diff --git a/.github/actions/check-dependency-ranges/action.yml b/.github/actions/check-dependency-ranges/action.yml
new file mode 100644
index 0000000..808bd07
--- /dev/null
+++ b/.github/actions/check-dependency-ranges/action.yml
@@ -0,0 +1,13 @@
+name: Check dependency version ranges
+description: Fails when package.json declares version ranges with no upper bound (*, latest, bare >= / >)
+inputs:
+  working-directory:
+    required: false
+    default: '.'
+runs:
+  using: composite
+  steps:
+    - shell: sh
+      env:
+        WORKING_DIR: ${{ inputs.working-directory }}
+      run: node "$GITHUB_ACTION_PATH/check-version-ranges.mjs" "$WORKING_DIR/package.json"
diff --git a/.github/actions/check-dependency-ranges/check-version-ranges.mjs b/.github/actions/check-dependency-ranges/check-version-ranges.mjs
new file mode 100644
index 0000000..86dbba7
--- /dev/null
+++ b/.github/actions/check-dependency-ranges/check-version-ranges.mjs
@@ -0,0 +1,79 @@
+// Fails when package.json declares dependency ranges with no upper bound.
+// An uncapped range silently adopts future majors, breaking changes included:
+// a ">=" security override can float a transitive dependency onto an ESM-only
+// major and break a CommonJS consumer at runtime, with CI still green.
+import { readFileSync } from 'fs';
+
+// peerDependencies are deliberately NOT checked: open-ended peer ranges are a
+// legitimate library convention — the consumer's own dependency range bounds them.
+const DEPENDENCY_SECTIONS = [
+  'dependencies',
+  'devDependencies',
+  'optionalDependencies',
+  'overrides',
+];
+
+// Specifiers that are not semver ranges (git URLs, local paths, workspace and
+// catalog protocols) are skipped — the check only judges registry version ranges.
+const NON_RANGE_SPECIFIER = /^(file:|git|http|link:|workspace:|catalog:)/;
+
+// An alternative is unbounded when nothing caps it from above: a bare "*",
+// "x", "latest", or a ">"/">=" comparator with no "<"/"<=" companion.
+const isUnboundedAlternative = (alternative) => {
+  const trimmed = alternative.trim();
+  if (trimmed === '' || trimmed === '*' || trimmed === 'x' || trimmed === 'latest') {
+    return true;
+  }
+  if (trimmed.includes('>') && !trimmed.includes('<')) {
+    return true;
+  }
+  return false;
+};
+
+const isUnboundedRange = (range) => {
+  if (NON_RANGE_SPECIFIER.test(range)) {
+    return false;
+  }
+  // npm: aliases delegate to the aliased range ("npm:package@^1.0.0")
+  const effectiveRange = range.startsWith('npm:') ? range.slice(range.lastIndexOf('@') + 1) : range;
+  return effectiveRange.split('||').some(isUnboundedAlternative);
+};
+
+export const findUnboundedRanges = (packageDefinition) => {
+  const findings = [];
+  const inspect = (section, entries) => {
+    for (const [name, specifier] of Object.entries(entries)) {
+      if (typeof specifier === 'object' && specifier !== null) {
+        // overrides admit nested objects ({".": "...", child: "..."})
+        inspect(`${section} > ${name}`, specifier);
+      }
+      if (typeof specifier === 'string' && isUnboundedRange(specifier)) {
+        findings.push({ section, name, specifier });
+      }
+    }
+  };
+  for (const section of DEPENDENCY_SECTIONS) {
+    if (packageDefinition[section]) {
+      inspect(section, packageDefinition[section]);
+    }
+  }
+  return findings;
+};
+
+const isMainModule = process.argv[1]?.endsWith('check-version-ranges.mjs');
+if (isMainModule) {
+  const packageJsonPath = process.argv[2] ?? 'package.json';
+  const packageDefinition = JSON.parse(readFileSync(packageJsonPath, 'utf8'));
+  const findings = findUnboundedRanges(packageDefinition);
+  if (findings.length > 0) {
+    console.error('Unbounded version ranges are not allowed — an uncapped range adopts');
+    console.error('future majors silently, including breaking ones: a ">=" override can');
+    console.error('float a dependency onto an ESM-only major and break a CommonJS consumer.');
+    for (const { section, name, specifier } of findings) {
+      console.error(`  ${section}: "${name}": "${specifier}" — pin a ceiling (^, ~ or an exact version)`);
+    }
+    console.error('To bypass while fixing: pass check-dependency-ranges: false to the reusable workflow.');
+    process.exit(1);
+  }
+  console.log('All dependency ranges have an upper bound.');
+}
diff --git a/.github/workflows/pr-checks-node-npm.yml b/.github/workflows/pr-checks-node-npm.yml
index 37b1e79..a911ccf 100644
--- a/.github/workflows/pr-checks-node-npm.yml
+++ b/.github/workflows/pr-checks-node-npm.yml
@@ -13,6 +13,11 @@ on:
         required: false
         type: string
         default: ''
+      check-dependency-ranges:
+        description: 'Fail when package.json has unbounded version ranges (*, latest, bare >= / >)'
+        required: false
+        type: boolean
+        default: true
 
 permissions:
   contents: read
@@ -44,6 +49,12 @@ jobs:
           cache: 'npm'
           cache-dependency-path: ${{ inputs.working-directory }}/package-lock.json
 
+      - name: Check dependency ranges
+        if: inputs.check-dependency-ranges
+        uses: nullplatform/actions-nullplatform/.github/actions/check-dependency-ranges@main
+        with:
+          working-directory: ${{ inputs.working-directory }}
+
       - name: Install dependencies
         working-directory: ${{ inputs.working-directory }}
         run: npm ci
diff --git a/.github/workflows/pr-checks-node-pnpm.yml b/.github/workflows/pr-checks-node-pnpm.yml
index 326cac4..5eba7cb 100644
--- a/.github/workflows/pr-checks-node-pnpm.yml
+++ b/.github/workflows/pr-checks-node-pnpm.yml
@@ -13,6 +13,11 @@ on:
         required: false
         type: string
         default: ''
+      check-dependency-ranges:
+        description: 'Fail when package.json has unbounded version ranges (*, latest, bare >= / >)'
+        required: false
+        type: boolean
+        default: true
 
 permissions:
   contents: read
@@ -47,6 +52,12 @@ jobs:
           cache: 'pnpm'
           cache-dependency-path: ${{ inputs.working-directory }}/pnpm-lock.yaml
 
+      - name: Check dependency ranges
+        if: inputs.check-dependency-ranges
+        uses: nullplatform/actions-nullplatform/.github/actions/check-dependency-ranges@main
+        with:
+          working-directory: ${{ inputs.working-directory }}
+
       - name: Install dependencies
         working-directory: ${{ inputs.working-directory }}
         run: pnpm install