diff --git a/src/core/AnonymizedFile.ts b/src/core/AnonymizedFile.ts
index 19703ce..2a7e81f 100644
--- a/src/core/AnonymizedFile.ts
+++ b/src/core/AnonymizedFile.ts
@@ -362,7 +362,16 @@ export default class AnonymizedFile {
     return true;
   }
 
+  /** Tree entries have no size; zero-byte files are still valid files. */
+  async assertContentFile(): Promise<void> {
+    const file = await this.getFileInfo();
+    if (file.size == null || file.submodule) {
+      throw new AnonymousError("folder_not_supported", { httpStatus: 404 });
+    }
+  }
+
   async content(): Promise<Readable> {
+    await this.assertContentFile();
     if (this.anonymizedPath.includes(config.ANONYMIZATION_MASK)) {
       await this.originalPath();
     }
@@ -388,6 +397,7 @@ export default class AnonymizedFile {
   }
 
   async anonymizedContent() {
+    await this.assertContentFile();
     const anonymizer = this.repository.generateAnonymizeTransformer(
       await this.originalPath()
     );
@@ -439,6 +449,7 @@ export default class AnonymizedFile {
   }
 
   async send(res: Response): Promise<void> {
+    await this.assertContentFile();
     const anonymizer = this.repository.generateAnonymizeTransformer(
       await this.originalPath()
     );
diff --git a/src/core/source/GitHubStream.ts b/src/core/source/GitHubStream.ts
index b53f6d8..f9de7ae 100644
--- a/src/core/source/GitHubStream.ts
+++ b/src/core/source/GitHubStream.ts
@@ -139,16 +139,30 @@ export default class GitHubStream extends GitHubBase {
       filePath
     );
     logger.debug("downloading via raw URL (LFS)", { url });
+    const assertDownloadUrl = (value: URL) => {
+      if (value.protocol !== "https:" || value.username || value.password || value.port ||
+          !["raw.githubusercontent.com", "media.githubusercontent.com"].includes(value.hostname)) {
+        throw new AnonymousError("upstream_error", { httpStatus: 502 });
+      }
+    };
     const data = this.data;
     const cancellation = new AbortController();
     let active: stream.Readable | undefined;
     const output = stream.Readable.from((async function* () {
       try {
         active = got.stream(url, {
-          hooks: { beforeRequest: [async () => { await githubTokenContext(token)?.renew(); }] },
+          hooks: {
+            beforeRequest: [async () => { await githubTokenContext(token)?.renew(); }],
+            beforeRedirect: [options => { assertDownloadUrl(options.url); }],
+          },
           headers: !token || githubTokenContext(token)?.publicRepository ? {} : { authorization: `token ${token}` },
           followRedirect: true,
         });
+        active.once("response", response => {
+          if (response.statusCode !== 200) return;
+          try { assertDownloadUrl(new URL(response.url)); }
+          catch (error) { active?.destroy(error as Error); }
+        });
         for await (const chunk of active) yield chunk;
         return;
       } catch (error) {
@@ -173,12 +187,6 @@ export default class GitHubStream extends GitHubBase {
         if (cancellation.signal.aborted) return;
         const metadata = response.data as { type?: string; download_url?: string };
         if (metadata.type !== "file" || !metadata.download_url) throw error;
-        const assertDownloadUrl = (value: URL) => {
-          if (value.protocol !== "https:" || value.username || value.password || value.port ||
-              !["raw.githubusercontent.com", "media.githubusercontent.com"].includes(value.hostname)) {
-            throw new AnonymousError("upstream_error", { httpStatus: 502 });
-          }
-        };
         assertDownloadUrl(new URL(metadata.download_url));
         // The API URL supplies its own authorization. Never forward the
         // owner's token to the download host or a redirect destination.
