diff --git a/signer/local/local_test.go b/signer/local/local_test.go
index 1cb36fba1..914763689 100644
--- a/signer/local/local_test.go
+++ b/signer/local/local_test.go
@@ -1636,3 +1636,307 @@ func TestLint(t *testing.T) {
 		})
 	}
 }
+
+// createCSRWithExtensions generates a CSR containing the given raw X.509v3
+// extensions. This simulates an attacker injecting extensions into a CSR.
+func createCSRWithExtensions(t *testing.T, extraExts []pkix.Extension) []byte {
+	t.Helper()
+	key, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		t.Fatalf("generating key: %v", err)
+	}
+
+	tmpl := &x509.CertificateRequest{
+		Subject:         pkix.Name{CommonName: "test.example.com"},
+		DNSNames:        []string{"test.example.com"},
+		ExtraExtensions: extraExts,
+	}
+
+	csrDER, err := x509.CreateCertificateRequest(rand.Reader, tmpl, key)
+	if err != nil {
+		t.Fatalf("creating CSR: %v", err)
+	}
+
+	return pem.EncodeToMemory(&pem.Block{Type: "CERTIFICATE REQUEST", Bytes: csrDER})
+}
+
+// marshalKeyUsage encodes a KeyUsage bitstring into the DER value expected
+// inside an X.509v3 KeyUsage extension (OID 2.5.29.15).
+func marshalKeyUsage(t *testing.T, ku x509.KeyUsage) []byte {
+	t.Helper()
+	// KeyUsage is a bit string where bits are numbered from the MSB.
+	// Go's x509.KeyUsage constants are already bit positions from MSB.
+	var b [2]byte
+	b[0] = byte(ku >> 8) // not used for standard usages below bit 8
+	// Standard key usages fit in one byte for the common ones:
+	// digitalSignature(0), contentCommitment(1), keyEncipherment(2),
+	// dataEncipherment(3), keyAgreement(4), keyCertSign(5), cRLSign(6),
+	// encipherOnly(7)
+	b[0] = 0
+	b[1] = 0
+	if ku&x509.KeyUsageDigitalSignature != 0 {
+		b[0] |= 0x80
+	}
+	if ku&x509.KeyUsageContentCommitment != 0 {
+		b[0] |= 0x40
+	}
+	if ku&x509.KeyUsageKeyEncipherment != 0 {
+		b[0] |= 0x20
+	}
+	if ku&x509.KeyUsageDataEncipherment != 0 {
+		b[0] |= 0x10
+	}
+	if ku&x509.KeyUsageKeyAgreement != 0 {
+		b[0] |= 0x08
+	}
+	if ku&x509.KeyUsageCertSign != 0 {
+		b[0] |= 0x04
+	}
+	if ku&x509.KeyUsageCRLSign != 0 {
+		b[0] |= 0x02
+	}
+	if ku&x509.KeyUsageEncipherOnly != 0 {
+		b[0] |= 0x01
+	}
+	if ku&x509.KeyUsageDecipherOnly != 0 {
+		b[1] |= 0x80
+	}
+
+	// Determine padding bits: find the lowest set bit
+	var usedBytes int
+	var padBits int
+	if b[1] != 0 {
+		usedBytes = 2
+		for i := 0; i < 8; i++ {
+			if b[1]&(1<<uint(i)) != 0 {
+				padBits = i
+				break
+			}
+		}
+	} else {
+		usedBytes = 1
+		for i := 0; i < 8; i++ {
+			if b[0]&(1<<uint(i)) != 0 {
+				padBits = i
+				break
+			}
+		}
+	}
+
+	bs := asn1.BitString{
+		Bytes:     b[:usedBytes],
+		BitLength: usedBytes*8 - padBits,
+	}
+	val, err := asn1.Marshal(bs)
+	if err != nil {
+		t.Fatalf("marshaling KeyUsage: %v", err)
+	}
+	return val
+}
+
+// marshalExtKeyUsage encodes ExtKeyUsage OIDs into the DER value expected
+// inside an X.509v3 ExtKeyUsage extension (OID 2.5.29.37).
