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duolingo.com

Name: duolingo.com Email Security Posture Assessment
Creator: RacterMX

Posture Score: 80/100

Scanned May 8, 2026 at 5:17 PM UTC

Identity

Attack Surface

Reputation

Identity Findings

Weak DKIM RSA key (~1296 bits) for selector 'google'

RSA keys shorter than 2048 bits are considered weak. Current key is approximately 1296 bits.

💡 Generate a new 2048-bit (or larger) RSA key pair and update the DKIM record.

high

Weak DKIM RSA key (~1296 bits) for selector 'k1'

RSA keys shorter than 2048 bits are considered weak. Current key is approximately 1296 bits.

💡 Generate a new 2048-bit (or larger) RSA key pair and update the DKIM record.

high

No MTA-STS record found

Without MTA-STS, mail transport encryption cannot be enforced and is vulnerable to downgrade attacks.

💡 Publish a _mta-sts TXT record and host a policy file at https://mta-sts.duolingo.com/.well-known/mta-sts.txt

medium

STARTTLS without MTA-STS or DANE protection

STARTTLS alone is vulnerable to downgrade attacks. Neither MTA-STS nor DANE is configured to enforce encryption.

💡 Deploy MTA-STS and/or DANE to protect against STARTTLS stripping attacks.

medium

No CAA records found

No CAA records are configured. Any certificate authority can issue certificates for this domain.

💡 Add CAA records to restrict certificate issuance. Example: 0 issue "letsencrypt.org"

medium

No TLS-RPT record found

Without TLS-RPT, you will not receive reports about TLS negotiation failures.

💡 Publish a _smtp._tls TXT record (e.g. "v=TLSRPTv1; rua=mailto:tls-reports@duolingo.com").

low

SPF has ip4 mechanisms but no ip6

The SPF record authorizes IPv4 addresses but not IPv6. Your MX servers have AAAA records, so IPv6 mail delivery is possible without SPF authorization.

💡 Add ip6: mechanisms to the SPF record for your IPv6 mail server addresses.

low

Shared IP 173.194.76.26 used by multiple MX servers

The IP 173.194.76.26 is shared across 2 MX hostnames, reducing redundancy.

💡 Use unique IP addresses for each MX server for better redundancy.

low

Very low TTL on MX records: 272s

7 MX records have a TTL of 272s or less (below 300s).

💡 Increase the TTL to at least 300 seconds unless frequent changes are expected.

low

Very low TTL on A records: 60s

2 A records have a TTL of 60s or less (below 300s).

💡 Increase the TTL to at least 300 seconds unless frequent changes are expected.

low

Very low TTL on DMARC record: 33s

The DMARC record has a TTL of 33s or less (below 300s).

💡 Increase the TTL to at least 300 seconds unless frequent changes are expected.

low

NS TTL mismatch between parent and authoritative zone

The NS record TTL in the parent zone (~7173s) differs significantly from the authoritative zone (~172800s).

💡 Align NS record TTLs between the parent zone delegation and the authoritative zone.

low

SPF record is valid

The SPF record is properly configured with valid syntax and within lookup limits.

pass

DKIM cryptographic configuration is compliant

No RFC 8301 compliance issues detected in DKIM key configuration.

pass

No deprecated DMARCbis tags present

The DMARC record does not use any tags deprecated in DMARCbis (pct, rf, ri).

pass

BIMI record is valid

BIMI is properly configured with a valid logo and DMARC enforcement.

pass

ARC support likely available

MX server software appears to support ARC headers for authentication preservation.

pass

SMTP security checks passed

No open relay, VRFY/EXPN, or banner disclosure issues detected.

pass

SMTP RFC 5321 compliance checks passed

The mail server accepts postmaster@ and abuse@ and uses a valid FQDN in EHLO.

pass

NS delegation is properly configured

All nameservers are responsive, consistent, and properly secured.

pass

DNS responses are consistent across all nameservers

All authoritative nameservers return consistent answers with proper AA flags.

