all-in-one privacy solution":["Proton Unlimited ist eine All-in-One-Datenschutzlösung"],"Black Friday":["Black Friday"],"No ads. Privacy by default.":["Keine Werbung. Privatsphäre als Standard."],"People before profits":["Menschen sind uns wichtiger als Gewinne"],"Security through transparency":["Sicherheit durch Transparenz"],"The best Proton Mail ${ BLACK_FRIDAY } deals":["Die besten Proton Mail-Angebote zum ${ BLACK_FRIDAY }"],"The world’s only community- supported email service":["Der weltweit einzige von der Community unterstützte E-Mail-Dienst"]},"specialoffer:limited":{"${ hours } hour":["${ hours } Stunde","${ hours } Stunden"],"${ hoursLeft }, ${ minutesLeft } and ${ secondsLeft } left":["Nur noch ${ hoursLeft }, ${ minutesLeft } und ${ secondsLeft }"],"${ minutes } minute":["${ minutes } Minute","${ minutes } Minuten"],"${ seconds } second":["${ seconds } Sekunde","${ seconds } Sekunden"],"Limited time offer":["Zeitlich befristetes Angebot"]},"specialoffer:listitem":{"Create multiple addresses":["Erstelle mehrere Adressen"],"Hide-my-email aliases":["Hide-my-email-Aliase"],"Quickly unsubscribe from newsletters":["Newsletter schnell abbestellen"],"Use your own domain name":["Verwende deine eigene Domain"]},"specialoffer:logos":{"As featured in":["Vorgestellt in"]},"specialoffer:metadescription":{"Get an encrypted email that protects your privacy":["Nutze einen verschlüsselte E-Mail-Dienst, der deine Privatsphäre schützt"]},"specialoffer:metatitle":{"Proton Mail Black Friday Sale - Up to 40% off":["Proton Mail Black Friday-Angebot – Bis zu 40 % Rabatt"]},"specialoffer:newmetadescription":{"Get up to 40% off Proton Mail subscriptions this Black Friday. Find great deals on our secure end-to-end encrypted email plans.":["Erhalte an diesem Black Friday bis zu 40 % Rabatt auf Proton Mail-Abonnements. Mache tolle Schnäppchen bei unseren sicheren, Ende-zu-Ende-verschlüsselten E-Mail-Abonnements."]},"specialoffer:newmetatitle":{"Proton Mail Black Friday sale | Up to 40% off secure email":["Proton Mail-Black-Friday-Angebot | Bis zu 40 % Rabatt auf sichere E-Mails"]},"specialoffer:note":{"* Billed at ${ TOTAL_SUM } for the first year":["* Zum Preis von ${ TOTAL_SUM } im ersten Jahr"],"*Billed at ${ TOTAL_SUM } for the first 2 years":["*Zum Preis von ${ TOTAL_SUM } in den ersten zwei Jahren"],"30-day money-back guarantee":["30-tägige Geld-zurück-Garantie"],"Billed at ${ TOTAL_SUM } for the first 2 years":["Zum Preis von ${ TOTAL_SUM } in den ersten zwei Jahren"],"Billed at ${ TOTAL_SUM } for the first year":["Zum Preis von ${ TOTAL_SUM } im ersten Jahr"],"You save ${ SAVE_SUM }":["Du sparst ${ SAVE_SUM }"]},"specialoffer:off":{"${ DISCOUNT } off":["− ${ DISCOUNT }"],"${ PERCENT_OFF } off":["− ${ PERCENT_OFF }"]},"specialoffer:testimonial":{"I love my ProtonMail":["Ich liebe mein ProtonMail"],"My favorite email service":["Mein Lieblings-E-Mail-Dienst"],"Thanks Proton for keeping us all safe in the complicated internet universe.":["Danke Proton, dass du uns alle im komplizierten Internet-Universum beschützt."],"You get what you pay for. In the case of big tech, if you pay nothing, you get used. I quit using Gmail and switched to @ProtonMail":["Du bekommst, wofür du bezahlst. Wenn du nichts bezahlst, wirst du bei der Nutzung von Big Tech ausgenutzt. Ich habe Gmail aufgegeben und bin zu @ProtonMail gewechselt"]},"specialoffer:time":{"Days":["Tage"],"Hours":["Stunden"],"Min":["Min."]