diff --git a/src/web-ui/src/component-library/components/Markdown/MermaidBlock.test.tsx b/src/web-ui/src/component-library/components/Markdown/MermaidBlock.test.tsx
new file mode 100644
index 0000000000..b77bc0f383
--- /dev/null
+++ b/src/web-ui/src/component-library/components/Markdown/MermaidBlock.test.tsx
@@ -0,0 +1,95 @@
+/**
+ * @vitest-environment jsdom
+ */
+
+import { describe, expect, it } from 'vitest';
+
+import { sanitizeMermaidSvg } from './sanitizeMermaidSvg';
+
+describe('sanitizeMermaidSvg', () => {
+  describe('XSS vectors are removed', () => {
+    it('strips <script> elements entirely', () => {
+      const raw = '<svg xmlns="http://www.w3.org/2000/svg"><script>alert(1)</script><circle cx="1" cy="1" r="1"/></svg>';
+      const clean = sanitizeMermaidSvg(raw);
+      expect(clean).not.toContain('<script');
+      expect(clean).not.toContain('alert(1)');
+      expect(clean.toLowerCase()).toContain('<svg');
+      expect(clean.toLowerCase()).toContain('<circle');
+    });
+
+    it('strips on* event handlers from elements', () => {
+      const raw = '<svg xmlns="http://www.w3.org/2000/svg"><g onload="alert(1)"><circle cx="1" cy="1" r="1" onmouseover="alert(2)"/></g></svg>';
+      const clean = sanitizeMermaidSvg(raw);
+      expect(clean).not.toContain('onload');
+      expect(clean).not.toContain('onmouseover');
+      expect(clean).not.toContain('alert(2)');
+      expect(clean.toLowerCase()).toContain('<svg');
+    });
+
+    it('strips javascript: URLs from href/src/xlink:href', () => {
+      const raw = '<svg xmlns="http://www.w3.org/2000/svg"><a href="javascript:alert(1)">x</a><image xlink:href="javascript:alert(2)"/></svg>';
+      const clean = sanitizeMermaidSvg(raw);
+      expect(clean).not.toContain('javascript:');
+      expect(clean.toLowerCase()).toContain('<svg');
+    });
+
+    it('removes injected HTML/script inside foreignObject labels', () => {
+      const raw =
+        '<svg xmlns="http://www.w3.org/2000/svg"><foreignObject width="10" height="10">' +
+        '<div xmlns="http://www.w3.org/1999/xhtml"><img src="x" onerror="alert(1)">' +
+        '<script>alert(2)</script><p onclick="alert(3)">label</p></div></foreignObject></svg>';
+      const clean = sanitizeMermaidSvg(raw);
+      expect(clean).not.toContain('onerror');
+      expect(clean).not.toContain('onclick');
+      expect(clean).not.toContain('<script');
+      expect(clean).not.toContain('alert(1)');
+      expect(clean).not.toContain('<img');
+      expect(clean.toLowerCase()).toContain('label');
+    });
+
+    it('returns empty string for malformed / non-svg input', () => {
+      expect(sanitizeMermaidSvg('')).toBe('');
+      expect(sanitizeMermaidSvg('<div>not an svg</div>')).toBe('');
+    });
+  });
+
+  describe('normal mermaid output is preserved', () => {
+    it('keeps structural svg elements and drops nothing that a rendered diagram needs', () => {
+      const raw = `<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="500" height="300" viewBox="0 0 500 300" role="img" aria-roledescription="diagram">
+  <style>#mermaid-1 .node rect { fill: #fff; }</style>
+  <defs>
+    <marker id="mermaid-1_arrow" markerWidth="10" markerHeight="10" orient="auto" refX="9" refY="3">
+      <path d="M0,0 L0,6 L9,3 z" fill="black"/>
+    </marker>
+    <linearGradient id="mermaid-1_g" x1="0" y1="0" x2="1" y2="0">
+      <stop offset="0%" stop-color="#ffffff"/>
+      <stop offset="100%" stop-color="#eeeeee"/>
+    </linearGradient>
+  </defs>
+  <g id="mermaid-1" class="node" transform="translate(100,50)">
+    <rect width="120" height="40" rx="5" ry="5" fill="url(#mermaid-1_g)" stroke="#333" stroke-width="1.5"/>
+    <text x="60" y="25" text-anchor="middle" dominant-baseline="central" font-size="14" font-family="sans-serif">Process</text>
+  </g>
+  <g id="mermaid-1_label" class="edgeLabel">
+    <foreignObject width="100" height="30" x="0" y="0">
+      <div xmlns="http://www.w3.org/1999/xhtml" class="nodeLabel" style="text-align:center;"><p>Hello</p></div>
+    </foreignObject>
+  </g>
+</svg>`;