pass

DKIM2 not detected

No DKIM key record with v=2 was found across common selectors. DKIM2 is an emerging standard and not yet widely deployed.

informational

DMARC alignment: adkim=r, aspf=r

DKIM alignment is relaxed, SPF alignment is relaxed.

informational

DMARC failure reporting: fo=1

Failure reports generated when any authentication mechanism fails.

informational

DMARCbis np tag absent

The DMARC record does not include the np (non-existent subdomain policy) tag. Consider adding np=reject to protect non-existent subdomains under DMARCbis.

informational

DMARCbis psd tag absent

The DMARC record does not include the psd (public suffix domain) indicator. Consider adding psd=n to explicitly identify this as an organizational domain.

informational

Shared/free email hosting detected: aspmx.l.google.com

The MX record points to a known shared or free email hosting provider.

informational

Shared/free email hosting detected: alt1.aspmx.l.google.com

The MX record points to a known shared or free email hosting provider.

informational

Shared/free email hosting detected: alt2.aspmx.l.google.com

The MX record points to a known shared or free email hosting provider.

informational

No DANE/TLSA records found

DANE is not configured for any MX server. This is optional but provides strong certificate binding.

informational

Open mail ports on aspmx.l.google.com

Detected open mail-related ports: 25

informational

RFC 8058 one-click unsubscribe assessment

DKIM signing is configured. RFC 8058 compliance depends on List-Unsubscribe and List-Unsubscribe-Post headers being included in outbound messages and DKIM signatures.

informational

MX IP network diversity

MX IPs span 5 different /16 networks, providing good diversity.

informational

SOA record parameters

Serial: 1, Refresh: 7200s, Retry: 900s, Expire: 1209600s, Min TTL: 86400s

informational

Attack Surface Findings

Microsoft Autodiscover accessible without authentication

The Autodiscover endpoint at https://autodiscover.duolingo.com/autodiscover/autodiscover.xml returns configuration data without requiring authentication. This exposes email server details to unauthenticated users.

💡 Configure the Autodiscover endpoint to require authentication before returning configuration data.

high

DNS ANY query returns 37 record(s)

The DNS server responds to ANY queries with 37 record(s), creating significant DNS amplification risk.

💡 Configure the DNS server to refuse or minimize ANY query responses per RFC 8482.

medium

DNSSEC is not enabled

No DNSKEY or DS records were found. DNS responses are not cryptographically authenticated.

💡 Enable DNSSEC by generating DNSKEY records and publishing DS records at the registrar.

medium

Domain enumerability: low

The domain has low enumerability. Proper NSEC3 configuration and limited CT exposure reduce the attack surface.

low

No internal information leakage detected

No non-routable IP addresses or internal references were found in public DNS records.

pass

8 subdomain(s) discovered for duolingo.com

Subdomains were enumerated via Certificate Transparency logs, DNS wordlist brute-forcing, and DNS record analysis.

informational

0 certificate(s) found in CT logs

Certificate Transparency logs contain 0 certificate(s) for duolingo.com and its subdomains. Each certificate reveals subdomain names.

informational

Wildcard DNS record detected

A wildcard DNS record exists for *.duolingo.com, resolving to 32.194.42.30, 54.167.143.88. Wildcard records make subdomain brute-forcing less effective but may mask misconfigurations.

informational

Mozilla Autoconfig endpoint detected

The Autoconfig endpoint at autoconfig.duolingo.com is publicly accessible.

informational

Reputation Findings

Missing Strict-Transport-Security on duolingo.com

The Strict-Transport-Security header is not set on https://duolingo.com.

💡 Add the Strict-Transport-Security header with a max-age of at least 31536000 (1 year) and include the includeSubDomains directive.

medium

Missing Strict-Transport-Security on webmail.duolingo.com

The Strict-Transport-Security header is not set on https://webmail.duolingo.com.

