diff --git a/packages/comark-ansi/src/render.ts b/packages/comark-ansi/src/render.ts
index 61b0dd14..1a62372a 100644
--- a/packages/comark-ansi/src/render.ts
+++ b/packages/comark-ansi/src/render.ts
@@ -1,6 +1,7 @@
-import type { MarkdownDocument, RendererOptions } from 'comark'
+import type { ElementNode, MarkdownDocument, Node, RendererOptions } from 'comark'
 import { render } from 'comark/render'
 import { handlers as defaultHandlers } from './handlers/index.ts'
+import { stripControlChars } from './utils/escape.ts'
 
 export * from 'comark/render'
 
@@ -17,6 +18,28 @@ export interface AnsiRendererOptions extends RendererOptions {
   width?: number
 }
 
+/**
+ * The renderer concatenates node text, code bodies and hrefs into terminal
+ * output, so any control byte in the parsed document reaches the TTY. Strip
+ * them (keeping \t and \n) before rendering — returns a copy, the input
+ * document is not mutated.
+ */
+function sanitizeForTerminal(nodes: Node[]): Node[] {
+  return nodes.map((node): Node => {
+    if (typeof node === 'string') return stripControlChars(node)
+    if (node[0] === null) {
+      // Comment node — [null, attrs, content]
+      return [null, node[1], stripControlChars(String((node as unknown[])[2] ?? ''))] as Node
+    }
+    const [tag, attrs, ...children] = node as ElementNode
+    const cleanAttrs: Record<string, unknown> = {}
+    for (const [key, value] of Object.entries(attrs)) {
+      cleanAttrs[key] = typeof value === 'string' ? stripControlChars(value) : value
+    }
+    return [tag, cleanAttrs, ...sanitizeForTerminal(children as Node[])] as Node
+  })
+}
+
 /**
  * Render a Markdown document to an ANSI-styled terminal string.
  *
@@ -40,7 +63,8 @@ export async function renderAnsiFromDocument(
   const colors = options?.colors ?? (typeof process !== 'undefined' ? !process.env.NO_COLOR : true)
   const width = options?.width ?? 80
 
-  return render(document, {
+  const sanitized = { ...document, nodes: sanitizeForTerminal(document.nodes) }
+  return render(sanitized, {
     ...options,
     colors,
     width,
diff --git a/packages/comark-ansi/src/utils/escape.ts b/packages/comark-ansi/src/utils/escape.ts
index a1c748ab..3d2c03d3 100644
--- a/packages/comark-ansi/src/utils/escape.ts
+++ b/packages/comark-ansi/src/utils/escape.ts
@@ -25,6 +25,17 @@ export function stripAnsi(text: string): string {
   return text.replace(ANSI_RE, '')
 }
 
