diff --git a/packages/core/src/compiler/compositionScoping.test.ts b/packages/core/src/compiler/compositionScoping.test.ts
index 7985c9f809..b9d7e72d79 100644
--- a/packages/core/src/compiler/compositionScoping.test.ts
+++ b/packages/core/src/compiler/compositionScoping.test.ts
@@ -1,6 +1,7 @@
 import { describe, expect, it, vi } from "vitest";
 import { parseHTML } from "linkedom";
 import {
+  buildVariablesByCompScript,
   scopeCssToComposition,
   wrapInlineScriptWithErrorBoundary,
   wrapScopedCompositionScript,
@@ -699,13 +700,18 @@ window.__afterTimeline = window.__timelines.scene;
   });
 
   it("wraps unscoped composition script source as a string literal", () => {
+    const source = 'window.payload = "</script><script>window.pwned = true;</script>";';
     const wrapped = wrapInlineScriptWithErrorBoundary(
-      'window.payload = "</script><script>window.pwned = true;</script>";',
+      source,
       "[HyperFrames] composition script error:",
     );
 
     expect(wrapped).toContain("Function(");
-    expect(wrapped).toContain('\\"</script><script>window.pwned = true;</script>\\"');
+    // The literal carries the source verbatim, with `<` escaped so it cannot end the
+    // raw-text `<script>` this is emitted into.
+    expect(wrapped).not.toContain("</script");
+    const literal = /Function\((".*")\)/.exec(wrapped)?.[1];
+    expect(JSON.parse(literal ?? "")).toBe(source);
   });
 
