diff --git a/.github/actions/setup-test-app-dependencies/action.yml b/.github/actions/setup-test-app-dependencies/action.yml
index 993e517290..01d879299f 100644
--- a/.github/actions/setup-test-app-dependencies/action.yml
+++ b/.github/actions/setup-test-app-dependencies/action.yml
@@ -10,7 +10,8 @@ runs:
   using: 'composite'
   steps:
     # Expo's dependency graph contains native binaries, so cache it by operating
-    # system, architecture, Node major, pnpm version, and the exact manifest.
+    # system, architecture, Node major, pnpm version, the exact manifest, and the
+    # pnpm patch files applied into it.
     - name: Resolve test app dependency cache key
       id: key
       shell: bash
@@ -24,7 +25,7 @@ runs:
       uses: actions/cache/restore@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.2.3
       with:
         path: examples/test-app/node_modules
-        key: test-app-dependencies-v2-${{ runner.os }}-${{ runner.arch }}-node${{ steps.key.outputs.node_major }}-pnpm${{ steps.key.outputs.pnpm_version }}-${{ hashFiles('examples/test-app/package.json', 'examples/test-app/pnpm-lock.yaml', 'examples/test-app/pnpm-workspace.yaml', '.github/actions/setup-test-app-dependencies/action.yml') }}
+        key: test-app-dependencies-v2-${{ runner.os }}-${{ runner.arch }}-node${{ steps.key.outputs.node_major }}-pnpm${{ steps.key.outputs.pnpm_version }}-${{ hashFiles('examples/test-app/package.json', 'examples/test-app/pnpm-lock.yaml', 'examples/test-app/pnpm-workspace.yaml', 'examples/test-app/patches/**', '.github/actions/setup-test-app-dependencies/action.yml') }}
 
     - name: Install test app dependencies
       if: steps.restore.outputs.cache-hit != 'true'
@@ -36,4 +37,4 @@ runs:
       uses: actions/cache/save@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.2.3
       with:
         path: examples/test-app/node_modules
-        key: test-app-dependencies-v2-${{ runner.os }}-${{ runner.arch }}-node${{ steps.key.outputs.node_major }}-pnpm${{ steps.key.outputs.pnpm_version }}-${{ hashFiles('examples/test-app/package.json', 'examples/test-app/pnpm-lock.yaml', 'examples/test-app/pnpm-workspace.yaml', '.github/actions/setup-test-app-dependencies/action.yml') }}
+        key: test-app-dependencies-v2-${{ runner.os }}-${{ runner.arch }}-node${{ steps.key.outputs.node_major }}-pnpm${{ steps.key.outputs.pnpm_version }}-${{ hashFiles('examples/test-app/package.json', 'examples/test-app/pnpm-lock.yaml', 'examples/test-app/pnpm-workspace.yaml', 'examples/test-app/patches/**', '.github/actions/setup-test-app-dependencies/action.yml') }}
diff --git a/.github/workflows/test-app-build-cache.yml b/.github/workflows/test-app-build-cache.yml
index ba8f372cb7..44e223341a 100644
--- a/.github/workflows/test-app-build-cache.yml
+++ b/.github/workflows/test-app-build-cache.yml
@@ -50,6 +50,12 @@ jobs:
         uses: ./.github/actions/run-gate
         with: { gate: test-app-typecheck }
 
+      # image-size has no fixed release for its parser DoS advisories; this gate
+      # proves the in-tree pnpm patch still terminates the zero-length vectors.
+      - name: Run the image-size parser security test
+        uses: ./.github/actions/run-gate
+        with: { gate: test-app-security }
+
       - name: Resolve native fingerprint
         id: fingerprint
         env:
diff --git a/examples/test-app/security/image-size-security.test.mjs b/examples/test-app/security/image-size-security.test.mjs
index e67556905a..41556de980 100644
--- a/examples/test-app/security/image-size-security.test.mjs
+++ b/examples/test-app/security/image-size-security.test.mjs
@@ -5,14 +5,13 @@ import path from 'node:path';
 import test from 'node:test';
 
 const packageStore = path.join(process.cwd(), 'node_modules', '.pnpm');
+const patchHash = /^ {2}image-size@1\.2\.1: ([0-9a-f]{64})$/m.exec(
+  fs.readFileSync(path.join(process.cwd(), 'pnpm-lock.yaml'), 'utf8'),
+)?.[1];
+assert.ok(patchHash, 'the lockfile must record the image-size patch hash');
 const imageSizePath =
   process.env.IMAGE_SIZE_PACKAGE_DIR ??
-  path.join(
-    packageStore,
-    fs.readdirSync(packageStore).find((entry) => entry.startsWith('image-size@1.2.1_patch_hash=')),
-    'node_modules',
-    'image-size',
-  );
+  path.join(packageStore, `image-size@1.2.1_patch_hash=${patchHash}`, 'node_modules', 'image-size');
 assert.ok(fs.existsSync(imageSizePath), 'the image-size package must be installed');
 
