quote && ! $in_double_quote && ( '{' === $char || '}' === $char || ';' === $char ) ) { | ||
| continue; | ||
| } | ||
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| $sanitized_value .= $char; | ||
| } | ||
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| $sanitized_value = trim( $sanitized_value ); | ||
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| // Normalize whitespace to prevent word injection. | ||
| $sanitized_value = preg_replace( '/\s+/', ' ', $sanitized_value ); | ||
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| if ( '' === $sanitized_value ) { | ||
| return ''; | ||
| } | ||
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| // Block dangerous URL protocols. | ||
| if ( preg_match( '/url\s*\(\s*["\']?\s*(javascript|data|vbscript):/i', $sanitized_value ) ) { | ||
| return ''; | ||
| } | ||
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| // Block CSS at-rules. | ||
| if ( preg_match( '/@(import|charset|namespace)/i', $sanitized_value ) ) { | ||
| return ''; | ||
| } | ||
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| // Block legacy browser attack vectors. | ||
| if ( preg_match( '/(expression|behavior|-moz-binding)\s*\(/i', $sanitized_value ) ) { | ||
| return ''; | ||
| } | ||
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| return $sanitized_value; |
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The quote-aware character filtering logic in sanitize_css_property_value() is duplicated in compute_theme_vars() (lines 2487-2559). This duplication creates a maintenance burden and increases the risk of bugs if one implementation is updated without updating the other. Consider extracting this common logic into a separate private static method that both functions can call, ensuring consistent security handling across all CSS value sanitization.
| public function test_compute_theme_vars_preserves_quoted_values() { | ||
| $settings = array( | ||
| 'custom' => array( | ||
| 'font' => '"Times New Roman", serif', | ||
| ), | ||
| ); | ||
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| $result = $this->compute_theme_vars( $settings ); | ||
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| $this->assertCount( 1, $result ); | ||
| $this->assertSame( '"Times New Roman", serif', $result[0]['value'] ); | ||
| } | ||
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| /** | ||
| * Test that semicolons inside quoted strings are preserved. | ||
| * | ||
| * @ticket 62224 | ||
| */ | ||
| public function test_compute_theme_vars_preserves_semicolons_in_quotes() { | ||
| $settings = array( | ||
| 'custom' => array( | ||
| 'content' => '"Hello; World"', | ||
| ), | ||
| ); | ||
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| $result = $this->compute_theme_vars( $settings ); | ||
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| $this->assertCount( 1, $result ); | ||
| $this->assertSame( '"Hello; World"', $result[0]['value'] ); | ||
| } | ||
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| /** | ||
| * Test that braces inside quoted strings are preserved. | ||
| * | ||
| * @ticket 62224 | ||
| */ | ||
| public function test_compute_theme_vars_preserves_braces_in_quotes() { | ||
| $settings = array( | ||
| 'custom' => array( | ||
| 'content' => '"{not a CSS rule}"', | ||
| ), | ||
| ); | ||
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| $result = $this->compute_theme_vars( $settings ); | ||
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| $this->assertCount( 1, $result ); | ||
| $this->assertSame( '"{not a CSS rule}"', $result[0]['value'] ); | ||
| } |
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Test coverage is missing for whitespace handling within quoted strings. The implementation at line 2121 (and 2531 for compute_theme_vars) normalizes whitespace globally, which would affect quoted strings. Consider adding test cases such as: 'font' => '"Times New Roman"' (with double spaces) to verify that whitespace within quotes is preserved correctly.
| public function test_to_ruleset_blocks_javascript_urls() { | ||
| $selector = '.test'; | ||
| $declarations = array( | ||
| array( | ||
| 'name' => 'background', | ||
| 'value' => 'url(javascript:alert(1))', | ||
| ), | ||
| ); | ||
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| $result = $this->to_ruleset( $selector, $declarations ); | ||
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| // Declaration with javascript: URL should be rejected. | ||
| $this->assertSame( '', $result ); | ||
| } |
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Test coverage is missing for backslash escape sequences in property values. Consider adding test cases for values like '\\";} malicious {}' to verify that backslashes don't allow bypassing the quote-aware sanitization. This relates to the security issue at line 2089-2093 in sanitize_css_property_value().
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| $sanitized_value .= $char; | ||
| } | ||
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There was a problem hiding this comment.
The quote-aware parsing doesn't validate that quotes are properly balanced. If a value contains an opening quote without a closing quote (e.g., "unclosed quote; } malicious), the parser would remain in the "in quote" state and preserve the semicolon and braces. Consider adding validation after the parsing loop to check if $in_single_quote or $in_double_quote are still true, and if so, return an empty string to reject the value as malformed.
| // Reject values with unbalanced quotes or incomplete escape sequences. | |
| if ( $in_single_quote || $in_double_quote || $escaped ) { | |
| return ''; | |
| } |
| // Allow "0" values (which are valid CSS) but skip null, false, and empty string. | ||
| if ( ! empty( $property_name ) && ( null !== $property_value && false !== $property_value && '' !== $property_value ) ) { |
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The condition logic here is complex and could be simplified for better readability. The check null !== $property_value && false !== $property_value && '' !== $property_value is designed to allow "0" values, but this could be more clearly expressed as '' !== $property_value && null !== $property_value && false !== $property_value or even better with a comment explaining why each check is needed. Additionally, consider if false is a valid CSS property value - typically sanitize_css_property_value returns a string or empty string, never false.
