diff --git a/components/email/email-viewer.tsx b/components/email/email-viewer.tsx
index cc179c90f..53b5d9c46 100644
--- a/components/email/email-viewer.tsx
+++ b/components/email/email-viewer.tsx
@@ -4,9 +4,9 @@ import { Fragment, useState, useEffect, useLayoutEffect, useMemo, useRef, useCal
 import { Email, ContactCard, Mailbox } from "@/lib/jmap/types";
 import { emailExportFilename, attachmentDownloadFilename, attachmentsBundleFilename, DEFAULT_EMAIL_TEMPLATE, DEFAULT_ATTACHMENT_TEMPLATE } from "@/lib/download-filename";
 import { EML_IMPORT_ACCEPT, expandImportableEmails } from "@/lib/eml-import";
-import { applyNewTabToAnchor, emailIframeCsp, escapeHtml, isExternalResourceUrl, isOpenableLinkHref, plainTextToSafeHtml, sanitizeEmailBodyForIframe, sanitizeEmailHtml, sanitizePlainTextRenderedHtml } from "@/lib/email-sanitization";
+import { TRANSPARENT_BLOCKED_PIXEL, applyNewTabToAnchor, emailIframeCsp, escapeHtml, isExternalResourceUrl, isOpenableLinkHref, plainTextToSafeHtml, sanitizeEmailBodyForIframe, sanitizeEmailHtml, sanitizePlainTextRenderedHtml } from "@/lib/email-sanitization";
 import { getRenderableHtmlBody } from "@/lib/email-body-selection";
-import { collectReferencedCids, isEmbeddedInBody, normalizeCid } from "@/lib/attachment-visibility";
+import { collectReferencedCids, isEmbeddedInBody, normalizeCid, rewriteCidReferences } from "@/lib/attachment-visibility";
 import { collapsePlainTextQuotes, setupQuoteCollapse } from "@/lib/quote-collapse";
 import { DARK_PENDING_CLASS, applyEmailDarkMode, emailColorScheme, emailDarkModeCss, emailHasNativeDarkMode as htmlHasNativeDarkMode, resolveColorSchemeMedia } from "@/lib/email-dark-mode";
 import { fitEmailBodyWidth } from "@/lib/email-fit-width";
@@ -2492,12 +2492,7 @@ export function EmailViewer({
         // Replace cid: references with authenticated blob URLs (fetched via useEffect)
         // This prevents browser auth dialogs that occur when loading raw JMAP download URLs
         if (email.attachments) {
-          htmlContent = htmlContent.replace(
-            /\bcid:([^"'\s)]+)/gi,
-            (_match, cidRef) => {
-              return cidBlobUrls[cidRef] || 'data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7';
-            }
-          );
+          htmlContent = rewriteCidReferences(htmlContent, cidBlobUrls, TRANSPARENT_BLOCKED_PIXEL);
         }
 
         // Sanitize (no dark mode color transforms - emails render true-to-life
@@ -2594,13 +2589,7 @@ export function EmailViewer({
       };
     };
     if (pluginRenderedHtml) {
-      const htmlWithCidUrls = pluginRenderedHtml.replace(
-        /\bcid:([^"'\s)]+)/gi,
-        (_match, cidRef) => {
-          return cidBlobUrls[cidRef] || 'data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7';
-        }
-      );
-      return renderHtml(htmlWithCidUrls);
+      return renderHtml(rewriteCidReferences(pluginRenderedHtml, cidBlobUrls, TRANSPARENT_BLOCKED_PIXEL));
     }
     if (pluginRenderedText) {
       return { html: plainToHtml(pluginRenderedText), isHtml: false, hasStyleTag: false, externalBlocked: false };
diff --git a/components/email/thread-conversation-view.tsx b/components/email/thread-conversation-view.tsx
index 5047813a2..4cde880b6 100644
--- a/components/email/thread-conversation-view.tsx
+++ b/components/email/thread-conversation-view.tsx
@@ -3,7 +3,7 @@
 import React, { useState, useEffect, useLayoutEffect, useMemo, useRef, useCallback } from "react";
 import DOMPurify from "dompurify";
 import { Email, ThreadGroup } from "@/lib/jmap/types";
-import { EMAIL_SANITIZE_CONFIG, blockExternalResourcesOnNode, collapseBlockedImageContainers, emailIframeCsp, isOpenableLinkHref, plainTextToSafeHtml, restrictDataUriResourcesOnNode, sanitizeEmailBodyForIframe, sanitizePlainTextRenderedHtml } from "@/lib/email-sanitization";
+import { EMAIL_SANITIZE_CONFIG, TRANSPARENT_BLOCKED_PIXEL, blockExternalResourcesOnNode, collapseBlockedImageContainers, emailIframeCsp, isOpenableLinkHref, plainTextToSafeHtml, restrictDataUriResourcesOnNode, sanitizeEmailBodyForIframe, sanitizePlainTextRenderedHtml } from "@/lib/email-sanitization";
 import { resolveExternalLink } from "@/lib/plugin-hook-result";
 import { stripBidiControls } from "@/lib/bidi";
 import { requestInternalMailto } from "@/lib/protocol-handlers/mailto";
@@ -11,7 +11,7 @@ import { emailDisplayDate } from "@/lib/email-date";
 import { getRenderableHtmlBody } from "@/lib/email-body-selection";
 import { findVerificationCode, verificationCodeBodyText } from "@/lib/verification-code";
 import { VerificationCodeChip } from "./verification-code-chip";
-import { collectReferencedCids, isEmbeddedInBody, normalizeCid } from "@/lib/attachment-visibility";
+import { collectReferencedCids, isEmbeddedInBody, normalizeCid, rewriteCidReferences } from "@/lib/attachment-visibility";
 import { collapsePlainTextQuotes, setupQuoteCollapse } from "@/lib/quote-collapse";
 import { fitEmailBodyWidth } from "@/lib/email-fit-width";
 import { DARK_PENDING_CLASS, applyEmailDarkMode, emailDarkModeCss } from "@/lib/email-dark-mode";
@@ -617,10 +617,7 @@ export function ConversationEmailBody({
     // Plugin-rendered bodies get the reader's sanitizer and remote-content
     // blocking, like any other mail body (#797).
     if (pluginBody.html) {
-      const withCids = pluginBody.html.replace(
-        /\bcid:([^"'\s)]+)/gi,
-        (_match, cidRef) => pluginBody.cidUrls[cidRef] || cidBlobUrls[cidRef] || 'data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7',
-      );
+      const withCids = rewriteCidReferences(pluginBody.html, { ...cidBlobUrls, ...pluginBody.cidUrls }, TRANSPARENT_BLOCKED_PIXEL);
       const { html, blockedExternalContent } = sanitizeEmailBodyForIframe(withCids, !allowExternal);
       if (blockedExternalContent) {
         if (!hasBlockedContent) setHasBlockedContent(true);
@@ -645,12 +642,7 @@ export function ConversationEmailBody({
         // Replace cid: references with authenticated blob URLs (fetched via useEffect)
         // This prevents browser auth dialogs that occur when loading raw JMAP download URLs
         if (email.attachments) {
-          htmlContent = htmlContent.replace(
-            /\bcid:([^"'\s)]+)/gi,
-            (_match, cidRef) => {
-              return cidBlobUrls[cidRef] || 'data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7';
-            }
-          );
+          htmlContent = rewriteCidReferences(htmlContent, cidBlobUrls, TRANSPARENT_BLOCKED_PIXEL);
         }
 