 test('zero-length image records and boxes return within the parser budget', () => {
@@ -38,6 +37,17 @@ test('zero-length image records and boxes return within the parser budget', () =
       ]),
       'No codestream found in JXL container',
     ],
+    [
+      'JXL zero-size jxlp box',
+      Buffer.concat([
+        box(8, 'JXL '),
+        box(16, 'ftyp'),
+        Buffer.from('jxl '),
+        Buffer.alloc(4),
+        zeroBox('jxlp'),
+      ]),
+      'No codestream found in JXL container',
+    ],
   ];
   const selectedFixtures = process.env.IMAGE_SIZE_FIXTURE
     ? fixtures.filter(([name]) => name === process.env.IMAGE_SIZE_FIXTURE)
diff --git a/package.json b/package.json
index d622692000..5551f4e14b 100644
--- a/package.json
+++ b/package.json
@@ -172,6 +172,7 @@
     "test-app:ios": "pnpm --dir examples/test-app ios",
     "test-app:android": "pnpm --dir examples/test-app android",
     "test-app:typecheck": "pnpm --dir examples/test-app typecheck",
+    "test-app:security": "pnpm --dir examples/test-app security:test",
     "test-app:replay:ios": "pnpm ad test examples/test-app/replays --platform ios --artifacts-dir .tmp/test-app-replay/ios",
     "test-app:replay:android": "pnpm ad test examples/test-app/replays --platform android --artifacts-dir .tmp/test-app-replay/android",
     "test-app:maestro": "node scripts/run-test-app-maestro-suite.mjs",
diff --git a/scripts/check-affected/checks.ts b/scripts/check-affected/checks.ts
index ff13cdafb5..a68e03a8f3 100644
--- a/scripts/check-affected/checks.ts
+++ b/scripts/check-affected/checks.ts
@@ -38,6 +38,12 @@ export const CHECK_CATALOG: readonly CheckSpec[] = [
   // The test app intentionally owns a separate Expo dependency graph. Do not
   // make every root-checkout validation install it implicitly.
   gate('test-app-typecheck', 'Expo test app typecheck', 'test-app:typecheck', false),
+  gate(
+    'test-app-security',
+    'Expo test app image-size parser security test',
+    'test-app:security',
+    false,
+  ),
   gate('layering', 'Import-direction layering guard', 'check:layering'),
   gate('di-seams', 'Test-only DI seam guard', 'check:di-seams'),
   gate('fallow', 'Fallow code-quality audit', 'check:fallow'),
diff --git a/scripts/check-affected/model.test.ts b/scripts/check-affected/model.test.ts
index a0b2d9f1b3..12fb1d0efc 100644
--- a/scripts/check-affected/model.test.ts
+++ b/scripts/check-affected/model.test.ts
@@ -184,6 +184,21 @@ test('test app source selects root lint and format plus its isolated typecheck',
   ]);
 });
 
+test('the image-size parser mitigation is selected when its defining files change', () => {
+  for (const file of [
+    'examples/test-app/patches/image-size@1.2.1.patch',
+    'examples/test-app/security/image-size-security.test.mjs',
+    'examples/test-app/pnpm-workspace.yaml',
+  ]) {
+    const result = plan([file]);
+    assert.equal(result.failOpen, false, `${file} must not fail open`);
+    assert.ok(
+      result.checks.includes('test-app-security'),
+      `expected test-app-security for ${file}`,
+    );
+  }
+});
+
 test('unknown path fails open to the full check set', () => {
   const result = plan(['fixtures/unknown.data']);
   assert.equal(result.failOpen, true);
diff --git a/scripts/check-affected/model.ts b/scripts/check-affected/model.ts
index 65028f6f23..043d23fc05 100644
--- a/scripts/check-affected/model.ts
+++ b/scripts/check-affected/model.ts
@@ -33,6 +33,7 @@ export type CheckId =
   | 'lint'
   | 'typecheck'
   | 'test-app-typecheck'
+  | 'test-app-security'
   | 'layering'
   | 'di-seams'
   | 'fallow'
@@ -96,6 +97,7 @@ export const ALL_CHECKS: readonly CheckId[] = [
   'lint',
   'typecheck',
   'test-app-typecheck',
+  'test-app-security',
   'layering',
   'di-seams',
   'fallow',
@@ -496,6 +498,17 @@ const BUILD_OWNERSHIP: ReadonlyArray<{
     detail: 'MCP registry metadata must stay in sync',
     owns: (file) => file === 'server.json' || file === 'smithery.yaml',
   },
+  // image-size ships no fixed version for its parser DoS advisories; the in-tree
+  // pnpm patch is the mitigation, and only its defining files own the proof.
+  {
+    check: 'test-app-security',
+    rule: 'own:test-app-security',
+    detail: 'the image-size parser mitigation is proven by the test-app security suite',
+    owns: (file) =>
+      file.startsWith('examples/test-app/patches/') ||
+      file.startsWith('examples/test-app/security/') ||
+      file === 'examples/test-app/pnpm-workspace.yaml',
+  },
   // TS/Swift golden tables (`contracts/fixtures/*.json`): the vitest parity test and the
   // runner XCTest twin both read them, so a table edit owns the unit lane and both runner
   // builds. Without this a `.json` under contracts/ has no derivable owner and fails open.