+// C0 controls except \t and \n, plus DEL and the C1 range. These bytes drive
+// terminal control sequences (CSI/OSC/DCS), so author-controlled markdown
+// must never pass them to the TTY.
+// eslint-disable-next-line no-control-regex
+const CONTROL_CHARS_RE = /[\u0000-\u0008\u000B\u000C\u000E-\u001F\u007F-\u009F]/g
+
+/** Remove terminal control characters, keeping tab and newline. */
+export function stripControlChars(text: string): string {
+  return text.replace(CONTROL_CHARS_RE, '')
+}
+
 /** Visible character length of a string, ignoring ANSI escape codes. */
 export function visibleLength(text: string): number {
   return stripAnsi(text).length
diff --git a/packages/comark-ansi/test/control-chars.test.ts b/packages/comark-ansi/test/control-chars.test.ts
new file mode 100644
index 00000000..af8ab5f1
--- /dev/null
+++ b/packages/comark-ansi/test/control-chars.test.ts
@@ -0,0 +1,51 @@
+import { describe, it, expect } from 'vitest'
+import { parseMarkdown } from 'comark'
+import { renderAnsiFromDocument } from '../src/index'
+import { stripControlChars } from '../src/utils/escape'
+
+async function renderAnsiFromMarkdown(markdown: string, options?: Parameters<typeof renderAnsiFromDocument>[1]) {
+  const tree = await parseMarkdown(markdown)
+  return renderAnsiFromDocument(tree, options)
+}
+
+const plain = (markdown: string) => renderAnsiFromMarkdown(markdown, { colors: false })
+
+describe('control character sanitization', () => {
+  it('strips ANSI escape sequences from text content', async () => {
+    const out = await plain('safe \x1B[2J\x1B[H done')
+    expect(out).not.toContain('\x1B')
+    expect(out).toContain('safe')
+    expect(out).toContain('done')
+  })
+
+  it('strips OSC sequences from code block content', async () => {
+    const md =
+      '```\n' +
+      String.fromCharCode(27) +
+      ']8;;https://evil.com' +
+      String.fromCharCode(7) +
+      'click' +
+      String.fromCharCode(27) +
+      ']8;;' +
+      String.fromCharCode(7) +
+      '\n```'
+    const out = await plain(md)
+    expect(out).not.toContain(String.fromCharCode(27))
+    expect(out).not.toContain(String.fromCharCode(7))
+  })
+
+  it('strips escape characters from link hrefs', async () => {
+    const out = await plain('[x](https://example.com/\x1B)')
+    expect(out).not.toContain('\x1B')
+  })
+
+  it('strips C1 control characters', async () => {
+    const out = await plain('a\u009Bb')
+    expect(out).not.toContain('\u009B')
+    expect(out).toContain('ab')
+  })
+
+  it('keeps newlines and tabs intact', () => {
+    expect(stripControlChars('a\tb\nc')).toBe('a\tb\nc')
+  })
+})
diff --git a/packages/comark-html/src/plugins/binding.ts b/packages/comark-html/src/plugins/binding.ts
index a356729d..af9e6d46 100644
--- a/packages/comark-html/src/plugins/binding.ts
+++ b/packages/comark-html/src/plugins/binding.ts
@@ -1,11 +1,9 @@
 import type { NodeHandler } from 'comark/render'
+import { escapeHtml } from '../utils/index.ts'
 
 export * from 'comark/plugins/binding'
 export { default } from 'comark/plugins/binding'
 
-const escape = (s: string) =>
-  s.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;')
-
 /**
  * HTML handler for `binding` nodes produced by the `binding` plugin.
  *
@@ -30,5 +28,5 @@ export const Binding: NodeHandler = (node, state) => {
   const raw = (node[1] || {}) as Record<string, unknown>
   const out = resolved.value ?? raw.defaultValue
   if (out === undefined || out === null) return ''
-  return escape(String(out))
+  return escapeHtml(String(out))
 }
diff --git a/packages/comark-html/src/plugins/math.ts b/packages/comark-html/src/plugins/math.ts
index fa47eea6..ef303b51 100644
--- a/packages/comark-html/src/plugins/math.ts
+++ b/packages/comark-html/src/plugins/math.ts
@@ -1,5 +1,6 @@
 import type { ElementNode } from 'comark'
 import katex from 'katex'
+import { escapeHtml } from '../utils/index.ts'
 
 export * from 'comark/plugins/math'
 export { default } from 'comark/plugins/math'
@@ -32,6 +33,10 @@ export const Math = ([, attrs]: ElementNode): string => {
     })
     return isInline ? `<span class="math inline">${rendered}</span>` : `<div class="math block">${rendered}</div>`
   } catch {
-    return isInline ? `<span class="math inline">${content}</span>` : `<div class="math block">${content}</div>`
+    // KaTeX can still throw non-ParseErrors (e.g. RangeError on deeply nested
+    // input) — never interpolate the raw source unescaped.
+    return isInline
+      ? `<span class="math inline">${escapeHtml(content)}</span>`
+      : `<div class="math block">${escapeHtml(content)}</div>`
   }
 }
diff --git a/packages/comark-html/src/plugins/mermaid.ts b/packages/comark-html/src/plugins/mermaid.ts
index c874f347..6fbe5b15 100644
--- a/packages/comark-html/src/plugins/mermaid.ts
+++ b/packages/comark-html/src/plugins/mermaid.ts
@@ -1,6 +1,7 @@
 import type { ElementNode } from 'comark'
 import type { ThemeNames } from 'comark/plugins/mermaid'
 import { renderMermaidSVG, THEMES } from 'beautiful-mermaid'
+import { escapeHtml } from '../utils/index.ts'
 