},"specialoffer:title":{"And much more":["Und vieles mehr"],"Make your inbox yours":["Übernimm volle Kontrolle über deinen Posteingang"],"Safe from trackers":["Sicher vor Trackern"],"Stay organized":["Sorge für Ordnung"],"Black Friday email deals":["E-Mail-Angebote zum Black Friday"],"Don’t just take our word for it":["Verlasse dich nicht nur auf unser Wort"],"Our story":["Unsere Geschichte"],"Transfer your data from Google in one click":["Übertrage deine Daten von Google mit einem Klick"]},"specialoffer:tooltip":{"Access blocked content and browse privately. Includes ${ TOTAL_SERVERS }+ servers in ${ TOTAL_COUNTRIES }+ countries, connect up to 10 devices, access worldwide streaming services, malware and ad-blocker, and more.":["Greife auf blockierte Inhalte zu und surfe privat. Umfasst ${ TOTAL_SERVERS } Server in über ${ TOTAL_COUNTRIES } Ländern, die Verbindung \nvon bis zu 10 Geräten, weltweite Streaming-Dienste, Malware- und Werbeblocker und mehr."],"Easily share your calendar with your family, friends or colleagues, and view external calendars.":["Teile deinen Kalender ganz einfach mit Verwandten, Freunden oder Kollegen und rufe externe Kalender auf."],"Includes support for 1 custom email domain, 10 email addresses, 10 hide-my-email aliases, calendar sharing, and more.":["Beinhaltet Unterstützung für eine eigene E-Mail-Domain, 10 E-Mail-Adressen, 10 „hide-my-email“-Aliasse, Kalenderfreigabe und mehr."],"Includes support for 3 custom email domains, 15 email addresses, unlimited hide-my-email aliases, calendar sharing, and more.":["Beinhaltet Unterstützung für 3 benutzerdefinierte E-Mail-Domänen, 15 E-Mail-Adressen, unbegrenzte „hide-my-email“-Aliase, Kalenderfreigabe und mehr."],"Manage up to 25 calendars, mobile apps, secured with end-to-end encryption, 1-click calendar import from Google, and more.":["Verwalte bis zu 25 Kalender, Ende zu Ende verschlüsselte mobile Apps, 1-Klick-Kalenderimporte von Google und vieles mehr."]},"Status Banner":{"At the moment we are experiencing issues with the Proton VPN service":["Im Moment gibt es Probleme mit dem Proton VPN-Dienst"],"Learn more":["Mehr erfahren"]},"Status banner":{"Learn more":["Mehr erfahren"],"Please note that at the moment we are experiencing issues with the ${ issues[0] } service.":["Bitte beachte, dass wir im Moment Probleme mit dem Dienst ${ issues[0] } haben."],"We are experiencing issues with one or more services at the moment.":["Im Moment gibt es Probleme mit einem oder mehreren Diensten."]},"suggestions":{"Suggestions":["Vorschläge"]},"Support":{"Sub category":["Unterkategorie","Unterkategorien"]},"Support article":{"${ readingTime } min":["${ readingTime } Min.","${ readingTime } Min."],"Category":["Kategorie","Kategorien"],"Didn’t find what you were looking for?":["Hast du nicht gefunden, wonach du gesucht hast?"],"General contact":["Allgemeiner Kontakt"],"Get help":["Hilfe erhalten"],"Legal contact":["Kontakt für Rechtliches"],"Media contact":["Kontakt für Medien"],"Partnerships contact":["Kontakt für Partnerschaften"],"Reading":["Lesen"]},"Support categories":{"Browse Proton product support":["Produkt-Support von Proton durchsuchen"]},"Support category":{"There is no article in this category yet.":["In dieser Kategorie gibt es noch keinen Artikel."]},"Support troubleshooting":{"--- Select ---":["--- Auswählen ---"],"App version":["App-Version"],"Browser":["Browser"],"Check if this helps":["Schau, ob das hilft."],"Choose a category for your question":["Wähle eine Kategorie für deine Frage aus."],"Did this solve your issue ?":["Hat dies dein Problem gelöst?"],"Faster assistance is just a few clicks away — please make your selections":["Schnellere Hilfe ist nur ein paar Klicks entfernt. Bitte triff deine Auswahl"],"No, contact support":["Nein, Support kontaktieren"],"Thank you for your feedback":["Danke für dein Feedback"],"Troubleshooting":["Fehlerbehebung"],"What can we help with ?":["Wie können wir dir helfen?"],"Yes":["Ja"]},"support_modal_search_query":{"Search query":["Anfrage suchen"]},"support_search_button":{"Search":["Suchen"]},"support_search_i_am_looking_for":{"I'm looking for":["Ich suche"]},"SupportForm":{"For a faster resolution, please report the issue from the Bridge app: Help > Report a problem.":["Damit das Problem schneller gelöst werden kann, melde es bitte über die Bridge-App: Help > Report a problem (Hilfe > Problem melden)."],"Information":["Informationen"]},"SupportForm:option":{"Account Security":["Kontosicherheit"],"Contacts":["Kontakte"],"Custom