diff --git a/CHANGELOG.md b/CHANGELOG.md
index a2359f1812b0..96297b4d449d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - _Experimental_: Add `@container-size` utility ([#18901](https://github.com/tailwindlabs/tailwindcss/pull/18901))
 
+### Fixed
+
+- Guard object lookups against inherited prototype properties ([#19725](https://github.com/tailwindlabs/tailwindcss/pull/19725))
+
 ## [4.2.1] - 2026-02-23
 
 ### Fixed
diff --git a/packages/tailwindcss/src/compat/plugin-api.test.ts b/packages/tailwindcss/src/compat/plugin-api.test.ts
index 4e35afa5a84e..cc410995b442 100644
--- a/packages/tailwindcss/src/compat/plugin-api.test.ts
+++ b/packages/tailwindcss/src/compat/plugin-api.test.ts
@@ -4592,4 +4592,48 @@ describe('config()', () => {
 
     expect(fn).toHaveBeenCalledWith('defaultvalue')
   })
+
+  // https://github.com/tailwindlabs/tailwindcss/issues/19721
+  test('matchUtilities does not match Object.prototype properties as values', async ({
+    expect,
+  }) => {
+    let input = css`
+      @tailwind utilities;
+      @plugin "my-plugin";
+    `
+
+    let compiler = await compile(input, {
+      loadModule: async (id, base) => {
+        return {
+          path: '',
+          base,
+          module: plugin(function ({ matchUtilities }) {
+            matchUtilities(
+              {
+                test: (value) => ({ '--test': value }),
+              },
+              {
+                values: {
+                  foo: 'bar',
+                },
+              },
+            )
+          }),
+        }
+      },
+    })
+
+    // These should not crash or produce output
+    expect(
+      optimizeCss(
+        compiler.build([
+          'test-constructor',
+          'test-hasOwnProperty',
+          'test-toString',
+          'test-valueOf',
+          'test-__proto__',
+        ]),
+      ).trim(),
+    ).toEqual('')
+  })
 })
diff --git a/packages/tailwindcss/src/compat/plugin-api.ts b/packages/tailwindcss/src/compat/plugin-api.ts
index 5954040017d2..af6e98d770ff 100644
--- a/packages/tailwindcss/src/compat/plugin-api.ts
+++ b/packages/tailwindcss/src/compat/plugin-api.ts
@@ -202,6 +202,9 @@ export function buildPluginApi({
                 ruleNodes.nodes,
               )
             } else if (variant.value.kind === 'named' && options?.values) {
+              if (!Object.hasOwn(options.values, variant.value.value)) {
+                return null
+              }
               let defaultValue = options.values[variant.value.value]
               if (typeof defaultValue !== 'string') {
                 return null
@@ -223,8 +226,14 @@ export function buildPluginApi({
           let aValueKey = a.value ? a.value.value : 'DEFAULT'
           let zValueKey = z.value ? z.value.value : 'DEFAULT'
 
-          let aValue = options?.values?.[aValueKey] ?? aValueKey
-          let zValue = options?.values?.[zValueKey] ?? zValueKey
+          let aValue =
+            (options?.values && Object.hasOwn(options.values, aValueKey)
+              ? options.values[aValueKey]
+              : undefined) ?? aValueKey
+          let zValue =
+            (options?.values && Object.hasOwn(options.values, zValueKey)
+              ? options.values[zValueKey]
+              : undefined) ?? zValueKey
 
           if (options && typeof options.sort === 'function') {
             return options.sort(
@@ -406,10 +415,13 @@ export function buildPluginApi({
                 value = values.DEFAULT ?? null
               } else if (candidate.value.kind === 'arbitrary') {
                 value = candidate.value.value
-              } else if (candidate.value.fraction && values[candidate.value.fraction]) {
+              } else if (
+                candidate.value.fraction &&
+                Object.hasOwn(values, candidate.value.fraction)
+              ) {
                 value = values[candidate.value.fraction]
                 ignoreModifier = true
-              } else if (values[candidate.value.value]) {
+              } else if (Object.hasOwn(values, candidate.value.value)) {
                 value = values[candidate.value.value]
               } else if (values.__BARE_VALUE__) {
                 value = values.__BARE_VALUE__(candidate.value) ?? null
@@ -430,7 +442,7 @@ export function buildPluginApi({
                 modifier = null
               } else if (modifiers === 'any' || candidate.modifier.kind === 'arbitrary') {
                 modifier = candidate.modifier.value
-              } else if (modifiers?.[candidate.modifier.value]) {
+              } else if (modifiers && Object.hasOwn(modifiers, candidate.modifier.value)) {
                 modifier = modifiers[candidate.modifier.value]
               } else if (isColor && !Number.isNaN(Number(candidate.modifier.value))) {
                 modifier = `${candidate.modifier.value}%`
diff --git a/packages/tailwindcss/src/compat/plugin-functions.ts b/packages/tailwindcss/src/compat/plugin-functions.ts
index f311ad1e8c43..40b8c93e89f1 100644
--- a/packages/tailwindcss/src/compat/plugin-functions.ts
+++ b/packages/tailwindcss/src/compat/plugin-functions.ts
@@ -223,8 +223,10 @@ function get(obj: any, path: string[]) {
   for (let i = 0; i < path.length; ++i) {
     let key = path[i]
 
-    // The key does not exist so concatenate it with the next key
-    if (obj?.[key] === undefined) {
+    // The key does not exist so concatenate it with the next key.
+    // We use Object.hasOwn to avoid matching inherited prototype properties
+    // (e.g. "constructor", "toString") when traversing config objects.
+    if (obj === null || obj === undefined || typeof obj !== 'object' || !Object.hasOwn(obj, key)) {
       if (path[i + 1] === undefined) {
         return undefined
       }
@@ -233,11 +235,6 @@ function get(obj: any, path: string[]) {
       continue
     }
 
-    // We never want to index into strings
-    if (typeof obj === 'string') {
-      return undefined
-    }
-
     obj = obj[key]
   }
 
diff --git a/packages/tailwindcss/src/utilities.test.ts b/packages/tailwindcss/src/utilities.test.ts
index 360acd9e29a6..e562cd3be15a 100644
--- a/packages/tailwindcss/src/utilities.test.ts
+++ b/packages/tailwindcss/src/utilities.test.ts
@@ -1646,6 +1646,13 @@ test('row', async () => {
       'row-span-full/foo',
       'row-[span_123/span_123]/foo',
       'row-span-[var(--my-variable)]/foo',
+
+      // Candidates matching Object.prototype properties should not crash or
+      // produce output (see: https://github.com/tailwindlabs/tailwindcss/issues/19721)
+      'row-constructor',
+      'row-hasOwnProperty',
+      'row-toString',
+      'row-valueOf',
     ]),
   ).toEqual('')
 
diff --git a/packages/tailwindcss/src/utilities.ts b/packages/tailwindcss/src/utilities.ts
index aa2187887405..62e4510b7309 100644
--- a/packages/tailwindcss/src/utilities.ts
+++ b/packages/tailwindcss/src/utilities.ts
@@ -391,6 +391,8 @@ export function createUtilities(theme: Theme) {
    * user's theme.
    */
   function functionalUtility(classRoot: string, desc: UtilityDescription) {
+    if (desc.staticValues) desc.staticValues = Object.assign(Object.create(null), desc.staticValues)
+
     function handleFunctionalUtility({ negative }: { negative: boolean }) {
       return (candidate: Extract<Candidate, { kind: 'functional' }>) => {
         let value: string | null = null