   it("rewrites #id CSS selectors to [data-hf-authored-id] when authoredRootId is provided", () => {
@@ -886,3 +892,132 @@ window.__timelines['intro'] = tl;
     expect(gsapTargets).toEqual([["HELLO"]]);
   });
 });
+
+/**
+ * The emitted statement is placed inside a `<script>` element, and `<script>` is a
+ * RAW TEXT element: HTML serialization does not escape its content and the tokenizer
+ * closes it at the first `</script`. `JSON.stringify` escapes `"` and `\` but not `/`,
+ * so an unescaped variable value could close the element and have the remainder parsed
+ * as markup — turning composition data into executable script.
+ */
+/**
+ * Every payload leads with a benign `<` before its `</script`, so escaping only the
+ * first `<` is not enough to pass: that pins the `/g` flag on the escape rather than
+ * merely "an escape ran". A lone `<` in a value is the common case (`a < b`, `<em>`),
+ * so a payload whose breakout is not the first `<` is the realistic one.
+ */
+const SCRIPT_BREAKOUT = "x<y</script><script>window.__pwned=1//";
+
+/** Serialize into a document the way the compilers do, then re-parse it. */
+function scriptsAfterRoundTrip(body: string): string[] {
+  const { document } = parseHTML("<!doctype html><html><head></head><body></body></html>");
+  const el = document.createElement("script");
+  el.textContent = body;
+  document.body.appendChild(el);
+  const { document: reparsed } = parseHTML(document.toString());
+  return [...reparsed.querySelectorAll("script")].map((s) => s.textContent ?? "");
+}
+
+describe("buildVariablesByCompScript — <script> breakout", () => {
+  it("does not let a variable VALUE close the script element", () => {
+    const body = buildVariablesByCompScript({
+      "comp-a": { greeting: SCRIPT_BREAKOUT },
+    });
+    expect(body).not.toBeNull();
+    expect(body).not.toContain("</script");
+    expect(scriptsAfterRoundTrip(body ?? "")).toHaveLength(1);
+  });
+
+  it("does not let a variable KEY close the script element", () => {
+    const body = buildVariablesByCompScript({
+      "comp-a": { [SCRIPT_BREAKOUT]: "x" },
+    });
+    expect(body).not.toContain("</script");
+    expect(scriptsAfterRoundTrip(body ?? "")).toHaveLength(1);
+  });
+
+  it("does not let a COMP ID close the script element", () => {
+    const body = buildVariablesByCompScript({
+      [SCRIPT_BREAKOUT]: { a: "x" },
+    });
+    expect(body).not.toContain("</script");
+    expect(scriptsAfterRoundTrip(body ?? "")).toHaveLength(1);
+  });
+
+  it("keeps the value byte-identical once executed — the escape is transparent", () => {
+    // Run the statement the way the browser does rather than string-slicing it.
+    const variables = { "comp-a": { greeting: "a </script> b <em>c</em>" } };
+    const body = buildVariablesByCompScript(variables) ?? "";
+    const fakeWindow: Record<string, unknown> = {};
+    new Function("window", body)(fakeWindow);
+    expect(fakeWindow.__hfVariablesByComp).toEqual(variables);
+  });
+
+  it("returns null when there are no per-instance values", () => {
+    expect(buildVariablesByCompScript({})).toBeNull();
+  });
+});
+
+/**
+ * The variables table is not the only attacker-reachable literal emitted into a
+ * `<script>`: the wrapper the sub-composition scripts run inside embeds the
+ * composition id four times over (directly, as the timeline id, and inside two
+ * derived selector patterns), plus the authored root id, the scope-selector
+ * override and the error label. All of them are emitted into the same raw-text
+ * element, so each has to survive a serialize/reparse round trip.
+ */
+describe("wrapScopedCompositionScript — <script> breakout via the wrapper literals", () => {
+  const LABEL = "[HyperFrames] composition script error:";
+
+  it("does not let a COMP ID close the script element", () => {
+    const body = wrapScopedCompositionScript("console.log(1);", SCRIPT_BREAKOUT);
+    expect(body).not.toContain("</script");
+    expect(scriptsAfterRoundTrip(body)).toHaveLength(1);
+  });
+
+  it("keeps the comp id byte-identical — the escape is transparent", () => {
+    const body = wrapScopedCompositionScript("console.log(1);", SCRIPT_BREAKOUT);
+    const literal = /var __hfCompId = (.*);/.exec(body)?.[1];
+    expect(literal).toBeDefined();
+    expect(JSON.parse(literal ?? "")).toBe(SCRIPT_BREAKOUT);
+  });
+
+  it("does not let the AUTHORED ROOT ID close the script element", () => {
+    const body = wrapScopedCompositionScript(
+      "console.log(1);",
+      "comp-a",
+      LABEL,
+      undefined,
+      "comp-a",
+      SCRIPT_BREAKOUT,
+    );
+    expect(body).not.toContain("</script");
+    expect(scriptsAfterRoundTrip(body)).toHaveLength(1);
+  });
+
+  it("does not let the SCOPE SELECTOR override close the script element", () => {
+    const body = wrapScopedCompositionScript("console.log(1);", "comp-a", LABEL, SCRIPT_BREAKOUT);
+    expect(body).not.toContain("</script");
+    expect(scriptsAfterRoundTrip(body)).toHaveLength(1);
+  });
+
+  it("does not let the ERROR LABEL close the script element", () => {
+    const body = wrapScopedCompositionScript("console.log(1);", "comp-a", SCRIPT_BREAKOUT);
+    expect(body).not.toContain("</script");
+    expect(scriptsAfterRoundTrip(body)).toHaveLength(1);
+  });
+});
+
+describe("wrapInlineScriptWithErrorBoundary — <script> breakout", () => {
+  it("does not let the wrapped SOURCE close the script element", () => {
+    const body = wrapInlineScriptWithErrorBoundary(`var a = "${SCRIPT_BREAKOUT}";`, "[err]");
+    expect(body).not.toContain("</script");
+    expect(scriptsAfterRoundTrip(body)).toHaveLength(1);
+  });
+
+  it("does not let the ERROR LABEL close the script element", () => {
+    const body = wrapInlineScriptWithErrorBoundary("var a = 1;", SCRIPT_BREAKOUT);
+    expect(body).not.toContain("</script");
+    expect(scriptsAfterRoundTrip(body)).toHaveLength(1);
+  });
+});
diff --git a/packages/core/src/compiler/compositionScoping.ts b/packages/core/src/compiler/compositionScoping.ts
index cbe75b0f87..795b801f08 100644
--- a/packages/core/src/compiler/compositionScoping.ts
+++ b/packages/core/src/compiler/compositionScoping.ts
@@ -250,6 +250,26 @@ export function scopeCssToComposition(
   return root.toResult({ map: false }).css;
 }
 