email domain":["Benutzerdefinierte E-Mail-Domain"],"Email delivery and Spam":["E-Mail-Zustellung und Spam"],"Encryption":["Verschlüsselung"],"Login and password":["Anmeldung und Passwort"],"Merge aliases and accounts":["Zusammenführung von Aliassen und Konten"],"Migrate to Proton":["Migration zu Proton"],"Notifications":["Benachrichtigungen"],"Other":["Sonstiges"],"Plans and billing":["Abonnements und Abrechnung"],"Proton for Business":["Proton for Business"],"Sign up":["Registrierung"],"Storage":["Speicher"],"Users, addresses, and identities":["Benutzer, Adressen und Identitäten"]},"SupportForm:optionIntro":{"Select a topic":["Thema auswählen"]},"swiss_baseed_feature":{"Swiss based":["In der Schweiz ansässig"]},"Testimonial":{"Awards":["Auszeichnungen"],"Customers":["Kunden"],"Featured":["Empfohlen"],"Go to testimonial source":["Zur Referenzquelle wechseln"],"Reviews":["Bewertungen"],"Videos":["Videos"]},"Text":{"Find the plan that's right for you":["Finde das passende Abonnement"],"If you need help, check out our ${ supportLink }.":["Hilfe erhältst du in unserem ${ supportLink }."],"The page you’re looking for might have been removed, or it could be an\nold link.":["Die von dir gesuchte Seite wurde möglicherweise entfernt, oder es könnte sich um einen alten Link handeln."]},"Title":{"On this page":["Auf dieser Seite"],"Related articles":["Verwandte Artikel"],"Share ${ thisPage }":["${ thisPage } teilen"],"Thank you!":["Vielen Dank!"],"this page":["diese Seite"]},"Tooltip":{"More information":["Weitere Informationen"]},"tooltip_calendar":{"Create up to 20 custom & shareable encrypted calendars. On top of that, add up to 5 calendars from friends, family, colleagues, and organizations.":["Erstelle bis zu 20 benutzerdefinierte und teilbare verschlüsselte Kalender. Darüber hinaus kannst du bis zu 5 Kalender von Freunden, Familie, Kollegen und Organisationen hinzufügen."]},"tooltip_vpn":{"Access blocked content and browse privately. Includes ${ TOTAL_SERVERS }+ servers in ${ TOTAL_COUNTRIES }+ countries, highest VPN speed, ${ TOTAL_VPN_CONNECTIONS } VPN connections, worldwide streaming services, malware and ad-blocker, and more.":["Greife auf blockierte Inhalte zu und surfe privat. Enthält über ${ TOTAL_SERVERS } Server in mehr als ${ TOTAL_COUNTRIES } Ländern, höchste VPN-Geschwindigkeiten, ${ TOTAL_VPN_CONNECTIONS } VPN-Verbindungen, weltweite Streaming-Dienste, Malware- und Werbeblocker und mehr."],"Access blocked content and browse privately. Includes ${ TOTAL_SERVERS }+ servers in ${ TOTAL_COUNTRIES }+ countries, highest VPN speed, 10 VPN connections, worldwide streaming services, malware and ad-blocker, and more.":["Greife auf blockierte Inhalte zu und surfe privat. Enthält über ${ TOTAL_SERVERS } Server in mehr als ${ TOTAL_COUNTRIES } Ländern, höchste VPN-Geschwindigkeiten, 10 VPN-Verbindungen, weltweite Streaming-Dienste, Malware- und Werbeblocker und mehr."]},"version_history_label":{"Version history":["Versionsverlauf"]},"version_history_tooltip":{"Store up to ${ versionHistoryNumber } versions of each file for up to ${ years } years":["Speichere bis zu ${ versionHistoryNumber } Versionen jeder Datei für einen Zeitraum von bis zu ${ years } Jahren"]},"vpn_features_link":{"View VPN plans":["VPN-Abonnements anzeigen"]},"vpn_features_useCase":{"Access blocked content and browse privately":["Greife auf blockierte Inhalte zu und surfe privat"]}}},"unleashApi":"https://account.proton.me/api"};
// We need to import data (the framework context) from the server
// This Astro feature with define:vars works well but creates an inline script
// So we can't directly import the initFramework helper, we have to store the context
window.frameworkContext = frameworkContext;
})();
On 1 December 2021, we began receiving sporadic reports of delivery failures from proton.me addresses to Gmail. This corresponded with a dramatic decline in proton.me’s domain reputation as seen via Gmail Postmaster Tools and an increase in sending from known bad IP addresses.
