diff --git a/app/api/share/route.ts b/app/api/share/route.ts
index a0b33fc..79e1561 100644
--- a/app/api/share/route.ts
+++ b/app/api/share/route.ts
@@ -2,7 +2,10 @@ import { timingSafeEqual } from "node:crypto";
 import { cdnUrl } from "@/lib/cdn-images";
 import { ensureDiscordUser } from "@/lib/auth/discord-user";
 import { getCanonicalUrl, getShareBotSecret } from "@/lib/config/server";
-import { getDiscordMemberProfile } from "@/lib/discord/discord";
+import {
+  DiscordProfileLookupError,
+  getDiscordSenderProfile,
+} from "@/lib/discord/discord";
 import { createShare } from "@/lib/share/create";
 import { ShareHttpError, shareErrorResponse } from "@/lib/share/http-error";
 import { enforceShareRateLimit } from "@/lib/share/rate-limit";
@@ -44,7 +47,25 @@ export async function POST(request: Request) {
       windowSeconds: 60 * 60,
     });
 
-    const formData = await request.formData();
+    let formData: FormData;
+    try {
+      formData = await request.formData();
+    } catch {
+      throw new ShareHttpError("Malformed multipart form data", 400);
+    }
+    for (const field of [
+      "displayName",
+      "avatarUrl",
+      "authorName",
+      "authorAvatarUrl",
+    ]) {
+      if (formData.has(field)) {
+        throw new ShareHttpError(
+          `Profile field ${field} must not be supplied`,
+          400,
+        );
+      }
+    }
     const file = formData.get("file");
     const senderDiscordId = formData.get("senderDiscordId");
     if (!(file instanceof File)) {
@@ -62,7 +83,15 @@ export async function POST(request: Request) {
       validateOptionalImage(formData.get("image")),
     ]);
     const description = validateShareDescription(formData.get("description"));
-    const profile = await getDiscordMemberProfile(senderDiscordId);
+    let profile;
+    try {
+      profile = await getDiscordSenderProfile(senderDiscordId);
+    } catch (error) {
+      if (error instanceof DiscordProfileLookupError) {
+        throw new ShareHttpError("Discord profile lookup is unavailable", 503);
+      }
+      throw error;
+    }
     if (!profile) {
       throw new ShareHttpError(
         "Discord sender was not found in the configured guild",
diff --git a/lib/discord/discord.ts b/lib/discord/discord.ts
index 30dbc5f..f20d4ba 100644
--- a/lib/discord/discord.ts
+++ b/lib/discord/discord.ts
@@ -24,6 +24,18 @@ export interface DiscordMemberProfile {
   roles: DiscordRole[];
 }
 
+export type DiscordSenderProfile = Pick<
+  DiscordMemberProfile,
+  "id" | "username" | "globalName" | "avatarUrl"
+>;
+
+export class DiscordProfileLookupError extends Error {
+  constructor(message = "Discord profile lookup is unavailable") {
+    super(message);
+    this.name = "DiscordProfileLookupError";
+  }
+}
+
 interface CacheEntry<T> {
   data: T;
   expiresAt: number;
@@ -213,3 +225,49 @@ export async function getDiscordMemberProfile(discordId: string): Promise<Discor
     return null;
   }
 }
+
+export async function getDiscordSenderProfile(
+  discordId: string,
+): Promise<DiscordSenderProfile | null> {
+  try {
+    const { token, guildId } = getDiscordServerConfig();
+    const response = await fetch(
+      `https://discord.com/api/v10/guilds/${encodeURIComponent(guildId)}/members/${encodeURIComponent(discordId)}`,
+      {
+        headers: { Authorization: `Bot ${token}` },
+        cache: "no-store",
+        signal: AbortSignal.timeout(5000),
+      },
+    );
+
+    if (response.status === 404) return null;
+    if (!response.ok) throw new DiscordProfileLookupError();
+
+    const member = (await response.json()) as {
+      user?: {
+        id?: string;
+        username?: string;
+        global_name?: string | null;
+        avatar?: string | null;
+      };
+    };
+    const user = member.user;
+    if (!user?.id || !user.username || user.id !== discordId) {
+      throw new DiscordProfileLookupError("Discord returned an invalid sender profile");
+    }
+
+    const avatarUrl = user.avatar
+      ? `https://cdn.discordapp.com/avatars/${user.id}/${user.avatar}.${user.avatar.startsWith("a_") ? "gif" : "png"}?size=128`
+      : null;
+
+    return {
+      id: user.id,
+      username: user.username,
+      globalName: user.global_name ?? null,
+      avatarUrl,
+    };
+  } catch (error) {
+    if (error instanceof DiscordProfileLookupError) throw error;
+    throw new DiscordProfileLookupError();
+  }
+}
diff --git a/lib/share/validation.test.ts b/lib/share/validation.test.ts
index 206eb57..e432d34 100644
--- a/lib/share/validation.test.ts
+++ b/lib/share/validation.test.ts
@@ -24,8 +24,9 @@ describe("share validation", () => {
       throw new Error("Expected blank text to fail");
     } catch (error) {
       expect(error).toBeInstanceOf(ShareHttpError);
-      expect((error as ShareHttpError).status).toBe(422);
+      expect((error as ShareHttpError).status).toBe(400);
     }
+    expect(() => validateShareText("before\0after")).toThrow("NUL");
   });
 
   test("measures the text limit in UTF-8 bytes", () => {
@@ -65,7 +66,12 @@ describe("share validation", () => {
           type: "text/plain",
         })
       )
-    ).rejects.toMatchObject({ status: 422 });
+    ).rejects.toMatchObject({ status: 400 });
+    await expect(
+      validateTextFile(
+        new File(["before\0after"], "nul.txt", { type: "text/plain" })
+      )
+    ).rejects.toMatchObject({ status: 400 });
   });
 
   test("validates image contents instead of trusting the MIME type", async () => {
diff --git a/lib/share/validation.ts b/lib/share/validation.ts
index ab20bf7..827e1e8 100644
--- a/lib/share/validation.ts
+++ b/lib/share/validation.ts
@@ -18,7 +18,10 @@ export function validateShareText(content: unknown) {
     throw new ShareHttpError("Text is required", 400);
   }
   if (!content.trim()) {
-    throw new ShareHttpError("Text cannot be blank", 422);
+    throw new ShareHttpError("Text cannot be blank", 400);
+  }
+  if (content.includes("\0")) {
+    throw new ShareHttpError("Text cannot contain NUL bytes", 400);
   }
   if (Buffer.byteLength(content, "utf8") > MAX_SHARE_TEXT_BYTES) {
     throw new ShareHttpError("Text must be 5 MiB or smaller", 413);
@@ -55,7 +58,7 @@ export async function validateTextFile(file: File) {
     throw new ShareHttpError("Text file must be 5 MiB or smaller", 413);
   }
   if (file.size === 0) {
-    throw new ShareHttpError("Text file cannot be empty", 422);
+    throw new ShareHttpError("Text file cannot be empty", 400);
   }
 
   try {
@@ -64,7 +67,7 @@ export async function validateTextFile(file: File) {
     return validateShareText(content);
   } catch (error) {
     if (error instanceof ShareHttpError) throw error;
-    throw new ShareHttpError("Text file must contain valid UTF-8", 422);
+    throw new ShareHttpError("Text file must contain valid UTF-8", 400);
   }
 }