+func marshalExtKeyUsage(t *testing.T, ekus []asn1.ObjectIdentifier) []byte {
+	t.Helper()
+	val, err := asn1.Marshal(ekus)
+	if err != nil {
+		t.Fatalf("marshaling ExtKeyUsage: %v", err)
+	}
+	return val
+}
+
+// TestCopyExtensionsDoesNotOverrideKeyUsage verifies that when
+// copy_extensions is enabled, a CSR containing injected KeyUsage and
+// ExtKeyUsage extensions cannot override the signing profile's intended
+// key usage values. This is a regression test for a vulnerability where
+// Go's x509.CreateCertificate gave ExtraExtensions precedence over
+// struct fields, allowing CSR-provided KU/EKU to bypass profile restrictions.
+func TestCopyExtensionsDoesNotOverrideKeyUsage(t *testing.T) {
+	// Profile allows only: digitalSignature + serverAuth
+	profilePolicy := &config.Signing{
+		Default: &config.SigningProfile{
+			Usage:          []string{"digital signature", "server auth"},
+			Expiry:         1 * time.Hour,
+			ExpiryString:   "1h",
+			CopyExtensions: true,
+		},
+	}
+
+	s, err := NewSignerFromFile(testCaFile, testCaKeyFile, profilePolicy)
+	if err != nil {
+		t.Fatalf("creating signer: %v", err)
+	}
+
+	// Attacker CSR injects keyCertSign + cRLSign and codeSigning + emailProtection
+	attackerKU := x509.KeyUsageDigitalSignature | x509.KeyUsageCertSign | x509.KeyUsageCRLSign
+	attackerEKUs := []asn1.ObjectIdentifier{
+		{1, 3, 6, 1, 5, 5, 7, 3, 3}, // codeSigning
+		{1, 3, 6, 1, 5, 5, 7, 3, 4}, // emailProtection
+	}
+
+	csrPEM := createCSRWithExtensions(t, []pkix.Extension{
+		{
+			Id:       asn1.ObjectIdentifier{2, 5, 29, 15}, // KeyUsage
+			Critical: true,
+			Value:    marshalKeyUsage(t, attackerKU),
+		},
+		{
+			Id:       asn1.ObjectIdentifier{2, 5, 29, 37}, // ExtKeyUsage
+			Critical: false,
+			Value:    marshalExtKeyUsage(t, attackerEKUs),
+		},
+	})
+
+	certPEM, err := s.Sign(signer.SignRequest{
+		Hosts:   []string{"test.example.com"},
+		Request: string(csrPEM),
+	})
+	if err != nil {
+		t.Fatalf("signing: %v", err)
+	}
+
+	cert, err := helpers.ParseCertificatePEM(certPEM)
+	if err != nil {
+		t.Fatalf("parsing signed cert: %v", err)
+	}
+
+	// The cert MUST have only the profile's KeyUsage (digitalSignature),
+	// NOT the attacker's keyCertSign or cRLSign.
+	if cert.KeyUsage&x509.KeyUsageCertSign != 0 {
+		t.Errorf("certificate has keyCertSign — attacker's KeyUsage was not filtered")
+	}
+	if cert.KeyUsage&x509.KeyUsageCRLSign != 0 {
+		t.Errorf("certificate has cRLSign — attacker's KeyUsage was not filtered")
+	}
+	if cert.KeyUsage&x509.KeyUsageDigitalSignature == 0 {
+		t.Errorf("certificate missing digitalSignature from profile")
+	}
+
+	// The cert MUST have only the profile's EKU (serverAuth),
+	// NOT the attacker's codeSigning or emailProtection.