It was clear both from the bad sending IPs (mostly in Russia) and our own metrics that the spam emails damaging Proton Mail’s domain reputation were not coming from our servers. However, the Postmaster Tools indicated that all emails being received by Gmail from proton.me were “fully authenticated”, including the fraudulent ones.
This, in turn, caused the fraudulent emails to feed into Google’s algorithm for determining domain reputation and lowered it enough that the deliverability of legitimate emails from our servers was affected as well.
We suspected a DKIM(new window) replay attack, where a single spam email originally sent from Proton Mail was being resent to many Gmail users in an attempt to exploit our deliverability and reputation to get around Google’s anti-spam measures. At one point, roughly 98% of the emails Gmail received that claimed to be from Proton Mail were actually spam, meaning the spammers were sending an amount of emails that was equivalent to 50 times our normal outgoing traffic to Google.
We immediately rotated our DKIM signing key to (temporarily) prevent the emails from passing DKIM and contacted Google’s counter-abuse team, who quickly implemented a fix to Gmail’s spam filters and restored legitimate email delivery.
Emails sent from proton.me, proton.me/mail, and custom domains were not affected by this issue.
How DKIM replay attacks work
Before we can explain how this happened, we first need to outline how emails are structured, delivered, and authenticated on the internet.
Emails are MIME (Multipurpose Internet Mail Extensions) messages, consisting of headers(new window) and sections that contain the message body and possibly attachments. The headers contain some fields that will be familiar to any email user (To, CC, From, Subject) but also hidden information that is used to authenticate the email.
However, none of the headers are actually used to route the email to its final destination. The recipient and sender of the email are specified separately as part of the email envelope, which is a very suitable metaphor. If the email message, including its headers, is equivalent to a paper letter, then the email envelope in which the “letter” is placed contains the recipient and return addresses, like a real envelope.
The key point is that the recipient on the envelope does not have to match any recipient in the To or CC headers — typical examples of this are emails sent via BCC or addressed to “undisclosed recipients”.
Perhaps even more surprisingly, the envelope sender does not have to match the email From header. This also has a legitimate purpose — mass mailings often specify a different “return address” than the From header to analyze deliverability problems or use third-party services to send emails. This flexibility is also important for enabling users to forward messages from one mailbox to another.
These return addresses are authenticated via SPF (Sender Policy Framework)(new window), which authorizes the sending of messages by specific servers or IP addresses using special DNS records. But this only validates the server sending the email; it does not ensure that the email’s content has not been tampered with. For that, we need DKIM (Domain Keys Identified Mail)(new window).
DKIM also uses special DNS records, but rather than a list of IP addresses, these records contain keys used to sign the email content and certain associated headers. The resulting cryptographic signature is appended to the message as a special header, and the recipient mail server or client can verify this signature against the email content on delivery. But the DKIM domain in the signature can also be different from the one in the From header in the message itself. If the DKIM signature is verified, this only confirms that the message went through the signing domain’s mail servers and has not been modified since, not that the message originated from where it claims.