💡 Add the Strict-Transport-Security header with a max-age of at least 31536000 (1 year) and include the includeSubDomains directive.

medium

security.txt present

Found security.txt at https://duolingo.com/.well-known/security.txt.

pass

TLS certificate valid for 230 days

Certificate for duolingo.com expires on 2026-12-24.

pass

Certificate issued by Amazon RSA 2048 M01

The certificate for duolingo.com is issued by a third-party CA.

pass

Signature algorithm: rsa-sha256

The certificate for duolingo.com uses an acceptable signature algorithm.

pass

RSA key: 2048 bits

The certificate for duolingo.com uses an adequate RSA key of 2048 bits.

pass

SCTs present in certificate

The certificate for duolingo.com contains Signed Certificate Timestamps.

pass

No weak ciphers detected

No known weak cipher suites detected on duolingo.com.

pass

Domain valid for 263 days

Domain duolingo.com expires on 2027-01-26.

pass

Domain transfer and delete locks enabled

Domain duolingo.com has clientTransferProhibited and clientDeleteProhibited locks.

pass

Google Safe Browsing DNS check — no flag detected

DNS-based Safe Browsing lookup returned no results. Full verification requires the Google Safe Browsing API key.

informational

Abuse contact lookup failed

Could not query abuse contact information.

informational

PhishTank — API key required

PhishTank phishing database check requires an API key.

informational

Sender Score — API key required

Sender Score reputation check requires an API key.

informational

Cisco Talos — API key required

Cisco Talos IP reputation check requires an API key.

informational

WHOIS registration data

Registrar: Amazon Registrar, Inc. | Created: 2010-01-26 | Expires: 2027-01-26 | Updated: 2025-12-22

informational

WHOIS privacy protection not detected

Domain duolingo.com does not appear to have WHOIS privacy protection. Registrant details may be publicly visible.

informational

Dependency Chains

DMARC Authentication Chain (SPF)

DMARC uses SPF to verify that emails come from authorized servers

SPF Record SPF record found and configured → DMARC Policy DMARC policy: reject

DMARC Authentication Chain (DKIM)

DMARC uses DKIM to verify that emails have not been tampered with

DKIM Signature DKIM signature missing → DMARC Policy DMARC policy: reject

⚠️ DKIM signature is missing. DMARC relies on DKIM to verify email integrity and prevent tampering. Without DKIM, DMARC alignment will fail for cryptographic verification.

Mail Server Identity: aspmx.l.google.com

PTR record proves mail server legitimacy

MX: aspmx.l.google.com Mail server at 172.253.116.27 → PTR Record Reverse DNS: dj-in-f27.1e100.net

Mail Server Identity: alt1.aspmx.l.google.com

PTR record proves mail server legitimacy

MX: alt1.aspmx.l.google.com Mail server at 173.194.76.26 → PTR Record Reverse DNS: ws-in-f26.1e100.net

Mail Server Identity: alt2.aspmx.l.google.com

PTR record proves mail server legitimacy

MX: alt2.aspmx.l.google.com Mail server at 142.250.102.26 → PTR Record Reverse DNS: rb-in-f26.1e100.net

Mail Server Identity: aspmx2.googlemail.com

PTR record proves mail server legitimacy

MX: aspmx2.googlemail.com Mail server at 173.194.76.26 → PTR Record Reverse DNS: ws-in-f26.1e100.net

Mail Server Identity: aspmx3.googlemail.com

PTR record proves mail server legitimacy

MX: aspmx3.googlemail.com Mail server at 142.250.102.27 → PTR Record Reverse DNS: rb-in-f27.1e100.net

Mail Server Identity: aspmx4.googlemail.com

PTR record proves mail server legitimacy

MX: aspmx4.googlemail.com Mail server at 192.178.213.26 → PTR Record Reverse DNS: yugrqzs-in-f26.1e100.net

Mail Server Identity: aspmx5.googlemail.com

PTR record proves mail server legitimacy

MX: aspmx5.googlemail.com Mail server at 142.251.127.27 → PTR Record Reverse DNS: lcfrai-in-f27.1e100.net

Mail Server Reputation: aspmx.l.google.com