+/**
+ * Serialize a value as a JS literal safe to emit inside a `<script>` element.
+ *
+ * `<script>` is a RAW TEXT element: HTML serialization does not escape its
+ * content, and the tokenizer ends the element at the first `</script` — in any
+ * string, comment or regex context. `JSON.stringify` escapes `"` and `\` but
+ * neither `<` nor `/`, so any dynamic literal carrying `</script>` would close
+ * the element early and have the remainder parsed as markup. Rewriting every
+ * `<` to `<` removes the only byte that can start a closing tag, and is
+ * transparent to both `JSON.parse` and the JS string grammar, so the value the
+ * runtime reads is unchanged.
+ *
+ * Every dynamic literal in an emitted script body must go through here: a
+ * per-value guard on this surface has already been missed once, since the
+ * composition id reaches the emitted script through four separate literals.
+ */
+function jsonScriptLiteral(value: unknown): string {
+  return JSON.stringify(value).replace(/</g, "\\u003c");
+}
+
 export function wrapScopedCompositionScript(
   source: string,
   compositionId: string,
@@ -258,19 +278,19 @@ export function wrapScopedCompositionScript(
   timelineCompositionId = compositionId,
   authoredRootId?: string | null,
 ): string {
-  const compositionIdLiteral = JSON.stringify(compositionId);
-  const timelineCompositionIdLiteral = JSON.stringify(timelineCompositionId);
-  const errorLabelLiteral = JSON.stringify(errorLabel);
+  const compositionIdLiteral = jsonScriptLiteral(compositionId);
+  const timelineCompositionIdLiteral = jsonScriptLiteral(timelineCompositionId);
+  const errorLabelLiteral = jsonScriptLiteral(errorLabel);
   const escapedCompositionId = escapeRegExp(compositionId);
-  const authoredRootIdLiteral = JSON.stringify(authoredRootId?.trim() || null);
-  const scopeSelectorLiteral = JSON.stringify(scopeSelectorOverride ?? null);
-  const rootSelectorPatternLiteral = JSON.stringify(
+  const authoredRootIdLiteral = jsonScriptLiteral(authoredRootId?.trim() || null);
+  const scopeSelectorLiteral = jsonScriptLiteral(scopeSelectorOverride ?? null);
+  const rootSelectorPatternLiteral = jsonScriptLiteral(
     String.raw`\[\s*data-composition-id\s*=\s*(?:"${escapedCompositionId}"|'${escapedCompositionId}')\s*\]`,
   );
-  const timingSelectorPatternLiteral = JSON.stringify(
+  const timingSelectorPatternLiteral = jsonScriptLiteral(
     String.raw`\s*\[\s*data-(?:start|duration)\s*=\s*(?:"[^"]*"|'[^']*')\s*\]`,
   );
-  const authoredRootIdFormsLiteral = JSON.stringify(
+  const authoredRootIdFormsLiteral = jsonScriptLiteral(
     getAuthoredRootIdSelectorForms(authoredRootId?.trim() || ""),
   );
   return `(function(){
@@ -278,7 +298,7 @@ export function wrapScopedCompositionScript(
   var __hfTimelineCompId = ${timelineCompositionIdLiteral};
   var __hfErrorLabel = ${errorLabelLiteral};
   var __hfAuthoredRootId = ${authoredRootIdLiteral};
-  var __hfAuthoredRootAttr = ${JSON.stringify(AUTHORED_ROOT_ID_ATTR)};
+  var __hfAuthoredRootAttr = ${jsonScriptLiteral(AUTHORED_ROOT_ID_ATTR)};
   var __hfEscapeAttr = function(value) {
     return (value + "").replace(/\\\\/g, "\\\\\\\\").replace(/"/g, "\\\\\\"");
   };
@@ -585,7 +605,7 @@ ${source.replace(/<\/(script)/gi, "<\\/$1")}
 }
 
 export function wrapInlineScriptWithErrorBoundary(source: string, errorLabel: string): string {
-  return `(function(){ try { Function(${JSON.stringify(source)}).call(window); } catch (_err) { console.error(${JSON.stringify(errorLabel)}, _err); } })();`;
+  return `(function(){ try { Function(${jsonScriptLiteral(source)}).call(window); } catch (_err) { console.error(${jsonScriptLiteral(errorLabel)}, _err); } })();`;
 }
 
 /**
@@ -601,10 +621,14 @@ export function wrapInlineScriptWithErrorBoundary(source: string, errorLabel: st
  * `getVariables()` returned `{}` only during render — parametrized sub-comps
  * silently shipped blank/default text in the final MP4 while snapshot QA passed
  * (issue #2064). Both callers now share this one builder so they can't drift.
+ *
+ * Values, keys and composition ids are all attacker-reachable, so the whole
+ * table goes through `jsonScriptLiteral` — see there for why.
  */
 export function buildVariablesByCompScript(
   variablesByComp: Record<string, Record<string, unknown>>,
 ): string | null {
   if (!variablesByComp || Object.keys(variablesByComp).length === 0) return null;
-  return `window.__hfVariablesByComp = Object.assign({}, window.__hfVariablesByComp || {}, ${JSON.stringify(variablesByComp)});`;
+  const json = jsonScriptLiteral(variablesByComp);
+  return `window.__hfVariablesByComp = Object.assign({}, window.__hfVariablesByComp || {}, ${json});`;
 }
