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Category: Technical

Captchas

Captchas – those twisty distorted words you have to decipher and type in to access a website – have been around since the 1990s. Their original purpose was to tell the difference between a human user and an automated system, by requiring the user to answer a challenge – one that was supposedly hard for […]

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SPF and TXT records and Go

A few days ago Laura noticed a bug in one of our in-house tools – it was sometimes marking an email as SPF Neutral when it should have been a valid SPF pass. I got around to debugging it today and traced it back to a bug in the Go standard library. A DNS TXT […]

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DNS Flag Day

There are quite a lot of broken DNS servers out there. I’m sure that’s no surprise to you, but some of them might be yours. And you might not notice that until your domains stop working early next year. DNS is quite an old protocol, and when it was originally specified there wasn’t really a […]

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Check your abuse addresses

Even if you have excellent policies and an effective, empowered enforcement team you can still have technical problems that can cause you to drop abuse mail, and so lose the opportunity to get a bad actor off your network before they damage your reputation further. It’s not quite as simple as “We’re seeing email in […]

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The Problem With Affiliates (2)

On Friday I mentioned spam coming from a BarkBox affiliate programme. The original email is here. It’s not terribly exciting, it’s rather typical spam of the sort sent by professional spammers. It’s validly DKIM and SPF authenticated, and DMARC-aligned. It includes invisible white-on-white padding text so that it doesn’t look like image-only spam to naive […]

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Reading RFCs

We mention RFCs quite a lot, both explicitly (RFC 6376 is the specification for DKIM) and implicitly (the 822.From aka bounce address aka return path). And we have local copies of a bunch of them to make them easy to refer to (SMTP, MIME, Carrier Pigeons …). They use quite a lot of jargon and […]

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Minimal DMARC

The intent of DMARC is to cause emails to silently vanish. Ideally deploying DMARC would cause all malicious email that uses your domain in the From address, but which has absolutely nothing to with you to vanish, while still allowing all email you send, including mail that was sent through third parties or forwarded, to […]

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List the world!

We often say that a blacklist has “listed the world” when it shuts down ungracefully. What exactly does that mean, and why does it happen? Blacklists are queried by sending a DNS lookup for an A record, just the same as you’d find the address of a domain for opening a webpage there. The IP […]

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SpamCannibal is dead

The SpamCannibal blacklist – one that didn’t affect your email too much but which would panic users who found it on one of the “check all the blacklists!” websites – has gone away. It was silently abandoned by the operator at some point in the past year and the domain registration has finally expired. It’s […]

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EFAIL PGP / S/MIME "flaw" ?

There’s going to be a lot of hype today about something the security researchers who found it are calling “EFAIL”. Interviews, commemorative T-Shirts, press tours, hype. The technical details are interesting, but the un-hyped end-user advice would probably be “If you’re using a mail client that’s got bugs in it’s MIME handling, and you’ve configured it […]

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