--- a/third_party/heimdal/lib/hx509/crypto-ec.c	2025-01-30 13:57:45.000000000 +0000
+++ b/third_party/heimdal/lib/hx509/crypto-ec.c	2026-09-01 23:21:32.000000000 +0000
@@ -785,6 +785,30 @@
     48
 };
 
+/*
+ * Windows/CNG places the KEY algorithm OID (id-ecPublicKey) in the CMS
+ * SignerInfo.signatureAlgorithm field, where RFC 5652 expects the SIGNATURE
+ * algorithm OID (ecdsa-with-SHA384). Accept it and verify as ECDSA/SHA-384 so
+ * that Windows PKINIT smart-card logon interoperates.
+ *
+ * NOTE: the digest is fixed at SHA-384 here, which is only correct for P-384
+ * clients. A general fix should derive the digest from SignerInfo.digestAlgorithm.
+ */
+const struct signature_alg ecdsa_ecpubkey_sha384_alg = {
+    "ecdsa-ecpublickey-sha384",
+    ASN1_OID_ID_ECPUBLICKEY,
+    &_hx509_signature_ecPublicKey,
+    ASN1_OID_ID_ECPUBLICKEY,
+    &_hx509_signature_sha384_data,
+    PROVIDE_CONF|REQUIRE_SIGNER|RA_RSA_USES_DIGEST_INFO|
+        SIG_PUBLIC_SIG|SELF_SIGNED_OK,
+    0,
+    NULL,
+    ecdsa_verify_signature,
+    ecdsa_create_signature,
+    48
+};
+
 const struct signature_alg ecdsa_with_sha256_alg = {
     "ecdsa-with-sha256",
     ASN1_OID_ID_ECDSA_WITH_SHA256,
--- a/third_party/heimdal/lib/hx509/crypto.c	2025-01-30 13:57:45.000000000 +0000
+++ b/third_party/heimdal/lib/hx509/crypto.c	2026-09-01 23:21:32.000000000 +0000
@@ -760,6 +760,7 @@
 #ifdef HAVE_HCRYPTO_W_OPENSSL
 extern const struct signature_alg ecdsa_with_sha512_alg;
 extern const struct signature_alg ecdsa_with_sha384_alg;
+extern const struct signature_alg ecdsa_ecpubkey_sha384_alg;
 extern const struct signature_alg ecdsa_with_sha256_alg;
 extern const struct signature_alg ecdsa_with_sha1_alg;
 #endif
@@ -968,6 +969,7 @@
 static const struct signature_alg *sig_algs[] = {
 #ifdef HAVE_HCRYPTO_W_OPENSSL
     &ecdsa_with_sha512_alg,
+    &ecdsa_ecpubkey_sha384_alg,
     &ecdsa_with_sha384_alg,
     &ecdsa_with_sha256_alg,
     &ecdsa_with_sha1_alg,