+	for _, eku := range cert.ExtKeyUsage {
+		switch eku {
+		case x509.ExtKeyUsageCodeSigning:
+			t.Errorf("certificate has codeSigning EKU — attacker's ExtKeyUsage was not filtered")
+		case x509.ExtKeyUsageEmailProtection:
+			t.Errorf("certificate has emailProtection EKU — attacker's ExtKeyUsage was not filtered")
+		}
+	}
+
+	foundServerAuth := false
+	for _, eku := range cert.ExtKeyUsage {
+		if eku == x509.ExtKeyUsageServerAuth {
+			foundServerAuth = true
+			break
+		}
+	}
+	if !foundServerAuth {
+		t.Errorf("certificate missing serverAuth EKU from profile")
+	}
+}
+
+// TestCopyExtensionsAllowsNonManagedOIDs verifies that CopyExtensions
+// still copies through non-security-critical, non-CA-managed extensions
+// (e.g., private-use OIDs). This ensures the blocklist does not break
+// legitimate use cases for copy_extensions.
+func TestCopyExtensionsAllowsNonManagedOIDs(t *testing.T) {
+	profilePolicy := &config.Signing{
+		Default: &config.SigningProfile{
+			Usage:          []string{"digital signature", "server auth"},
+			Expiry:         1 * time.Hour,
+			ExpiryString:   "1h",
+			CopyExtensions: true,
+		},
+	}
+
+	s, err := NewSignerFromFile(testCaFile, testCaKeyFile, profilePolicy)
+	if err != nil {
+		t.Fatalf("creating signer: %v", err)
+	}
+
+	// A private-use extension that should be copied through
+	customOID := asn1.ObjectIdentifier{1, 2, 3, 4, 5, 6, 7}
+	customValue, _ := asn1.Marshal("custom-extension-value")
+
+	csrPEM := createCSRWithExtensions(t, []pkix.Extension{
+		{
+			Id:    customOID,
+			Value: customValue,
+		},
+	})
+
+	certPEM, err := s.Sign(signer.SignRequest{
+		Hosts:   []string{"test.example.com"},
+		Request: string(csrPEM),
+	})
+	if err != nil {
+		t.Fatalf("signing: %v", err)
+	}
+
+	cert, err := helpers.ParseCertificatePEM(certPEM)
+	if err != nil {
+		t.Fatalf("parsing signed cert: %v", err)
+	}
+
+	found := false
+	for _, ext := range cert.Extensions {
+		if ext.Id.Equal(customOID) {
+			found = true
+			if !bytes.Equal(ext.Value, customValue) {
+				t.Errorf("custom extension value mismatch: got %x, want %x", ext.Value, customValue)
+			}
+			break
+		}
+	}
+	if !found {
+		t.Errorf("custom extension (OID %s) was not copied into the certificate", customOID)
+	}
+}
+
+// TestCopyExtensionsDisabledDoesNotCopy verifies that when copy_extensions
+// is false (the default), CSR extensions are NOT copied — whether they are
+// CA-managed or not.