+      const clean = sanitizeMermaidSvg(raw);
+      const lower = clean.toLowerCase();
+      expect(lower).toContain('<svg');
+      expect(lower).toContain('<path');
+      expect(lower).toContain('<rect');
+      expect(lower).toContain('<text');
+      expect(lower).toContain('<foreignobject');
+      expect(lower).toContain('<div');
+      expect(lower).toContain('<style');
+      expect(lower).toContain('class="nodelabel"');
+      expect(lower).toContain('viewbox');
+      // safety invariants on a normal diagram
+      expect(clean).not.toContain('<script');
+      expect(clean).not.toMatch(/on\w+=/i);
+    });
+  });
+});
diff --git a/src/web-ui/src/component-library/components/Markdown/MermaidBlock.tsx b/src/web-ui/src/component-library/components/Markdown/MermaidBlock.tsx
index 0e11d31b0f..36f022f4fb 100644
--- a/src/web-ui/src/component-library/components/Markdown/MermaidBlock.tsx
+++ b/src/web-ui/src/component-library/components/Markdown/MermaidBlock.tsx
@@ -9,6 +9,7 @@ import { MermaidService } from '../../../tools/mermaid-editor/services/MermaidSe
 import { mermaidAppearanceAdapter } from '@/infrastructure/appearance/adapters/MermaidAppearanceAdapter';
 import { Loader2, AlertCircle, Code2, Copy, Check } from 'lucide-react';
 import { createLogger } from '@/shared/utils/logger';
+import { sanitizeMermaidSvg } from './sanitizeMermaidSvg';
 import './MermaidBlock.scss';
 
 const log = createLogger('MermaidBlock');
@@ -206,7 +207,7 @@ export const MermaidBlock: React.FC<MermaidBlockProps> = ({
               className="mermaid-block__diagram"
               data-bf-component="mermaid-block"
               data-bf-part="diagram"
-              dangerouslySetInnerHTML={{ __html: svgContent }}
+              dangerouslySetInnerHTML={{ __html: sanitizeMermaidSvg(svgContent) }}
             />
             
             <div data-bf-component="mermaid-block" data-bf-part="actions" className="mermaid-block__actions">
diff --git a/src/web-ui/src/component-library/components/Markdown/sanitizeMermaidSvg.ts b/src/web-ui/src/component-library/components/Markdown/sanitizeMermaidSvg.ts
new file mode 100644
index 0000000000..b6645570f5
--- /dev/null
+++ b/src/web-ui/src/component-library/components/Markdown/sanitizeMermaidSvg.ts
@@ -0,0 +1,134 @@
+/**
+ * Sanitizer for Mermaid-rendered SVG markup.
+ *
+ * Mermaid is initialized with `securityLevel: 'loose'`, which allows label
+ * markup (raw HTML in labels) to pass through rendering. A crafted diagram
+ * can therefore carry `<script>`, event-handler attributes, `javascript:` URLs
+ * or foreignObject payloads into the rendered output. This module strips those
+ * vectors while keeping the structural SVG and text-container elements that a
+ * rendered diagram needs.
+ */
+
+/** Elements that must never survive sanitization. */
+const DANGEROUS_TAGS = new Set([
+  'script', 'iframe', 'object', 'embed', 'form', 'meta', 'link', 'base',
+  'frame', 'frameset', 'noscript', 'template',
+]);
+
+/** Structural SVG / HTML elements that rendered diagrams legitimately use. */
+const ALLOWED_SVG_TAGS = new Set([
+  // svg structure / shapes
+  'svg', 'g', 'defs', 'style', 'marker', 'path', 'rect', 'circle', 'ellipse',
+  'line', 'polyline', 'polygon', 'text', 'tspan', 'title', 'desc', 'use',
+  'symbol', 'textpath', 'clippath', 'stop', 'pattern', 'mask', 'filter',
+  'fegaussianblur', 'fecolormatrix', 'feoffset', 'feflood', 'feblend',
+  'image', 'a',
+  // gradient / pattern
+  'lineargradient', 'radialgradient',
+  // foreignObject content (mermaid htmlLabels) + text containers
+  'foreignobject', 'div', 'span', 'p', 'br', 'b', 'strong', 'i', 'em', 'u',
+  'small', 'sub', 'sup', 'ul', 'ol', 'li', 'pre', 'code', 'font',
+  'table', 'thead', 'tbody', 'tfoot', 'tr', 'td', 'th', 'figure', 'h1', 'h2',
+  'h3', 'h4', 'h5', 'h6',
+]);
+
+/** Attributes that rendered diagrams legitimately use. */
+const ALLOWED_SVG_ATTRS = new Set([
+  // core + namespace
+  'id', 'class', 'xmlns', 'xmlns:xlink', 'xlink:href', 'xlink:title',
+  'xml:space', 'role', 'aria-label', 'aria-roledescription', 'aria-hidden',
