diff --git a/src/server/routes/webview.ts b/src/server/routes/webview.ts
index c33326a..d89db2c 100644
--- a/src/server/routes/webview.ts
+++ b/src/server/routes/webview.ts
@@ -83,13 +83,19 @@ async function webView(req: express.Request, res: express.Response) {
     const wRoot = repo.options.pageSource.path;
 
     const indexRepoId = req.path.indexOf(req.params.repoId);
-    const filePath = req.path.substring(
+    const rawPath = req.path.substring(
       indexRepoId + req.params.repoId.length + 1
     );
+    let filePath: string;
+    try {
+      filePath = decodeURIComponent(rawPath);
+    } catch {
+      throw new AnonymousError("invalid_path", { httpStatus: 400 });
+    }
     // Reject traversal in the URL-derived segment before joining it onto the
     // page-source root. Stripping a single leading "/" or "." is not enough
     // to stop "../../" sequences from climbing out of the repo (CWE-22).
-    if (filePath.split(/[\\/]/).some((segment) => segment === "..")) {
+    if (/^[\\/]/.test(filePath) || filePath.split(/[\\/]/).some((segment) => segment === "..")) {
       throw new AnonymousError("invalid_path", {
         httpStatus: 400,
         object: filePath,
diff --git a/test/production-regressions.test.js b/test/production-regressions.test.js
index e7a5ecd..b0b5fce 100644
--- a/test/production-regressions.test.js
+++ b/test/production-regressions.test.js
@@ -152,7 +152,7 @@ describe("production regressions", function () {
     expect(second.headers.ETag).not.to.equal(first.headers.ETag);
     expect(second.statusCode).not.to.equal(304);
   });
-  it("sandboxes rendered webview documents", async function () {
+  it("decodes webview filenames and sandboxes rendered documents", async function () {
     const File = require("../src/core/AnonymizedFile").default;
     const utils = require("../src/server/routes/route-utils");
     const repo = { options: { terms: [], page: true, pageSource: { path: "/", branch: "main" }, image: true }, model: { source: { branch: "main" } } };
@@ -162,7 +162,7 @@ describe("production regressions", function () {
     stub(File.prototype, "send", async () => {});
     const handler = require("../src/server/routes/webview").default.stack[0].route.stack[0].handle;
     const res = response();
-    await handler({ path: "/repo/my file.html", params: { repoId: "repo" }, headers: {} }, res);
+    await handler({ path: "/repo/my%20file.html", params: { repoId: "repo" }, headers: {} }, res);
     expect(path).to.equal("my file.html");
     expect(res.headers["Content-Security-Policy"]).to.include("sandbox");
     expect(res.headers["Content-Security-Policy"]).not.to.include("allow-same-origin");