+func TestCopyExtensionsDisabledDoesNotCopy(t *testing.T) {
+	profilePolicy := &config.Signing{
+		Default: &config.SigningProfile{
+			Usage:          []string{"digital signature", "server auth"},
+			Expiry:         1 * time.Hour,
+			ExpiryString:   "1h",
+			CopyExtensions: false,
+		},
+	}
+
+	s, err := NewSignerFromFile(testCaFile, testCaKeyFile, profilePolicy)
+	if err != nil {
+		t.Fatalf("creating signer: %v", err)
+	}
+
+	customOID := asn1.ObjectIdentifier{1, 2, 3, 4, 5, 6, 7}
+	customValue, _ := asn1.Marshal("should-not-appear")
+
+	csrPEM := createCSRWithExtensions(t, []pkix.Extension{
+		{
+			Id:    customOID,
+			Value: customValue,
+		},
+	})
+
+	certPEM, err := s.Sign(signer.SignRequest{
+		Hosts:   []string{"test.example.com"},
+		Request: string(csrPEM),
+	})
+	if err != nil {
+		t.Fatalf("signing: %v", err)
+	}
+
+	cert, err := helpers.ParseCertificatePEM(certPEM)
+	if err != nil {
+		t.Fatalf("parsing signed cert: %v", err)
+	}
+
+	for _, ext := range cert.Extensions {
+		if ext.Id.Equal(customOID) {
+			t.Errorf("custom extension (OID %s) should NOT be in cert when CopyExtensions is false", customOID)
+		}
+	}
+}
diff --git a/signer/signer.go b/signer/signer.go
index d5b1f96f0..cfffba1b0 100644
--- a/signer/signer.go
+++ b/signer/signer.go
@@ -23,6 +23,7 @@ import (
 	cferr "github.com/cloudflare/cfssl/errors"
 	"github.com/cloudflare/cfssl/helpers"
 	"github.com/cloudflare/cfssl/info"
+	"github.com/cloudflare/cfssl/log"
 )
 
 // Subject contains the information that should be used to override the
@@ -180,6 +181,47 @@ func isCommonAttr(t []int) bool {
 	return (len(t) == 4 && t[0] == 2 && t[1] == 5 && t[2] == 4 && (t[3] == 3 || (t[3] >= 5 && t[3] <= 11) || t[3] == 17))
 }
 
+// caMangedExtensionOIDs is the set of X.509v3 extension OIDs whose values are
+// authoritatively determined by the CA's signing profile (via FillTemplate) and
+// must never be copied from a CSR. Allowing a CSR to supply these via
+// ExtraExtensions would silently override the profile because Go's
+// x509.CreateCertificate gives ExtraExtensions precedence over struct fields
+// for the same OID.
+//
+// See: https://pkg.go.dev/crypto/x509#Certificate (ExtraExtensions field).
+var caManagedExtensionOIDs = map[string]bool{
+	// Key Usage (RFC 5280, 4.2.1.3) — set by profile.Usages()
+	asn1.ObjectIdentifier{2, 5, 29, 15}.String(): true,
+	// Extended Key Usage (RFC 5280, 4.2.1.12) — set by profile.Usages()
+	asn1.ObjectIdentifier{2, 5, 29, 37}.String(): true,
+	// Basic Constraints (RFC 5280, 4.2.1.9) — already handled specially above,
+	// but included for defense-in-depth
+	asn1.ObjectIdentifier{2, 5, 29, 19}.String(): true,
+	// Subject Key Identifier (RFC 5280, 4.2.1.2) — computed by FillTemplate
+	asn1.ObjectIdentifier{2, 5, 29, 14}.String(): true,
+	// Authority Key Identifier (RFC 5280, 4.2.1.1) — set by CreateCertificate
+	// from the issuer
+	asn1.ObjectIdentifier{2, 5, 29, 35}.String(): true,
+	// Authority Info Access / OCSP (RFC 5280, 4.2.2.1) — set by profile OCSP URL
+	asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 1, 1}.String(): true,
+	// CRL Distribution Points (RFC 5280, 4.2.1.13) — set by profile CRL URL
+	asn1.ObjectIdentifier{2, 5, 29, 31}.String(): true,
+	// Certificate Policies (RFC 5280, 4.2.1.4) — set by profile Policies
+	asn1.ObjectIdentifier{2, 5, 29, 32}.String(): true,
+	// Name Constraints (RFC 5280, 4.2.1.10) — CA-controlled
+	asn1.ObjectIdentifier{2, 5, 29, 30}.String(): true,
+	// Subject Alternative Name (RFC 5280, 4.2.1.6) — handled via Hosts/SANs
+	asn1.ObjectIdentifier{2, 5, 29, 17}.String(): true,
+	// Issuer Alternative Name (RFC 5280, 4.2.1.7) — CA-controlled
+	asn1.ObjectIdentifier{2, 5, 29, 18}.String(): true,
+}
+
+// isCAManagedExtension reports whether the given OID is for an extension whose
+// value is authoritatively set by the CA and must not be copied from a CSR.
