Allow serial number 0 for root CA certificates #9567

jackctj117 · 2025-12-19T22:32:58Z

This pull request updates the logic for validating X.509 certificate serial numbers in wolfcrypt/src/asn.c. The main change is to improve compliance with RFC 5280 while allowing for real-world exceptions involving root CAs. The previous strict check for zero serial numbers is now more nuanced, permitting serial number 0 for self-signed CA certificates but still rejecting it for other certificates.

Certificate serial number validation improvements:

Moved and updated the check for serial numbers of 0 to allow self-signed CA (root) certificates to have a serial number of 0, while still treating serial 0 as an error for all other certificates. This better aligns with RFC 5280 and real-world trust store practices. [1] [2]
Removed the previous check that always treated a serial number of 0 as an error, regardless of certificate type.

Testing

Tested with certificates generated using OpenSSL to verify all scenarios:

Root CA with serial 0 loads successfully (previously failed, now passes)
End-entity cert with serial 0 is correctly rejected
Normal end-entity cert signed by root CA with serial 0 verifies successfully
Self-signed non-CA cert with serial 0 is correctly rejected

kareem-wolfssl

Changes look good.

Can you add some test cases with a CA and leaf cert with a serial of 0?

jackctj117 · 2025-12-23T17:44:25Z

@kareem-wolfssl it looks like the failures are looking for an expected failure due to a self-signed CA certificate with serial number 0.

kareem-wolfssl · 2025-12-23T18:00:32Z

@kareem-wolfssl it looks like the failures are looking for an expected failure due to a self-signed CA certificate with serial number 0.

Yes, you will need to update the failing test test_MakeCertWith0Ser since we no longer expect a root CA with serial 0 to fail.

dgarske · 2025-12-26T15:54:31Z

@jackctj117 looks like some valid unit test failures you will need to look into. All related to --enable-asn=original.

FAILURES:
   390: test_SerialNumber0_RootCA

jackctj117 · 2026-01-14T23:19:18Z

Jenkins retest this please. History lost.

jackctj117 · 2026-01-16T18:32:24Z

Jenkins retest this please. History lost.

jackctj117 · 2026-01-17T00:12:48Z

Jenkins retest this please

jackctj117 · 2026-01-20T21:07:22Z

Jenkins retest this please

jackctj117 · 2026-01-21T16:19:17Z

Jenkins retest this

jackctj117 · 2026-01-21T17:01:02Z

Jenkins retest this please

kareem-wolfssl · 2026-01-22T21:44:34Z

tests/api/test_asn.c

+    ExpectIntNE(wolfSSL_CertManagerVerify(cm, eeSerial0File,
+                WOLFSSL_FILETYPE_PEM), WOLFSSL_SUCCESS);


This should test for the expected error code.

kareem-wolfssl · 2026-01-22T21:45:06Z

tests/api/test_asn.c

+    ExpectIntNE(wolfSSL_CertManagerLoadCA(cm, selfSignedNonCASerial0File, NULL),
+                WOLFSSL_SUCCESS);


Same as above, check for the expected error code.

kareem-wolfssl · 2026-01-30T22:50:38Z

Jenkins retest this please.

… test

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Comments suppressed due to low confidence (1)

wolfcrypt/src/asn.c:23904

The log message for cert->serialSz == 0 is confusing: it suggests a certificate might legitimately have “no serial number”, but X.509 certificates always have a serial number and RFC 5280 requires it to be present. Consider rewording to clearly state that an empty/zero-length serial is invalid, and (if appropriate in this file) emit the verbose error macro consistently with other parse failures.

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Copilot · 2026-02-06T18:20:30Z

wolfcrypt/src/asn.c

+    /* 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) 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 (!(cert->isCA && cert->selfSigned)
+#ifdef WOLFSSL_CERT_REQ
+            && !cert->isCSR
+#endif
+        ) {


The serial-0 exception is currently keyed off cert->isCA && cert->selfSigned, but selfSigned is determined by issuer/subject name hash equality (not by verifying the signature with the certificate's own public key). This makes the exception apply to any self-issued CA certificate (and in all parse contexts), not specifically to explicitly-trusted root CAs as the comment describes. Consider tightening the condition to only allow serial 0 when parsing a trust anchor context (e.g., CA_TYPE / TRUSTED_PEER_TYPE) or when the certificate is cryptographically self-signed, so nonconforming non-root/self-issued certs remain rejected under strict ASN settings.

Copilot · 2026-02-06T18:20:31Z

tests/api/test_asn.c

+#if !defined(NO_CERTS) && !defined(NO_FILESYSTEM) && !defined(NO_RSA) && \
+    defined(WOLFSSL_CERT_GEN) && defined(WOLFSSL_CERT_EXT)
+    /* Test that root CA certificates with serial number 0 are accepted,


This test only loads/verifies existing PEM files via the cert manager, but it is gated on WOLFSSL_CERT_GEN. Certificate generation support shouldn't be required for these APIs, so this guard will skip the test in builds that still include ASN/X.509 parsing/verification. Consider dropping defined(WOLFSSL_CERT_GEN) from the preprocessor condition (keeping only the features actually required, e.g., filesystem/certs/extensions) to improve coverage across configurations.

jackctj117 self-assigned this Dec 19, 2025

kareem-wolfssl reviewed Dec 19, 2025

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kareem-wolfssl mentioned this pull request Dec 19, 2025

[Bug]: WolfSSL accepts a certificate whose serial number is zero #8615

Open

jackctj117 requested a review from kareem-wolfssl December 23, 2025 17:43

tmael mentioned this pull request Dec 23, 2025

[Bug]: curl: (35) SSL_connect failed with error -140: ASN parsing error, invalid input #7844

Closed

kareem-wolfssl requested changes Jan 22, 2026

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kareem-wolfssl requested a review from wolfSSL-Bot January 22, 2026 21:47

jackctj117 requested a review from kareem-wolfssl January 30, 2026 22:38

jackctj117 added 8 commits February 3, 2026 14:33

Allow serial number 0 for root CA certificates

2ff3ccd

Added serial 0 tests to unit tests

56b7a05

Fix for old tests

18178a3

Add serial 0 validation to non-template code path

38a1ffc

Add serial 0 validation to original parser, fix CSR handling

7bafa04

Fix test_MakeCertWith0Ser: remove keyCertSign from non-CA certificate…

384471c

… test

Add generated test certificates for serial 0 testing

c529e8d

Added new certs to include.am

af2579c

jackctj117 force-pushed the serial-0 branch from 82c6f7e to af2579c Compare February 3, 2026 21:45

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jackctj117 added 2 commits February 5, 2026 16:15

removed trailing whitespaces

8ee52be

Fixes for tests

8c31d3a

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		ExpectIntNE(wolfSSL_CertManagerVerify(cm, eeSerial0File,
		WOLFSSL_FILETYPE_PEM), WOLFSSL_SUCCESS);

		ExpectIntNE(wolfSSL_CertManagerLoadCA(cm, selfSignedNonCASerial0File, NULL),
		WOLFSSL_SUCCESS);

Allow serial number 0 for root CA certificates #9567

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Allow serial number 0 for root CA certificates #9567

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jackctj117 commented Dec 19, 2025

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kareem-wolfssl commented Jan 30, 2026 •

edited by jackctj117

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