diff --git a/src/server/routes/file.ts b/src/server/routes/file.ts
index 67da556..180436b 100644
--- a/src/server/routes/file.ts
+++ b/src/server/routes/file.ts
@@ -115,6 +115,7 @@ router.get(
         anonymizedPath,
       });
       const originalPath = await f.originalPath();
+      await f.assertContentFile();
       if (!f.isFileSupported()) {
         throw new AnonymousError("file_not_supported", {
           httpStatus: 403,
diff --git a/test/folder-file-security.test.js b/test/folder-file-security.test.js
new file mode 100644
index 0000000..bf2b7e5
--- /dev/null
+++ b/test/folder-file-security.test.js
@@ -0,0 +1,86 @@
+const { expect } = require("chai");
+const express = require("express");
+const http = require("http");
+const { Readable } = require("stream");
+const { URL } = require("url");
+const got = require("got");
+require("ts-node/register/transpile-only");
+const AnonymizedFile = require("../src/core/AnonymizedFile").default;
+const GitHubStream = require("../src/core/source/GitHubStream").default;
+const utils = require("../src/server/routes/route-utils");
+const router = require("../src/server/routes/file").default;
+
+async function collect(input) { const chunks = []; for await (const chunk of input) chunks.push(Buffer.from(chunk)); return Buffer.concat(chunks).toString(); }
+async function listen(app) { return new Promise(resolve => { const server = app.listen(0, "127.0.0.1", () => resolve(server)); }); }
+async function close(server) { server.closeAllConnections(); await new Promise(resolve => server.close(resolve)); }
+
+describe("folder file endpoint disclosure", function () {
+  this.timeout(10000);
+  const restores = [];
+  function stub(object, key, value) { const old = object[key]; object[key] = value; restores.push(() => { object[key] = old; }); }
+  afterEach(() => restores.splice(0).reverse().forEach(restore => restore()));
+
+  for (const size of [undefined, null]) {
+    it(`rejects folder metadata with size ${size} before any cached or upstream content`, async () => {
+      const f = new AnonymizedFile({ repository: { options: { terms: [] }, model: { source: { type: "GitHubStream" } } }, anonymizedPath: "strategy1/.idea" });
+      f._file = { name: ".idea", path: "strategy1", sha: "tree-sha", size };
+      for (const call of [() => f.content(), () => f.anonymizedContent(), () => f.send({})]) {
+        const error = await call().catch(error => error);
+        expect(error.message).to.equal("folder_not_supported"); expect(error.httpStatus).to.equal(404);
+      }
+    });
+  }
+  it("keeps zero-byte files and repository HTML downloadable", async () => {
+    for (const [name, size] of [["empty.txt", 0], ["index.html", 123]]) {
+      const f = new AnonymizedFile({ repository: { options: { terms: [] }, model: { source: { type: "GitHubStream" } } }, anonymizedPath: name });
+      f._file = { name, path: "", size, sha: "blob-sha" };
+      await f.assertContentFile();
+    }
+  });
+  it("rejects GET, HEAD and conditional folder requests without trailing slash", async () => {
+    stub(utils, "getRepo", async () => ({ options: { terms: [] }, isReady: async () => true }));
+    stub(AnonymizedFile.prototype, "getFileInfo", async () => ({ name: ".idea", path: "strategy1", sha: "tree-sha" }));
+    stub(AnonymizedFile.prototype, "send", async () => { throw Error("must not serve cached content"); });
+    const app = express(); app.use("/api/repo", router); const server = await listen(app);
+    try {
+      for (const method of ["GET", "HEAD"]) {
+        const response = await got(`http://127.0.0.1:${server.address().port}/api/repo/test/file/strategy1/.idea`, { method, headers: { "if-none-match": "*" }, throwHttpErrors: false });
+        expect(response.statusCode).to.equal(404);
+        if (method === "GET") expect(JSON.parse(response.body).error).to.equal("folder_not_supported");
+      }
+    } finally { await close(server); }
+  });
+  it("blocks a real HTTP redirect to a GitHub tree page before fetching its HTML", async () => {
