Allow serial number 0 for root CA certificates

jackctj117 · jackctj117 · commit 2a252b8f4aed · 2026-03-20T12:43:34.000-06:00
diff --git a/.github/workflows/codespell.yml b/.github/workflows/codespell.yml
@@ -27,4 +27,4 @@ jobs:
           # The exclude_file contains lines of code that should be ignored. This is useful for individual lines which have non-words that can safely be ignored.
         exclude_file: '.codespellexcludelines'
           # To skip files entirely from being processed, add it to the following list:
-        skip: '*.cproject,*.der,*.mtpj,*.pem,*.vcxproj,.git,*.launch,*.scfg,*.revoked,./examples/asn1/dumpasn1.cfg,./examples/asn1/oid_names.h'
+        skip: '*.cproject,*.csr,*.der,*.mtpj,*.pem,*.vcxproj,.git,*.launch,*.scfg,*.revoked,./examples/asn1/dumpasn1.cfg,./examples/asn1/oid_names.h'
diff --git a/certs/include.am b/certs/include.am
@@ -155,6 +155,7 @@ include certs/ocsp/include.am
 include certs/statickeys/include.am
 include certs/test/include.am
 include certs/test-pathlen/include.am
+include certs/test-serial0/include.am
 include certs/intermediate/include.am
 include certs/falcon/include.am
 include certs/rsapss/include.am
diff --git a/certs/test-serial0/ee_normal.pem b/certs/test-serial0/ee_normal.pem
@@ -0,0 +1,21 @@
+-----BEGIN CERTIFICATE-----
+MIIDeDCCAmCgAwIBAgIBZDANBgkqhkiG9w0BAQsFADBEMR4wHAYDVQQDDBVUZXN0
+IFJvb3QgQ0EgU2VyaWFsIDAxFTATBgNVBAoMDHdvbGZTU0wgVGVzdDELMAkGA1UE
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+V3l1KwxiECev0CFul+J7OZE0fdkcEkscZ7ySrsA5cHiqtB9xzcCWMRMR/LYjcUDo
+R6o7yosbg6SzoDO78VUImqtwQhKXrvDy7kMwPw==
+-----END CERTIFICATE-----
diff --git a/certs/test-serial0/ee_serial0.pem b/certs/test-serial0/ee_serial0.pem
@@ -0,0 +1,21 @@
+-----BEGIN CERTIFICATE-----
+MIIDejCCAmKgAwIBAgIBADANBgkqhkiG9w0BAQsFADBEMR4wHAYDVQQDDBVUZXN0
+IFJvb3QgQ0EgU2VyaWFsIDAxFTATBgNVBAoMDHdvbGZTU0wgVGVzdDELMAkGA1UE
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+BgNVBAYTAlVTMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAzQSr4Wfu
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+uYn0tWZDeLq932X8gNWKuJQlM1mky1y+1RB/e0F2
+-----END CERTIFICATE-----
diff --git a/certs/test-serial0/generate_certs.sh b/certs/test-serial0/generate_certs.sh
@@ -0,0 +1,88 @@
+#!/bin/bash
+#
+# Generate test certificates for serial number 0 testing (issue #8615)
+#
+# Tests verify that root CAs (self-signed + CA:TRUE) with serial 0 are
+# accepted as trust anchors, while all other cert types with serial 0
+# are rejected per RFC 5280 section 4.1.2.2.
+#
+# Output files (certs only -- EE keys use temp files):
+#   root_serial0.pem / root_serial0_key.pem  - Root CA with serial 0
+#   ee_serial0.pem                           - EE cert with serial 0 (rejected)
+#   ee_normal.pem                            - Normal EE cert (serial 100)
+#   selfsigned_nonca_serial0.pem             - Self-signed non-CA, serial 0
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+cd "$SCRIPT_DIR"
+
+echo "==================================================="
+echo "Generating serial 0 test certificates in: $SCRIPT_DIR"
+echo "==================================================="
+
+# 1. Create Root CA with serial number 0
+echo ""
+echo "[1/4] Creating Root CA with serial number 0..."
+openssl req -x509 -newkey rsa:2048 -keyout root_serial0_key.pem -out root_serial0.pem \
+    -days 7300 -nodes -subj "/CN=Test Root CA Serial 0/O=wolfSSL Test/C=US" \
+    -set_serial 0 \
+    -addext "basicConstraints=critical,CA:TRUE" \
+    -addext "keyUsage=critical,keyCertSign,cRLSign"
+
+echo "   Root CA serial number:"
+openssl x509 -in root_serial0.pem -noout -serial
+
+# 2. Create end-entity cert with serial 0 signed by root_serial0
+echo ""
+echo "[2/4] Creating end-entity certificate with serial number 0..."