To actually verify that the domain in the From header sent the message, we need DMARC (Domain-based Message Authentication, Reporting, and Conformance)(new window). To pass DMARC, an email must pass either SPF or DKIM, and the domain in the From header must be “aligned” with the corresponding SPF or DKIM domain. The From header is what the user ultimately sees, so DMARC is a critical part of ensuring that an email originated from where it claims it did as it is the only policy that connects the From header to either the sending or signing domain.
Now let’s return to the attack. The reason that DKIM replay attacks work and why Gmail considered those replayed spam mails “authenticated” comes down to the fact that DMARC requires DKIM or SPF alignment, but not both. The replayed message itself had a valid DKIM signature from proton.me, which meant it passed DMARC. This message was then sent enough times that it influenced proton.me’s domain reputation in Gmail’s system, eventually becoming low enough to affect deliverability for legitimate email.
How we prevent replay attacks
The fact that DMARC passes if either the DKIM domain or the SPF domain align with the From domain is a feature of the specification, not a bug. In particular, it enables email forwarding and allows email sending by trusted third parties. However, for email service providers, such as Proton, Gmail, or Yahoo, it also enables these kinds of replay attacks, as any user can send an email, get it signed by the corresponding domain, then resend it with the signature intact.
This is one reason that Proton and other email providers invest heavily in their own anti-spam technology and systems. These systems are complex and often rely on complicated heuristics to separate spam from legitimate email. In this case, the attackers found a vulnerability in Gmail’s anti-spam system and were able to exploit it.
We appreciate Google’s responsiveness in addressing the issue.
How you can prevent domain impersonation on your domain
DKIM replay attacks are an issue primarily for email service providers or other organizations that offer email addresses on a shared domain. However, email authentication attacks in general are a risk for any organization. Here are some tips to help you ensure no one else can impersonate your domain or use it to send fraudulent messages.
Set up SPF, DKIM, and DMARC – Although imperfect, these policies are key to ensuring that your emails are delivered and resistant to being spoofed(new window). If you use Proton to host your domain, our domain setup wizard will explain how to set it up and protect it from being spoofed.
Rotate your DKIM keys regularly – Rotating our DKIM keys allowed us to quickly stop the attack and buy time for the permanent solution. Although tedious and risky to do manually, Proton’s DKIM key management system(new window) allowed us to easily do this in minutes, and this system is the same system used for all domains hosted at Proton. The system also automatically rotates keys regularly to reduce the risk of key compromise.
Oversign From, To, and CC headers – Most DKIM implementations always sign the From, To, and CC headers if they are present in an email, preventing them from being modified if the message is resent. However, if these headers are missing, they are often unsigned, opening the door to replay attacks with forged headers that make the fraudulent emails seem legitimate. Oversigning mitigates these attacks by signing these sensitive headers in all cases, even if they are blank. If you use Proton to send your email, this oversigning is done for you automatically by our mail servers.
Be careful of subdomains – If you use CNAME records to delegate parts of your website to third parties, you may be also allowing these third parties to send email on behalf of your main domain. This is because, by default, DMARC considers domains aligned if they have a parent-child relationship — that is, sub.example.com aligns with example.com. You can force exact match alignment for SPF, DKIM, or both with the aspf and adkim options in your DMARC policy. However, be aware that this can affect third-party mailing service integrations.
A lot happens after you click Send in Proton Mail
If we’re doing everything correctly on our end, email delivery seems as easy as a single click, but the truth is that it relies on a complex web of interlocking, interdependent policies. Events like this replay attack demonstrate just how complicated it can be to verify something as seemingly simple as “Who sent this email?”
At Proton Mail, we are constantly investigating new protocols and policies to ensure that the millions of emails sent using our platform every day are delivered reliably and securely. We also continuously monitor incoming mail to ensure our authentication checking has been optimized and we have systems and analysts in place 24/7 to mitigate spam and phishing attacks.
This is a vital part of creating an internet where privacy is the default, and we could not do it without the support of the Proton community. Thank you.
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