fix(上传): 头像和题面插图按文件头认格式，不信扩展名

原来只看扩展名：改个后缀什么都能落盘，再由 /public/* 按扩展名配 content-type
发出去。线上 Caddy 有 nosniff 兜着，但不该把非图片存到盘上。

- services/image.ts：按 PNG / JPEG / GIF / BMP 的文件头识别，返回落盘用的扩展名 ——
  以内容为准，改名成 .png 的 JPEG 存成 .jpg，content-type 才对得上
- 头像上传、后台插图上传都走它；插图那边扩展名白名单保留当第一道快速筛
- serveUpload 自己也带 x-content-type-options: nosniff，不只靠 Caddy

实跑核过：真 PNG / GIF 正常；改名成 .png 的 JPEG 存成 .jpg；改名成 .png 的 HTML
两个入口都拒；/public/upload 回 image/png + nosniff。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/src/index.ts

@@ -78,7 +78,12 @@ async function serveUpload(
   if (await file.exists()) {
     // 文件名由后端生成且内容不变，可以放心长缓存
     return new Response(file, {
-      headers: { "cache-control": "public, max-age=86400" },
+      headers: {
+        "cache-control": "public, max-age=86400",
+        // 线上 Caddy 也加了这个头，这里再加一道：content-type 按扩展名配，
+        // 不让浏览器自己嗅探成 HTML 去执行
+        "x-content-type-options": "nosniff",
+      },
     })
   }
   return null

apps/api/src/routes/account.ts

@@ -1,5 +1,5 @@
 import { randomBytes } from "node:crypto"
-import { extname, resolve } from "node:path"
+import { resolve } from "node:path"
 
 import {
   rankProfileSchema,
@@ -46,6 +46,7 @@ import { db, schema } from "../db"
 import { failure, parseBody, success } from "../http"
 import { JudgeStatus } from "../judge/status"
 import { getBooleanOption } from "../services/options"
+import { sniffImageExtension } from "../services/image"
 import { getUserProfileById } from "../services/profile"
 import {
   clientIp,
@@ -205,8 +206,8 @@ accountRoutes.post("/me/avatar", requireAuth, async (c) => {
     return failure(c, 400, "invalid-file", "Invalid file content")
   if (image.size > 2 * 1024 * 1024)
     return failure(c, 400, "file-too-large", "Picture is too large")
-  const extension = extname(image.name).toLowerCase()
-  if (![".gif", ".jpg", ".jpeg", ".bmp", ".png"].includes(extension)) {
+  const extension = await sniffImageExtension(image)
+  if (!extension) {
     return failure(c, 400, "unsupported-file", "Unsupported file format")
   }
   const filename = `${randomBytes(10).toString("hex")}${extension}`

apps/api/src/routes/admin/conf.ts

@@ -23,6 +23,7 @@ import { config } from "../../config"
 import { db, schema } from "../../db"
 import { publishConfigUpdate } from "../../events"
 import { failure, parseBody, success } from "../../http"
+import { sniffImageExtension } from "../../services/image"
 import { getWebsiteOptions } from "../../services/options"
 import { todayStart } from "../../time"
 import { queryInteger } from "../helpers"
@@ -292,8 +293,12 @@ adminConfRoutes.post("/upload-image", requireAdmin, async (c) => {
       filePath: "",
     } satisfies UploadImageResponse)
   }
-  const suffix = image.name.slice(image.name.lastIndexOf(".")).toLowerCase()
-  if (!IMAGE_SUFFIXES.includes(suffix)) {
+  // 以文件头为准，扩展名只是第一道快速筛
+  const claimed = image.name.slice(image.name.lastIndexOf(".")).toLowerCase()
+  const suffix = IMAGE_SUFFIXES.includes(claimed)
+    ? await sniffImageExtension(image)
+    : null
+  if (!suffix) {
     return success(c, {
       success: false,
       msg: "Unsupported file format",

apps/api/src/services/image.ts

@@ -0,0 +1,21 @@
+/**
+ * 按文件头认图片格式，不信文件名。头像和题面插图原来只看扩展名：改个后缀什么都能
+ * 落盘，再由 /public/* 按扩展名配 content-type 发出去。
+ *
+ * 返回值就是落盘该用的扩展名 —— 以内容为准，一张改名成 .png 的 JPEG 会存成 .jpg，
+ * 浏览器拿到的 content-type 才对得上。
+ */
+const SIGNATURES: [extension: string, bytes: number[]][] = [
+  [".png", [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]],
+  [".jpg", [0xff, 0xd8, 0xff]],
+  [".gif", [0x47, 0x49, 0x46, 0x38]], // GIF8（GIF87a / GIF89a）
+  [".bmp", [0x42, 0x4d]], // BM
+]
+
+export async function sniffImageExtension(file: Blob) {
+  const head = new Uint8Array(await file.slice(0, 8).arrayBuffer())
+  for (const [extension, bytes] of SIGNATURES) {
+    if (bytes.every((byte, index) => head[index] === byte)) return extension
+  }
+  return null
+}