+openssl req -newkey rsa:2048 -keyout ee_serial0_key.tmp -out ee_serial0.csr.tmp -nodes \
+    -subj "/CN=End Entity Serial 0/O=wolfSSL Test/C=US"
+
+openssl x509 -req -in ee_serial0.csr.tmp -CA root_serial0.pem -CAkey root_serial0_key.pem \
+    -out ee_serial0.pem -days 3650 -set_serial 0 \
+    -extfile <(echo "basicConstraints=CA:FALSE
+keyUsage=digitalSignature,keyEncipherment
+extendedKeyUsage=serverAuth,clientAuth")
+
+rm -f ee_serial0_key.tmp ee_serial0.csr.tmp
+
+echo "   End-entity cert serial number:"
+openssl x509 -in ee_serial0.pem -noout -serial
+
+# 3. Create normal end-entity cert signed by root CA with serial 0
+echo ""
+echo "[3/4] Creating normal end-entity certificate (signed by serial 0 root)..."
+openssl req -newkey rsa:2048 -keyout ee_normal_key.tmp -out ee_normal.csr.tmp -nodes \
+    -subj "/CN=End Entity Normal/O=wolfSSL Test/C=US"
+
+openssl x509 -req -in ee_normal.csr.tmp -CA root_serial0.pem -CAkey root_serial0_key.pem \
+    -out ee_normal.pem -days 3650 -set_serial 100 \
+    -extfile <(echo "basicConstraints=CA:FALSE
+keyUsage=digitalSignature,keyEncipherment
+extendedKeyUsage=serverAuth,clientAuth")
+
+rm -f ee_normal_key.tmp ee_normal.csr.tmp
+
+echo "   Normal end-entity cert serial number:"
+openssl x509 -in ee_normal.pem -noout -serial
+
+# 4. Create self-signed non-CA certificate with serial 0
+echo ""
+echo "[4/4] Creating self-signed non-CA certificate with serial number 0..."
+openssl req -x509 -newkey rsa:2048 -keyout selfsigned_nonca_serial0_key.tmp \
+    -out selfsigned_nonca_serial0.pem -days 3650 -nodes \
+    -subj "/CN=Self-Signed Non-CA Serial 0/O=wolfSSL Test/C=US" \
+    -set_serial 0 \
+    -addext "basicConstraints=CA:FALSE" \
+    -addext "keyUsage=digitalSignature,keyEncipherment"
+
+rm -f selfsigned_nonca_serial0_key.tmp
+
+echo "   Self-signed non-CA cert serial number:"
+openssl x509 -in selfsigned_nonca_serial0.pem -noout -serial
+
+echo ""
+echo "==================================================="
+echo "Certificate generation complete!"
+echo "==================================================="
diff --git a/certs/test-serial0/include.am b/certs/test-serial0/include.am
@@ -0,0 +1,11 @@
+# vim:ft=automake
+# included from Top Level Makefile.am
+# All paths should be given relative to the root
+
+EXTRA_DIST += certs/test-serial0/generate_certs.sh \
+             certs/test-serial0/root_serial0.pem \
+             certs/test-serial0/root_serial0_key.pem \
+             certs/test-serial0/ee_serial0.pem \
+             certs/test-serial0/ee_normal.pem \
+             certs/test-serial0/selfsigned_nonca_serial0.pem
+
diff --git a/certs/test-serial0/root_serial0.pem b/certs/test-serial0/root_serial0.pem
@@ -0,0 +1,21 @@
+-----BEGIN CERTIFICATE-----
+MIIDZjCCAk6gAwIBAgIBADANBgkqhkiG9w0BAQsFADBEMR4wHAYDVQQDDBVUZXN0
+IFJvb3QgQ0EgU2VyaWFsIDAxFTATBgNVBAoMDHdvbGZTU0wgVGVzdDELMAkGA1UE
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+ujFE1ZTiQhm+4A==
+-----END CERTIFICATE-----
diff --git a/certs/test-serial0/root_serial0_key.pem b/certs/test-serial0/root_serial0_key.pem
@@ -0,0 +1,28 @@
+-----BEGIN PRIVATE KEY-----
+MIIEugIBADANBgkqhkiG9w0BAQEFAASCBKQwggSgAgEAAoIBAQChQKbHG0su7Bpy
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+Vowu3kDcLFcpu3v6368=
+-----END PRIVATE KEY-----
diff --git a/certs/test-serial0/selfsigned_nonca_serial0.pem b/certs/test-serial0/selfsigned_nonca_serial0.pem
@@ -0,0 +1,21 @@
+-----BEGIN CERTIFICATE-----
+MIIDaTCCAlGgAwIBAgIBADANBgkqhkiG9w0BAQsFADBKMSQwIgYDVQQDDBtTZWxm
+LVNpZ25lZCBOb24tQ0EgU2VyaWFsIDAxFTATBgNVBAoMDHdvbGZTU0wgVGVzdDEL
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+dgos00E67av5sbeXDw==
+-----END CERTIFICATE-----
diff --git a/tests/api.c b/tests/api.c
@@ -20456,12 +20456,14 @@ static int test_MakeCertWith0Ser(void)
         CTC_NAME_SIZE);
 