+  'focusable', 'tabindex', 'title',
+  // geometry / viewport
+  'viewbox', 'preserveaspectratio', 'width', 'height', 'x', 'y', 'x1', 'y1',
+  'x2', 'y2', 'cx', 'cy', 'r', 'rx', 'ry', 'd', 'points', 'pathlength',
+  'transform', 'translate', 'scale', 'rotate', 'skewx', 'skewy', 'matrix',
+  'offset', 'refx', 'refy', 'refwidth', 'refheight',
+  // painting
+  'fill', 'fill-opacity', 'fill-rule', 'stroke', 'stroke-width',
+  'stroke-linecap', 'stroke-linejoin', 'stroke-miterlimit', 'stroke-dasharray',
+  'stroke-dashoffset', 'stroke-opacity', 'opacity', 'color', 'flood-color',
+  'flood-opacity', 'stop-color', 'stop-opacity',
+  // typography / text
+  'font-family', 'font-size', 'font-weight', 'font-style', 'font-variant',
+  'text-anchor', 'dominant-baseline', 'baseline-shift', 'letter-spacing',
+  'word-spacing', 'text-decoration', 'direction', 'unicode-bidi',
+  'writing-mode', 'alignment-baseline', 'line-height', 'text-transform',
+  'white-space', 'font-stretch',
+  // markers / gradients / patterns / filters
+  'marker-start', 'marker-mid', 'marker-end', 'marker', 'markerwidth',
+  'markerheight', 'markerunits', 'orient', 'gradientunits', 'gradienttransform',
+  'spreadmethod', 'patternunits', 'patterncontentunits', 'patterntransform',
+  'maskunits', 'maskcontentunits', 'in', 'in2', 'result', 'stddeviation',
+  'edgemode', 'kernelunitlength', 'tablevalues', 'values', 'type', 'mode',
+  'interpolate', 'numoctaves', 'basefrequency', 'stitchtiles',
+  // styles / display
+  'style', 'display', 'visibility', 'overflow', 'clip', 'clip-path', 'clip-rule',
+  'vector-effect', 'shape-rendering', 'text-rendering', 'image-rendering',
+  'color-rendering', 'color-interpolation', 'color-interpolation-filters',
+  'paint-order', 'mix-blend-mode', 'isolation', 'pointer-events', 'cursor',
+  'filter', 'mask', 'enable-background',
+  // animation
+  'attributename', 'attributetype', 'begin', 'dur', 'end', 'repeatcount',
+  'repeatdur', 'from', 'to', 'by', 'calcmode', 'additive', 'keytimes',
+  'keysplines', 'keypoints', 'restart', 'fill-freeze',
+  // html container attributes
+  'align', 'valign', 'colspan', 'rowspan', 'cellpadding', 'cellspacing',
+  'border', 'bgcolor', 'nowrap', 'dir', 'lang', 'col', 'row', 'span',
+  'start', 'reversed', 'data-label',
+]);
+
+const EVENT_ATTR_RE = /^on/i;
+const URL_ATTRS = new Set(['href', 'src', 'xlink:href']);
+const UNSAFE_URL_RE = /^\s*(?:javascript|vbscript|data:text\/html)\s*:/i;
+
+/**
+ * Sanitize Mermaid-rendered SVG markup before it is injected via
+ * dangerouslySetInnerHTML. Anything outside the allowlist (dangerous tags,
+ * unknown elements injected through labels, `on*` handlers, unsafe URL
+ * schemes) is removed. Input that does not parse into a single root `<svg>`
+ * element is dropped entirely.
+ */
+export function sanitizeMermaidSvg(svgRaw: string): string {
+  if (!svgRaw) return svgRaw;
+
+  let doc: Document;
+  try {
+    doc = new DOMParser().parseFromString(svgRaw, 'text/html');
+  } catch {
+    return '';
+  }
+
+  const root = doc.body.firstElementChild;
+  if (!root || root.localName.toLowerCase() !== 'svg') return '';
+
+  const sanitizeNode = (node: Element) => {
+    const tag = node.localName.toLowerCase();
+    // Any element outside the allowed set (scripts, iframes, unknown HTML
+    // injected via labels, etc.) is removed together with its subtree.
+    if (DANGEROUS_TAGS.has(tag) || !ALLOWED_SVG_TAGS.has(tag)) {
+      node.remove();
+      return;
+    }
+
+    for (const attr of Array.from(node.attributes)) {
+      const name = attr.name;
+      const lowerName = name.toLowerCase();
+      if (EVENT_ATTR_RE.test(lowerName)) {
+        node.removeAttribute(name);
+        continue;
+      }
+      if (URL_ATTRS.has(lowerName) && UNSAFE_URL_RE.test(attr.value)) {
+        node.removeAttribute(name);
+        continue;
+      }
+      if (!ALLOWED_SVG_ATTRS.has(lowerName)) {
+        node.removeAttribute(name);
+      }
+    }
+
+    Array.from(node.children).forEach((child) => sanitizeNode(child));
+  };
+
+  sanitizeNode(root);
+  return root.outerHTML;
+}