 export * from 'comark/plugins/mermaid'
 export { default } from 'comark/plugins/mermaid'
@@ -29,6 +30,7 @@ export const Mermaid = ([, attrs]: ElementNode): string => {
     const svg = renderMermaidSVG(content, theme)
     return `<div class="mermaid">${svg}</div>`
   } catch {
-    return `<pre class="mermaid">${content}</pre>`
+    // Invalid diagram source is author-controlled — escape before fallback.
+    return `<pre class="mermaid">${escapeHtml(content)}</pre>`
   }
 }
diff --git a/packages/comark-html/src/utils/index.ts b/packages/comark-html/src/utils/index.ts
index 705cc782..dd2d304e 100644
--- a/packages/comark-html/src/utils/index.ts
+++ b/packages/comark-html/src/utils/index.ts
@@ -1 +1,9 @@
 export * from 'comark/utils'
+
+/**
+ * Escape a string for safe interpolation into HTML markup. Used by plugin
+ * renderers whose fallback output includes author-controlled source.
+ */
+export function escapeHtml(value: string): string {
+  return value.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;')
+}
diff --git a/packages/comark-html/test/plugin-fallbacks.test.ts b/packages/comark-html/test/plugin-fallbacks.test.ts
new file mode 100644
index 00000000..05bfca9d
--- /dev/null
+++ b/packages/comark-html/test/plugin-fallbacks.test.ts
@@ -0,0 +1,21 @@
+import { describe, expect, it } from 'vitest'
+import { Math } from '../src/plugins/math'
+import { Mermaid } from '../src/plugins/mermaid'
+
+describe('@comark/html plugin fallback escaping', () => {
+  it('escapes raw math source when KaTeX throws', () => {
+    // Deeply nested groups make KaTeX rethrow a RangeError even with
+    // throwOnError: false — the catch branch must not emit raw markup.
+    const content = '<img src=x onerror=alert(1)>' + '{'.repeat(50_000)
+    const html = Math(['math', { class: 'math inline', content }] as any)
+    expect(html).not.toContain('<img src=x onerror=alert(1)>')
+    expect(html).toContain('&lt;img src=x onerror=alert(1)&gt;')
+  })
+
+  it('escapes raw mermaid source when rendering fails', () => {
+    const content = '</pre><img src=x onerror=alert(1)>'
+    const html = Mermaid(['mermaid', { content }] as any)
+    expect(html).not.toContain('<img src=x onerror=alert(1)>')
+    expect(html).toContain('&lt;/pre&gt;&lt;img src=x onerror=alert(1)&gt;')
+  })
+})
diff --git a/test/bundle.test.ts b/test/bundle.test.ts
index 831642d3..03d7bb67 100644
--- a/test/bundle.test.ts
+++ b/test/bundle.test.ts
@@ -61,8 +61,8 @@ describe('package bundle size', { timeout: 60_000 }, () => {
     expect(report).toMatchInlineSnapshot(`
       {
         "@comark/angular": "54.3k (70 files)",
-        "@comark/ansi": "39.9k (98 files)",
-        "@comark/html": "18.6k (58 files)",
+        "@comark/ansi": "41.5k (98 files)",
+        "@comark/html": "19.4k (58 files)",
         "@comark/nuxt": "11.8k (58 files)",
         "@comark/react": "43.6k (74 files)",
         "@comark/svelte": "43.9k (82 files)",