     cert.selfSigned = 1;
-    cert.isCA       = 1;
+    /* Changed from isCA=1 to isCA=0 to test non-root certificate.
+     * Serial 0 is now allowed for root CAs (selfSigned && isCA),
+     * but should still be rejected for non-CA certificates. */
+    cert.isCA       = 0;
     cert.sigType    = CTC_SHA256wECDSA;
 
-#ifdef WOLFSSL_CERT_EXT
-    cert.keyUsage |= KEYUSE_KEY_CERT_SIGN;
-#endif
+    /* Note: KEYUSE_KEY_CERT_SIGN is not set here because it's only valid for
+     * CA certificates. This test creates a non-CA certificate (isCA=0). */
 
     /* set serial number to 0 */
     cert.serialSz  = 1;
diff --git a/tests/api/test_asn.c b/tests/api/test_asn.c
@@ -920,3 +920,65 @@ int test_wc_DecodeRsaPssParams(void)
 #endif /* WC_RSA_PSS && !NO_RSA && !NO_ASN */
     return EXPECT_RESULT();
 }
+
+int test_SerialNumber0_RootCA(void)
+{
+    EXPECT_DECLS;
+
+#if !defined(NO_CERTS) && !defined(NO_FILESYSTEM) && !defined(NO_RSA) && \
+    !defined(WOLFSSL_NO_PEM) && defined(WOLFSSL_PEM_TO_DER)
+    /* Test that root CA certificates with serial number 0 are accepted,
+     * while non-root certificates with serial 0 are rejected (issue #8615) */
+
+#if !defined(WOLFSSL_NO_ASN_STRICT) && !defined(WOLFSSL_PYTHON) && \
+    !defined(WOLFSSL_ASN_ALLOW_0_SERIAL) && \
+    !defined(WOLFSSL_TEST_APPLE_NATIVE_CERT_VALIDATION)
+    WOLFSSL_CERT_MANAGER* cm = NULL;
+    const char* rootSerial0File = "./certs/test-serial0/root_serial0.pem";
+    const char* selfSignedNonCASerial0File =
+        "./certs/test-serial0/selfsigned_nonca_serial0.pem";
+
+    /* Test 1: Root CA with serial 0 should load successfully */
+    ExpectNotNull(cm = wolfSSL_CertManagerNew());
+    ExpectIntEQ(wolfSSL_CertManagerLoadCA(cm, rootSerial0File, NULL),
+                WOLFSSL_SUCCESS);
+
+#if (!defined(NO_WOLFSSL_CLIENT) || !defined(WOLFSSL_NO_CLIENT_AUTH)) || \
+    defined(OPENSSL_EXTRA)
+    {
+        const char* eeSerial0File = "./certs/test-serial0/ee_serial0.pem";
+        const char* eeNormalFile = "./certs/test-serial0/ee_normal.pem";
+
+        /* Test 2: End-entity cert with serial 0 should be rejected during
+         * verify */
+        ExpectIntEQ(wolfSSL_CertManagerVerify(cm, eeSerial0File,
+                    WOLFSSL_FILETYPE_PEM), WC_NO_ERR_TRACE(ASN_PARSE_E));
+
+        /* Test 3: Normal end-entity cert signed by root CA with serial 0
+         * should verify successfully */
+        ExpectIntEQ(wolfSSL_CertManagerVerify(cm, eeNormalFile,
+                    WOLFSSL_FILETYPE_PEM), WOLFSSL_SUCCESS);
+    }
+#endif
+
+    if (cm != NULL) {
+        wolfSSL_CertManagerFree(cm);
+        cm = NULL;
+    }
+
+    /* Test 4: Self-signed non-CA certificate with serial 0 should be rejected */
+    ExpectNotNull(cm = wolfSSL_CertManagerNew());
+    ExpectIntNE(wolfSSL_CertManagerLoadCA(cm, selfSignedNonCASerial0File, NULL),
+                WOLFSSL_SUCCESS);
+
+    if (cm != NULL) {
+        wolfSSL_CertManagerFree(cm);
+        cm = NULL;
+    }
+#endif /* !WOLFSSL_NO_ASN_STRICT && !WOLFSSL_PYTHON &&
+          !WOLFSSL_ASN_ALLOW_0_SERIAL &&
+          !WOLFSSL_TEST_APPLE_NATIVE_CERT_VALIDATION */
+#endif /* !NO_CERTS && !NO_FILESYSTEM && !NO_RSA && !WOLFSSL_NO_PEM */
+
+    return EXPECT_RESULT();
+}
diff --git a/tests/api/test_asn.h b/tests/api/test_asn.h
@@ -29,12 +29,14 @@ int test_GetSetShortInt(void);
 int test_wc_IndexSequenceOf(void);
 int test_wolfssl_local_MatchBaseName(void);
 int test_wc_DecodeRsaPssParams(void);
+int test_SerialNumber0_RootCA(void);
 
