Outlook Security Bug - RFC 791 Exploit

Question

Outlook Security Bug - RFC 791 Exploit

SteveB 0

Security bug, RFC 791 Exploit bug. Outlook has comprised my email address due to a bug. This issue results from Outlook not recognizing a hexadecimal encoded domain name in an image source URL and download images when the email is in the Junk folder, comprising my email address. This bug also downloads images from emails in any mail folder with hexadecimal encoded domain name when 'Block attachments, pictures and links from anyone not in my Safe senders and domains list' is enabled. Steps to reproduce: 1. Create HTML email with tag like attached screenshot. Enable Reading Pane. Put email into Junk folder and select email (I selected emails in Junk folder to report phishing). Images are downloaded. 2. Enable 'Block attachments...' setting. Using same encoded email, place in any email folder. Click on encoded email, images are downloaded. My email address was comprised by this bug, FIX IT!!!!

Screenshot 2026-03-10 134509

WISLabs Inc 10 Reputation points

2026-04-11T19:47:51.74+00:00
Hi @SteveB,

What you’re observing is not typically considered an RFC 791 exploit or account compromise. In Outlook, automatic image loading can still occur under certain conditions even when “block external content” policies are enabled, especially when:

The message is rendered in the Reading Pane (HTML preview engine still processes content)

The image URL is interpreted as a valid external request by the renderer

Security settings apply to attachments/links but not all embedded HTML rendering paths equally

In other words, this is usually a content rendering and privacy protection limitation, not evidence that your email account has been compromised.

Seeing images load from a message does not mean the sender gained access to your account — it only means Outlook processed an external resource referenced in the email body.

If this answers your question, please consider accepting it.

Regards,

WIS Labs Inc