auth.dkim.weak_key

DKIM RSA Key Too Small

What we check

We read the RSA key length from the published DKIM records. RFC 8301 sets a hard floor: signers must use at least 1024-bit keys, and verifiers must not treat signatures from keys under 1024 bits as valid. We flag keys below the recommended 2048-bit standard.

What this finding means

A DKIM key on this domain is shorter than 2048 bits. Short RSA keys can be factored with modern computing resources — and a factored key lets an attacker forge valid DKIM signatures for your domain, defeating the protection entirely. Keys under 1024 bits are rejected outright by conformant verifiers.

Historically, 512-bit keys were common because their short base64 fit older DNS panels' 255-character limit. That constraint is long gone: DNS TXT records can hold a 2048-bit key across concatenated strings.

Why it matters

Forgeable signatures. A weak key undermines the whole point of DKIM. If it can be factored, an attacker signs mail as you and it passes.
Outright rejection under 1024 bits. RFC 8301 requires verifiers to reject sub-1024-bit signatures as invalid — so a too-short key can also mean your legitimate mail simply fails DKIM.
Easy to strengthen. Rotating to a 2048-bit key is a routine key-generation-and-publish step with no downside.

How to fix

Generate a new 2048-bit RSA key at your provider or MTA (Microsoft 365, Google Workspace, OpenDKIM, rspamd all support this).

Publish the new public key, ideally under a new selector so you can rotate without a gap:

selector2._domainkey.example.com.  IN  TXT  "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8A..."

Switch your signer to the new selector, confirm outgoing mail verifies, then retire the old key.

How it's graded

A DKIM RSA key below the minimum carries a 20-point deduction in the Authentication category. See Grading Methodology for the full scoring model.

Evidence example

$ dig +short TXT selector1._domainkey.example.com | tr -d '"'
v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQ...   ← 1024-bit key
Key length: 1024 bits (RFC 8301 recommends 2048)