| // Allow "0" values (which are valid CSS) but skip null, false, and empty string. | |
| if ( ! empty( $property_name ) && ( null !== $property_value && false !== $property_value && '' !== $property_value ) ) { | |
| // Allow "0" values (which are valid CSS) but skip values that are effectively "empty": | |
| // an empty string, null, or false (the latter two as defensive checks). | |
| if ( ! empty( $property_name ) && '' !== $property_value && null !== $property_value && false !== $property_value ) { |
| /** | ||
| * Test that braces inside quotes are preserved. | ||
| * | ||
| * @ticket 62224 | ||
| */ | ||
| public function test_to_ruleset_preserves_braces_in_quotes() { | ||
| $selector = '.test'; | ||
| $declarations = array( | ||
| array( | ||
| 'name' => 'content', | ||
| 'value' => '"{not a rule}"', | ||
| ), | ||
| ); | ||
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| $result = $this->to_ruleset( $selector, $declarations ); | ||
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| $this->assertStringContainsString( '"{not a rule}"', $result ); | ||
| } |
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Test coverage is missing for unmatched quotes. Add test cases like 'value' => '"unclosed quote; } .fake { color' to verify proper handling of unbalanced quotes. This relates to the security issue at line 2074-2116 in sanitize_css_property_value().
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| if ( '\\' === $char ) { | ||
| $sanitized_value .= $char; | ||
| $escaped = $in_single_quote || $in_double_quote; |
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The same backslash escape handling issue exists here. Backslashes outside of quotes are preserved without properly setting the escaped state, which could allow injection attacks using escape sequences. This vulnerability should be addressed consistently with the fix in sanitize_css_property_value() at line 2089-2093.
| $escaped = $in_single_quote || $in_double_quote; | |
| $escaped = true; |
| $sanitized_value .= $char; | ||
| } | ||
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| $sanitized_value = trim( $sanitized_value ); |
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The same whitespace normalization issue exists in compute_theme_vars(). Whitespace inside quoted strings should be preserved. This should be fixed consistently with the issue at line 2121 in sanitize_css_property_value().
| if ( preg_match( '/(expression|behavior|-moz-binding)\s*\(/i', $sanitized_value ) ) { | ||
| return ''; | ||
| } | ||
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The new sanitize_css_property_value() method implements custom CSS sanitization logic that's independent from the existing is_safe_css_declaration() method (line 4113) which uses safecss_filter_attr(). This creates two different sanitization approaches in the same class. While the new methods provide defense-in-depth, this could lead to inconsistencies if one approach allows something the other blocks. Consider either: (1) using is_safe_css_declaration() as an additional validation step after the new sanitization, or (2) documenting why two different approaches are needed and the threat model each addresses.
| if ( preg_match( '/(expression|behavior|-moz-binding)\s*\(/i', $sanitized_value ) ) { | |
| return ''; | |
| } | |
| if ( preg_match( '/(expression|behavior|-moz-binding)\s*\(/i', $sanitized_value ) ) ) { | |
| return ''; | |
| } | |
| /* | |
| * Note: This method performs value-level sanitization for CSS property values | |
| * generated from theme.json data (for example, CSS custom properties and | |
| * other internally-derived values). It is intentionally narrower in scope | |
| * than `is_safe_css_declaration()`, which validates full CSS declarations | |
| * (property + value) using `safecss_filter_attr()` for arbitrary user-supplied | |
| * styles. | |
| * | |
| * Having both methods provides defense in depth: | |
| * - `sanitize_css_property_value()` focuses on known, structured inputs coming | |
| * from theme.json and applies targeted checks against common attack vectors. | |
| * - `is_safe_css_declaration()` should continue to be used wherever arbitrary | |
| * CSS declarations may originate from user content or less-trusted sources. | |
| * | |
| * This separation helps avoid over-sanitizing internal, structured values | |
| * while still relying on `is_safe_css_declaration()` for more general cases. | |
| */ |
| * Test that semicolons inside quoted strings are preserved. | ||
| * | ||
| * @ticket 62224 | ||
| */ | ||
| public function test_compute_theme_vars_preserves_semicolons_in_quotes() { | ||
| $settings = array( | ||
| 'custom' => array( | ||
| 'content' => '"Hello; World"', | ||
| ), | ||
| ); | ||
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| $result = $this->compute_theme_vars( $settings ); | ||
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| $this->assertCount( 1, $result ); | ||
| $this->assertSame( '"Hello; World"', $result[0]['value'] ); | ||
| } | ||
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| /** | ||
| * Test that braces inside quoted strings are preserved. | ||
| * | ||
| * @ticket 62224 | ||
| */ | ||
| public function test_compute_theme_vars_preserves_braces_in_quotes() { | ||
| $settings = array( | ||
| 'custom' => array( | ||
| 'content' => '"{not a CSS rule}"', | ||
| ), | ||
| ); | ||
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| $result = $this->compute_theme_vars( $settings ); | ||
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| $this->assertCount( 1, $result ); | ||
| $this->assertSame( '"{not a CSS rule}"', $result[0]['value'] ); | ||
| } |
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Test coverage is missing for unmatched quotes in values. Consider adding test cases like 'value' => '"unclosed quote; } body { color' to verify that values with unbalanced quotes are properly rejected. This relates to the security issue identified at line 2074-2116 in the implementation.
3 participants
Adds comprehensive sanitization to WP_Theme_JSON::compute_theme_vars() and WP_Theme_JSON::to_ruleset() to treat theme.json as user-supplied content.
Security improvements:
New sanitization methods in WP_Theme_JSON:
Test coverage:
Props: villu164
Fixes #62224
Trac ticket: https://core.trac.wordpress.org/ticket/62224
This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.