diff --git a/src/devtoolsPanel.ts b/src/devtoolsPanel.ts
index 96a1322d..7077c4a5 100644
--- a/src/devtoolsPanel.ts
+++ b/src/devtoolsPanel.ts
@@ -414,6 +414,12 @@ export class DevToolsPanel {
 
         const uri = await this.parseUrlToUri(url);
 
+        // The stylesheet url comes from the inspected page, so it must never be able to steer
+        // this write outside the developer's project.
+        if (uri && !this.isWithinTrustedRoot(uri)) {
+            return;
+        }
+
         // Finally open and edit the document if it exists
         if (uri) {
             const textEditor = await this.openEditorFromUri(uri);
@@ -539,6 +545,35 @@ export class DevToolsPanel {
         });
     }
 
+    private getTrustedRoots(): string[] {
+        const folders = vscode.workspace.workspaceFolders;
+        if (folders && folders.length > 0) {
+            return folders.map(folder => folder.uri.fsPath).filter(fsPath => Boolean(fsPath));
+        }
+
+        // Single-file debugging (e.g. "Launch HTML file") has no workspace folder, so the
+        // developer-chosen target file's own directory is the only trusted root.
+        if (this.targetUrl.startsWith('file://')) {
+            try {
+                return [path.dirname(vscode.Uri.parse(this.targetUrl).fsPath)];
+            } catch {
+                return [];
+            }
+        }
+
+        return [];
+    }
+
+    private isWithinTrustedRoot(uri: vscode.Uri): boolean {
+        const target = path.resolve(uri.fsPath);
+        return this.getTrustedRoots().some(root => {
+            // path.relative escapes with '..' (or an absolute path) whenever target is outside root,
+            // and compares case-insensitively on Windows.
+            const relative = path.relative(path.resolve(root), target);
+            return relative === '' || (!relative.startsWith(`..${path.sep}`) && relative !== '..' && !path.isAbsolute(relative));
+        });
+    }
+
     private async showCssMirroringWarning() {
         if (!this.cssWarningActive) {
             this.cssWarningActive = true;
diff --git a/test/devtoolsPanel.test.ts b/test/devtoolsPanel.test.ts
index b29b1c87..f051291b 100644
--- a/test/devtoolsPanel.test.ts
+++ b/test/devtoolsPanel.test.ts
@@ -643,7 +643,7 @@ describe("devtoolsPanel", () => {
 
             it("calls openTextDocument for onSocketCssMirrorContent", async () => {
                 const expectedRequest = {
-                    url: "app.js",
+                    url: "g:\\GIT\\testPage\\styles.css",
                     newContent: ".body{color: blue;}"
                 };
 
@@ -665,6 +665,55 @@ describe("devtoolsPanel", () => {
                 expect(mockVsCode.workspace.openTextDocument).toHaveBeenCalled();
             });
 
+            it("does not mirror css to a path outside the workspace", async () => {
+                // The url originates from the inspected page via a `/*# sourceURL= */` comment.
+                const expectedRequest = {
+                    url: "c:\\Users\\test\\AppData\\Roaming\\Code\\User\\settings.json",
+                    newContent: ".body{color: blue;}"
+                };
+
+                const mockVsCode = jest.requireMock("vscode");
+                const mockUtils = {
+                    applyPathMapping: jest.fn().mockImplementation((x) => x),
+                    fetchUri: jest.fn().mockRejectedValue(null),
+                    isHeadlessEnabled: jest.fn(),
+                    getCSSMirrorContentEnabled: jest.fn().mockImplementation(() => true),
+                };
+                jest.doMock("../src/utils", () => mockUtils);
+
+                const dtp = await import("../src/devtoolsPanel");
+                const { TextEncoder } = require('util');
+                global.TextEncoder = TextEncoder;
+                dtp.DevToolsPanel.createOrShow(context, mockTelemetry, "", mockRuntimeConfig);
+
+                await hookedEvents.get("cssMirrorContent")!(JSON.stringify(expectedRequest));
+                expect(mockVsCode.workspace.openTextDocument).not.toHaveBeenCalled();
+            });
+
+            it("does not mirror css to a path that escapes the workspace via traversal", async () => {
+                const expectedRequest = {
+                    url: "g:\\GIT\\testPage\\..\\..\\..\\Windows\\system32\\drivers\\etc\\hosts",
+                    newContent: ".body{color: blue;}"
+                };
+
+                const mockVsCode = jest.requireMock("vscode");
+                const mockUtils = {
+                    applyPathMapping: jest.fn().mockImplementation((x) => x),
+                    fetchUri: jest.fn().mockRejectedValue(null),
+                    isHeadlessEnabled: jest.fn(),
+                    getCSSMirrorContentEnabled: jest.fn().mockImplementation(() => true),
+                };
+                jest.doMock("../src/utils", () => mockUtils);
+
+                const dtp = await import("../src/devtoolsPanel");
+                const { TextEncoder } = require('util');
+                global.TextEncoder = TextEncoder;
+                dtp.DevToolsPanel.createOrShow(context, mockTelemetry, "", mockRuntimeConfig);
+
+                await hookedEvents.get("cssMirrorContent")!(JSON.stringify(expectedRequest));
+                expect(mockVsCode.workspace.openTextDocument).not.toHaveBeenCalled();
+            });
+
             it("calls getVscodeSettings", async () => {
                 jest.dontMock("../src/common/settingsProvider.ts");
                 const expectedId = { id: 0 };
diff --git a/test/helpers/helpers.ts b/test/helpers/helpers.ts
index f6cb494a..93b1a244 100644
--- a/test/helpers/helpers.ts
+++ b/test/helpers/helpers.ts
@@ -138,7 +138,10 @@ export function createFakeVSCode() {
             openTextDocument: jest.fn().mockResolvedValue(null),
             workspaceFolders: [
                 {
-                    uri:  'file:///g%3A/GIT/testPage'
+                    uri: {
+                        fsPath: 'g:\\GIT\\testPage',
+                        toString: () => 'file:///g%3A/GIT/testPage',
+                    }
                 }
             ],
             fs: {