+func isCaManagedExtension(oid asn1.ObjectIdentifier) bool {
+	return caManagedExtensionOIDs[oid.String()]
+}
+
 // ParseCertificateRequest takes an incoming certificate request and
 // builds a certificate template from it.
 func ParseCertificateRequest(s Signer, p *config.SigningProfile, csrBytes []byte) (template *x509.Certificate, err error) {
@@ -247,8 +289,16 @@ func ParseCertificateRequest(s Signer, p *config.SigningProfile, csrBytes []byte
 		} else if val.Id.Equal(helpers.DelegationUsage) {
 			template.ExtraExtensions = append(template.ExtraExtensions, val)
 		} else {
-			// If the profile has 'copy_extensions' to true then lets add it
+			// If the profile has 'copy_extensions' to true then copy the
+			// extension, but never copy CA-managed extensions (KeyUsage,
+			// ExtKeyUsage, SKI, AKI, etc.) whose values are set by the
+			// signing profile. Allowing them through would let a CSR
+			// override the CA's policy via ExtraExtensions precedence.
 			if p.CopyExtensions {
+				if isCaManagedExtension(val.Id) {
+					log.Warningf("copy_extensions: skipping CA-managed extension OID %s from CSR", val.Id)
+					continue
+				}
 				template.ExtraExtensions = append(template.ExtraExtensions, val)
 			}
 		}
diff --git a/signer/signer_test.go b/signer/signer_test.go
index bab6ef745..acdea1ee2 100644
--- a/signer/signer_test.go
+++ b/signer/signer_test.go
@@ -107,6 +107,38 @@ func TestAddPoliciesWithQualifiers(t *testing.T) {
 	}
 }
 
+func TestIsCaManagedExtension(t *testing.T) {
+	tests := []struct {
+		name string
+		oid  asn1.ObjectIdentifier
+		want bool
+	}{
+		{"KeyUsage", asn1.ObjectIdentifier{2, 5, 29, 15}, true},
+		{"ExtKeyUsage", asn1.ObjectIdentifier{2, 5, 29, 37}, true},
+		{"BasicConstraints", asn1.ObjectIdentifier{2, 5, 29, 19}, true},
+		{"SubjectKeyIdentifier", asn1.ObjectIdentifier{2, 5, 29, 14}, true},
+		{"AuthorityKeyIdentifier", asn1.ObjectIdentifier{2, 5, 29, 35}, true},
+		{"AuthorityInfoAccess", asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 1, 1}, true},
+		{"CRLDistributionPoints", asn1.ObjectIdentifier{2, 5, 29, 31}, true},
+		{"CertificatePolicies", asn1.ObjectIdentifier{2, 5, 29, 32}, true},
+		{"NameConstraints", asn1.ObjectIdentifier{2, 5, 29, 30}, true},
+		{"SubjectAltName", asn1.ObjectIdentifier{2, 5, 29, 17}, true},
+		{"IssuerAltName", asn1.ObjectIdentifier{2, 5, 29, 18}, true},
+		{"private use OID", asn1.ObjectIdentifier{1, 2, 3, 4, 5}, false},
+		{"DelegationUsage", asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 44363, 44}, false},
+		{"CT Poison", asn1.ObjectIdentifier{1, 3, 6, 1, 4, 1, 11129, 2, 4, 3}, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := isCaManagedExtension(tt.oid)
+			if got != tt.want {
+				t.Errorf("isCaManagedExtension(%v) = %v, want %v", tt.oid, got, tt.want)
+			}
+		})
+	}
+}
+
 func TestName(t *testing.T) {
 	sub := &Subject{
 		CN: "foobar",