git » chasquid » commit 71af456

diff --git a/internal/dkim/dns.go b/internal/dkim/dns.go
index 930f14b..8a2fdaa 100644
--- a/internal/dkim/dns.go
+++ b/internal/dkim/dns.go
@@ -13,6 +13,10 @@ import (
 	"strings"
 )
 
+// Maximum number of TXT records we consider when looking up the public key.
+// See findPublicKeys for details.
+const maxKeyRecords = 3
+
 func findPublicKeys(ctx context.Context, domain, selector string) ([]*publicKey, error) {
 	// Subdomain where the key lives.
 	// https://datatracker.ietf.org/doc/html/rfc6376#section-3.6.2
@@ -33,8 +37,16 @@ func findPublicKeys(ctx context.Context, domain, selector string) ([]*publicKey,
 	//  - driusan/dkim: Use the first TXT record that can be parsed as a key.
 	//  - go-msgauth: Reject if there are multiple records.
 	//
-	// What we do: use _all_ TXT records that can be parsed as keys. This is
-	// possibly too much, and we could reconsider this in the future.
+	// What we do: use the TXT records that can be parsed as keys, but only
+	// look at the first few. The records are attacker-controlled, and each
+	// key can cost a signature verification, so we need to cap them.
+	// This is still more lenient than the implementations above, and we
+	// could reconsider this in the future.
+	if len(values) > maxKeyRecords {
+		trace(ctx, "Too many TXT records for %q (%d), using the first %d",
+			d, len(values), maxKeyRecords)
+		values = values[:maxKeyRecords]
+	}
 
 	pks := []*publicKey{}
 	for _, v := range values {
diff --git a/internal/dkim/dns_test.go b/internal/dkim/dns_test.go
index 549023c..6615224 100644
--- a/internal/dkim/dns_test.go
+++ b/internal/dkim/dns_test.go
@@ -66,6 +66,34 @@ func TestSkipBadRecords(t *testing.T) {
 	}
 }
 
+func TestMaxKeyRecords(t *testing.T) {
+	ctx := WithLookupTXTFunc(t.Context(), testLookupTXT)
+	defer clear(results)
+
+	// Only the first maxKeyRecords records are used, even if the rest are
+	// valid keys.
+	key := "v=DKIM1; p=" + exampleRSAKeyB64
+	results["selector._domainkey.example.com"] = []string{
+		key, key, key, key, key,
+	}
+	pks, err := findPublicKeys(ctx, "example.com", "selector")
+	if err != nil || len(pks) != maxKeyRecords {
+		t.Errorf("findPublicKeys expected %d keys / nil, got %d / %v",
+			maxKeyRecords, len(pks), err)
+	}
+
+	// Records after the first maxKeyRecords are ignored, even if the first
+	// ones are not valid keys.
+	results["selector._domainkey.example.com"] = []string{
+		"not a tag", "not a tag", "not a tag", key,
+	}
+	pks, err = findPublicKeys(ctx, "example.com", "selector")
+	if err != nil || len(pks) != 0 {
+		t.Errorf("findPublicKeys expected 0 keys / nil, got %d / %v",
+			len(pks), err)
+	}
+}
+
 func TestParsePublicKey(t *testing.T) {
 	cases := []struct {
 		in  string