From bde1bad2c1798afdaaacabfdd3ec42feff4537cd Mon Sep 17 00:00:00 2001
From: Vincent Koc <vincentkoc@ieee.org>
Date: Wed, 27 May 2026 20:40:43 +0200
Subject: [PATCH] fix(gateway): bound webchat image data scans

---
 .../server-methods/chat-webchat-media.test.ts | 23 +++++++++++
 .../server-methods/chat-webchat-media.ts      | 41 +++++++++++++++----
 2 files changed, 57 insertions(+), 7 deletions(-)

diff --git a/src/gateway/server-methods/chat-webchat-media.test.ts b/src/gateway/server-methods/chat-webchat-media.test.ts
index a7b97015b20..180a8cfdc94 100644
--- a/src/gateway/server-methods/chat-webchat-media.test.ts
+++ b/src/gateway/server-methods/chat-webchat-media.test.ts
@@ -297,6 +297,29 @@ describe("buildWebchatAssistantMessageFromReplyPayloads", () => {
     expect(message).toBeNull();
   });
 
+  it("keeps valid whitespace in data image payloads", async () => {
+    const imageUrl = "data:image/png;base64,cG 5n\n";
+    const message = await buildWebchatAssistantMessageFromReplyPayloads([
+      {
+        text: "whitespace image",
+        mediaUrl: imageUrl,
+      },
+    ]);
+
+    expect(message?.content).toContainEqual({ type: "input_image", image_url: imageUrl.trim() });
+  });
+
+  it("rejects invalid data image payload characters", async () => {
+    const message = await buildWebchatAssistantMessageFromReplyPayloads([
+      {
+        text: "invalid",
+        mediaUrl: "data:image/png;base64,cG5n!",
+      },
+    ]);
+
+    expect(message).toBeNull();
+  });
+
   it("rejects remote image URLs", async () => {
     const message = await buildWebchatAssistantMessageFromReplyPayloads([
       {
diff --git a/src/gateway/server-methods/chat-webchat-media.ts b/src/gateway/server-methods/chat-webchat-media.ts
index 052807dabd6..33bc373bfd6 100644
--- a/src/gateway/server-methods/chat-webchat-media.ts
+++ b/src/gateway/server-methods/chat-webchat-media.ts
@@ -2,6 +2,7 @@ import path from "node:path";
 import type { ReplyPayload } from "../../auto-reply/reply-payload.js";
 import { openLocalFileSafely } from "../../infra/fs-safe.js";
 import { assertNoWindowsNetworkPath, safeFileURLToPath } from "../../infra/local-file-access.js";
+import { estimateBase64DecodedBytes } from "../../media/base64.js";
 import { assertLocalMediaAllowed, LocalMediaAccessError } from "../../media/local-media-access.js";
 import { isAudioFileName } from "../../media/mime.js";
 import { resolveSendableOutboundReplyParts } from "../../plugin-sdk/reply-payload.js";
@@ -131,10 +132,31 @@ function mimeTypeForPath(filePath: string): string {
   return MIME_BY_EXT[ext] ?? "audio/mpeg";
 }
 
-function estimateBase64DecodedBytes(base64: string): number {
-  const sanitized = base64.replace(/\s+/g, "");
-  const padding = sanitized.endsWith("==") ? 2 : sanitized.endsWith("=") ? 1 : 0;
-  return Math.floor((sanitized.length * 3) / 4) - padding;
+function isBase64DataPayload(value: string): boolean {
+  if (value.length === 0) {
+    return false;
+  }
+  for (let index = 0; index < value.length; index += 1) {
+    const code = value.charCodeAt(index);
+    const isBase64Char =
+      (code >= 0x41 && code <= 0x5a) ||
+      (code >= 0x61 && code <= 0x7a) ||
+      (code >= 0x30 && code <= 0x39) ||
+      code === 0x2b ||
+      code === 0x2f ||
+      code === 0x3d;
+    const isWhitespace =
+      code === 0x09 ||
+      code === 0x0a ||
+      code === 0x0b ||
+      code === 0x0c ||
+      code === 0x0d ||
+      code === 0x20;
+    if (!isBase64Char && !isWhitespace) {
+      return false;
+    }
+  }
+  return true;
 }
 
 function resolveEmbeddableImageUrl(url: string): string | null {
@@ -145,12 +167,17 @@ function resolveEmbeddableImageUrl(url: string): string | null {
   if (trimmed.length > MAX_WEBCHAT_IMAGE_DATA_URL_CHARS) {
     return null;
   }
-  const match = /^data:(image\/[a-z0-9.+-]+);base64,([a-z0-9+/=\s]+)$/i.exec(trimmed);
-  if (!match) {
+  const commaIndex = trimmed.indexOf(",");
+  if (commaIndex < 0) {
+    return null;
+  }
+  const metadata = trimmed.slice(0, commaIndex);
+  const match = /^data:(image\/[a-z0-9.+-]+);base64$/i.exec(metadata);
+  const base64Data = trimmed.slice(commaIndex + 1);
+  if (!match || !isBase64DataPayload(base64Data)) {
     return null;
   }
   const mediaType = normalizeLowercaseStringOrEmpty(match[1]);
-  const base64Data = match[2];
   if (!ALLOWED_WEBCHAT_DATA_IMAGE_MEDIA_TYPES.has(mediaType)) {
     return null;
   }