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Fix lax basicConstraints parsing

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When parsing the basicConstraints extension, reject junk after the SEQUENCE
inside the extension (which is probably benign), and reject a SEQUENCE that
extends beyond the extension (could be very dangerous).

Add a non-regression test where a certificate that is technically malformed,
but accepted as a leaf certificate by OpenSSL and other X.509
implementations, to be accepted as a CA certificate by Mbed TLS.

Signed-off-by: Gilles Peskine <[email protected]>
This commit is contained in:
Gilles Peskine
2026-05-26 20:46:23 +02:00
parent 412b8d91e0
commit 07f45b8768
3 changed files with 22 additions and 0 deletions
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ChangeLog.d/x509-crt-parse-basicConstraints.txt
+7
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@@ -0,0 +1,7 @@
Security
* Fix a bug in the X.509 certificate parser that caused some inputs
with a malformed basicConstraints extension to be accepted. This
could have dangerous results, in particular causing some certificates
to be parsed as CA certificates when other X.509 implementations would
parse them as end-entity certificates. Reported by Mohammad Seet
(mhdsait101). CVE-2026-TODO
library/x509_crt.c
+7
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@@ -516,6 +516,13 @@ static int x509_get_basic_constraints(unsigned char **p,
return 0;
}
if ((size_t) (end - *p) != len) {
/* Reject junk after the SEQUENCE inside the extension (which is
* probably benign), and reject a SEQUENCE that extends beyond
* the extension (could be very dangerous). */
return MBEDTLS_ERR_X509_INVALID_EXTENSIONS;
}
if ((ret = mbedtls_asn1_get_bool(p, end, ca_istrue)) != 0) {
if (ret == MBEDTLS_ERR_ASN1_UNEXPECTED_TAG) {
ret = mbedtls_asn1_get_int(p, end, ca_istrue);
tests/suites/test_suite_x509parse.data
+8
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@@ -2060,6 +2060,14 @@ X509 CRT ASN1 (TBSCertificate v3, inv CertificatePolicies, policy qualifier leng
depends_on:MBEDTLS_RSA_C:MBEDTLS_MD_CAN_SHA256
x509parse_crt:"3081b030819aa0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092a864886f70d010101050003190030160210ffffffffffffffffffffffffffffffff0202ffffa100a200a314301230100603551d200409300730050601003001300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERR_X509_INVALID_EXTENSIONS + MBEDTLS_ERR_ASN1_OUT_OF_DATA
X509 CRT ASN1: basicConstraints overflow would make CA=1 (bad signature)
depends_on:MBEDTLS_PK_CAN_ECDSA_SOME:MBEDTLS_ECP_HAVE_SECP256R1:MBEDTLS_MD_CAN_SHA256
mbedtls_x509_crt_parse_file:"../framework/data_files/parse_input/server5.basic-constraints-sequence-overflow.badsign.crt":MBEDTLS_ERR_X509_INVALID_EXTENSIONS:0
X509 CRT ASN1: basicConstraints overflow would make CA=1 (good self signature)
depends_on:MBEDTLS_PK_CAN_ECDSA_SOME:MBEDTLS_ECP_HAVE_SECP256R1:MBEDTLS_MD_CAN_SHA256
mbedtls_x509_crt_parse_file:"../framework/data_files/parse_input/server5.basic-constraints-sequence-overflow.selfsigned.crt":MBEDTLS_ERR_X509_INVALID_EXTENSIONS:0
X509 CRT ASN1 (TBS, inv extBasicConstraint, no pathlen length)
depends_on:MBEDTLS_RSA_C:MBEDTLS_MD_CAN_SHA256
x509parse_crt:"3081b030819aa0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D010101050003190030160210ffffffffffffffffffffffffffffffff0202ffffa100a200a314301230100603551d130101010406300402010102300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERR_X509_INVALID_EXTENSIONS + MBEDTLS_ERR_ASN1_OUT_OF_DATA