+    const server = await listen(http.createServer((_req, res) => { res.writeHead(302, { location: "https://github.com/example/private-repo/tree/main/folder" }); res.end(); }));
+    const original = got.stream;
+    stub(got, "stream", (_url, options) => original(`http://127.0.0.1:${server.address().port}/raw`, options));
+    const source = new GitHubStream({ repoId: "test", organization: "example", repoName: "repo", commit: "main", getToken: () => "" });
+    try {
+      const error = await collect(await source.downloadWithFallback("", "tree-sha", "folder")).catch(error => error);
+      expect(error.message).to.equal("upstream_error");
+    } finally { await close(server); }
+  });
+  it("rejects a successful web-page response before emitting any bytes", async () => {
+    const server = await listen(http.createServer((_req, res) => { res.writeHead(200, { "content-type": "text/html" }); res.end("<html>owner metadata</html>"); }));
+    const original = got.stream;
+    stub(got, "stream", (_url, options) => original(`http://127.0.0.1:${server.address().port}/raw`, options));
+    const source = new GitHubStream({ repoId: "test", organization: "example", repoName: "repo", commit: "main", getToken: () => "" });
+    let bytes = 0;
+    try {
+      const stream = await source.downloadWithFallback("", "tree-sha", "folder");
+      stream.on("data", chunk => { bytes += chunk.length; });
+      const error = await collect(stream).catch(error => error);
+      expect(error.message).to.equal("upstream_error"); expect(bytes).to.equal(0);
+    } finally { await close(server); }
+  });
+  it("allows raw/LFS redirects and legitimate HTML file bodies", async () => {
+    stub(got, "stream", (_url, options) => {
+      for (const host of ["raw.githubusercontent.com", "media.githubusercontent.com"]) {
+        options.hooks.beforeRedirect[0]({ url: new URL(`https://${host}/owner/repo/main/index.html`) });
+      }
+      return Readable.from(["<html>repository file</html>"]);
+    });
+    const source = new GitHubStream({ repoId: "test", organization: "example", repoName: "repo", commit: "main", getToken: () => "" });
+    expect(await collect(await source.downloadWithFallback("", "blob-sha", "index.html"))).to.equal("<html>repository file</html>");
+  });
+});
diff --git a/test/owner-notifications.test.js b/test/owner-notifications.test.js
index 49756cc..26aa1ad 100644
--- a/test/owner-notifications.test.js
+++ b/test/owner-notifications.test.js
@@ -484,6 +484,7 @@ describe("streamed file access alerts", () => {
     repository = { owner: { id: "saved-owner" }, options: { terms: [] }, model: { source: {} }, repoId: "anon-id", status: "ready", source: {},
       getToken: async () => "token", generateAnonymizeTransformer: () => new PassThrough() };
     file = new File({ repository, anonymizedPath: "README.md" });
+    file._file = { name: "README.md", path: "", sha: "sha", size: 10 };
   });
   afterEach(() => { while (restores.length) restores.pop()(); });
   for (const status of [403, 404, 503]) it(`handles local file download failure ${status}`, async () => {
diff --git a/test/production-regressions.test.js b/test/production-regressions.test.js
index d5b1c5c..d3c72c8 100644
--- a/test/production-regressions.test.js
+++ b/test/production-regressions.test.js
@@ -226,7 +226,7 @@ describe("production regressions", function () {
     const utils = require("../src/server/routes/route-utils");
     const repo = { options: { pdf: false, image: false, terms: [] }, model: { source: { commit: "commit-1" }, options: {} }, isReady: async () => true, countView: async () => {} };
     stub(utils, "getRepo", async () => repo);
-    stub(File.prototype, "originalPath", async function () { this._file = { name: originalName, path: "" }; return originalName; });
+    stub(File.prototype, "originalPath", async function () { this._file = { name: originalName, path: "", size: 10 }; return originalName; });
     stub(File.prototype, "sha", async () => "sha");
     stub(File.prototype, "send", async () => {});
     const handler = require("../src/server/routes/file").default.stack[0].route.stack[0].handle;