         let blockedExternalContent = false;
diff --git a/lib/__tests__/attachment-visibility.test.ts b/lib/__tests__/attachment-visibility.test.ts
index d922b180b..fc163236b 100644
--- a/lib/__tests__/attachment-visibility.test.ts
+++ b/lib/__tests__/attachment-visibility.test.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { collectReferencedCids, isEmbeddedInBody } from '@/lib/attachment-visibility';
+import { collectReferencedCids, isEmbeddedInBody, rewriteCidReferences } from '@/lib/attachment-visibility';
 
 describe('collectReferencedCids', () => {
   it('finds cid: references wherever the body uses them', () => {
@@ -72,3 +72,20 @@ describe('isEmbeddedInBody', () => {
     expect(isEmbeddedInBody({ cid: 'logoImage', type: 'application/octet-stream' }, none)).toBe(false);
   });
 });
+
+describe('rewriteCidReferences', () => {
+  it('swaps a cid: reference for its URL and an unknown one for the placeholder', () => {
+    expect(rewriteCidReferences('<img src="cid:logo"><img src="cid:gone">', { logo: 'blob:x/1' }, 'data:,p'))
+      .toBe('<img src="blob:x/1"><img src="data:,p">');
+  });
+
+  it('ends an unquoted reference at the tag instead of swallowing the markup after it', () => {
+    const html = '<img src=cid:logo><p>Hello</p>';
+    expect(rewriteCidReferences(html, { logo: 'blob:x/1' }, 'data:,p')).toBe('<img src=blob:x/1><p>Hello</p>');
+    expect(collectReferencedCids(html)).toEqual(new Set(['logo']));
+  });
+
+  it('never resolves a reference through the object prototype', () => {
+    expect(rewriteCidReferences('<img src="cid:constructor">', {}, 'data:,p')).toBe('<img src="data:,p">');
+  });
+});
diff --git a/lib/attachment-visibility.ts b/lib/attachment-visibility.ts
index 85fc8e746..bcd05c4b0 100644
--- a/lib/attachment-visibility.ts
+++ b/lib/attachment-visibility.ts
@@ -14,10 +14,21 @@
  */
 
 /**
- * The exact pattern the viewers rewrite to blob URLs, so "referenced" here
- * means "rendered" and a chip is only ever hidden for a part the body shows.
+ * The exact pattern the viewers rewrite to blob URLs (rewriteCidReferences),
+ * so "referenced" here means "rendered" and a chip is only ever hidden for a
+ * part the body shows. A reference ends at `<`/`>` too: an unquoted
+ * `<img src=cid:logo>` would otherwise take the rest of the markup with it.
  */
-const CID_REFERENCE = /\bcid:([^"'\s)]+)/gi;
+const CID_REFERENCE = /\bcid:([^"'\s)<>]+)/gi;
+
+/**
+ * Swaps every `cid:` reference in a raw HTML body for the URL fetched for that
+ * part, or `fallback` when there is none.
+ */
+export function rewriteCidReferences(html: string, urls: Readonly<Record<string, string>>, fallback: string): string {
+  return html.replace(CID_REFERENCE, (_match, cid: string) =>
+    (Object.prototype.hasOwnProperty.call(urls, cid) ? urls[cid] : fallback));
+}
 
 const EMPTY: ReadonlySet<string> = new Set();