Email bombs exploit lax authentication in Zendesk

https://krebsonsecurity.com/2025/10/email-bombs-exploit-lax-authentication-in-zendesk/

by todsacerdoti • 9 months ago

69 17 9 months ago

dboreham• 9 months ago

Ah. This explains a bunch of odd emails I received all at the same time last week.

Volundr• 9 months ago

Yeah I got enough of these from discord, that I emailed their abuse@ and put in a support ticket, but they ignored me. Nice to have it confirmed. I ended up doing a password rotation on the off chance it was me.

whatamidoingyo• 9 months ago

Yeah, I got like 50 from bugcrowd. I figured someone found a bug somewhere, lol.

Ekaros• 9 months ago

I was kinda confused why I got one from company that really doesn't even operate here and what was the vector with it...

ianhawes• 9 months ago

Brian Krebs is a saint for being the perennial punching bag and target of cybercriminals but continuing to publish important information independently.

bombcar• 9 months ago

You know, combing "bomb" with LAX makes me think really different things for awhile until my parser finally woke up ...

fckgw• 9 months ago

If you start getting an email bombed out of nowhere, being signed up for hundreds of newsletters or other email notifications, take a quick look at your credit card statements for any unknown purchases. Email bombs are often used by card thieves to hide legitimate purchase notifcation email from retailers when they use your stolen creds.

OptionOfT• 9 months ago

Another reason to actually get your credit card statement via snail-mail.

I understand it is wasteful, but I go on an evening walk and pick up the mail.

The effort for me to pick up the mail and read my credit card statement is actually quite nice.

It doesn't require you to sign in, and search my house for my phone or my YubiKey, it doesn't prompt me for other credit card offers, doesn't require me to download a PDF reader.

mcast• 9 months ago

Better yet, setup transaction alerts on all your credit cards, and use a budgeting app like Monarch/YNAB to review all your household transactions each month or receive weekly email summaries.

YeBanKo• 9 months ago

Monarch/YNAB

Yeah, right. Let some thirds party app collect all your info in their secure cloud. Do you also give Monarch login to your bank account?

tlonny• 9 months ago

Also check airline miles haven’t gone missing.

A friend of mine recently had his BA account compromised, all his Avios stolen and he was none the wiser after receiving about 60 emails a minute

bgc• 9 months ago

Another fun Zendesk “feature,” that, to my knowledge, has never been fixed is if you CC it on a thread with any other email address that auto-replies, it will get stuck in a loop and ping-pong emails back and forth until the mailbox fills up.

vachina• 9 months ago

This attack is called email amplification. Any open form that triggers email sending is vulnerable. Fortunately these bots are pretty basic in my experience, putting a captcha (or anything unexpected) in front is enough to stop these bots.

jtokoph• 9 months ago

Unless I missed it, the article doesn’t explain how this works. It seems like the spammer sends an email to support@somecompany.com but spoofs the From address to be the target of the spam. The Zendesk ticket system then sends the auto reply to the spoofed From address

ssalmon• 9 months ago

This doesn't surprise me since Zendesk uses the same DKIM key for all customers. I have multiple domains that I support and they all point to the same CNAME record.

mobeigi• 9 months ago

This spam campaign drove me nuts! I received so many of these emails from so many random companies.

The key takeaway is to always have a email verification loop (or something stronger like phone verification) when using an anonymous user feature. You need to prove you own an email address before you use it.

Email bombs exploit lax authentication in Zendesk

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