 #define TEST_ASN_DECLS                                              \
     TEST_DECL_GROUP("asn", test_SetAsymKeyDer),                     \
     TEST_DECL_GROUP("asn", test_GetSetShortInt),                    \
     TEST_DECL_GROUP("asn", test_wc_IndexSequenceOf),                \
     TEST_DECL_GROUP("asn", test_wolfssl_local_MatchBaseName),       \
-    TEST_DECL_GROUP("asn", test_wc_DecodeRsaPssParams)
+    TEST_DECL_GROUP("asn", test_wc_DecodeRsaPssParams),            \
+    TEST_DECL_GROUP("asn", test_SerialNumber0_RootCA)
 
 #endif /* WOLFCRYPT_TEST_ASN_H */
diff --git a/wolfcrypt/src/asn.c b/wolfcrypt/src/asn.c
@@ -24206,21 +24206,10 @@ static int DecodeCertInternal(DecodedCert* cert, int verify, int* criticalExt,
         cert->version = version;
         cert->serialSz = (int)serialSz;
 
-    #if !defined(WOLFSSL_NO_ASN_STRICT) && !defined(WOLFSSL_PYTHON) && \
-        !defined(WOLFSSL_ASN_ALLOW_0_SERIAL)
-        /* RFC 5280 section 4.1.2.2 states that non-conforming CAs may issue
-         * a negative or zero serial number and should be handled gracefully.
-         * Since it is a non-conforming CA that issues a serial of 0 then we
-         * treat it as an error here. */
-        if (cert->serialSz == 1 && cert->serial[0] == 0) {
-            WOLFSSL_MSG("Error serial number of 0, use WOLFSSL_NO_ASN_STRICT "
-                "if wanted");
-            ret = ASN_PARSE_E;
-        }
-    #endif
+        /* RFC 5280 requires serial number to be present and at least 1 byte */
         if (cert->serialSz == 0) {
-            WOLFSSL_MSG("Error serial size is zero. Should be at least one "
-                        "even with no serial number.");
+            WOLFSSL_MSG("Error: certificate serial number is empty "
+                        "(zero-length serial is invalid per RFC 5280)");
             ret = ASN_PARSE_E;
         }
 
@@ -26121,6 +26110,26 @@ int ParseCertRelative(DecodedCert* cert, int type, int verify, void* cm,
         }
 #endif
 
+#if !defined(WOLFSSL_NO_ASN_STRICT) && !defined(WOLFSSL_PYTHON) && \
+    !defined(WOLFSSL_ASN_ALLOW_0_SERIAL)
+        /* Check for serial number of 0. RFC 5280 section 4.1.2.2 requires
+         * positive serial numbers. However, allow zero for self-signed CA
+         * certificates (root CAs) being loaded as trust anchors since they
+         * are explicitly trusted and some legacy root CAs in real-world
+         * trust stores have serial number 0. */
+        if ((ret == 0) && (cert->serialSz == 1) && (cert->serial[0] == 0)) {
+            if (!((type == CA_TYPE || type == TRUSTED_PEER_TYPE) &&
+                  cert->isCA && cert->selfSigned)
+#ifdef WOLFSSL_CERT_REQ
+                && !cert->isCSR
+#endif
+            ) {
+                WOLFSSL_MSG("Error serial number of 0 for non-root certificate");
+                return ASN_PARSE_E;
+            }
+        }
+#endif
+
     #ifndef ALLOW_INVALID_CERTSIGN
         /* https://datatracker.ietf.org/doc/html/rfc5280#section-4.2.1.9
          *   If the cA boolean is not asserted, then the keyCertSign bit in the
