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x509: Accept basicConstraints with omitted cA

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RFC 5280 defines cA as BOOLEAN DEFAULT FALSE, so a basicConstraints
extension containing only pathLenConstraint (SEQUENCE { INTEGER n })
is syntactically valid and must be interpreted as cA=FALSE. The fix
for CVE-2026-49300 rejected this encoding outright, breaking
compatibility with certificates emitted by devices such as
Eaton/CODESYS PLCs.

Tolerate MBEDTLS_ERR_ASN1_UNEXPECTED_TAG from the cA BOOLEAN parse,
keeping cA FALSE and letting the existing INTEGER parser validate
pathLenConstraint. The INTEGER value is never used to determine cA,
so the CVE fix is preserved.

Fixes #11006

Signed-off-by: v1rtu3x <149050962+v1rtu3x@users.noreply.github.com>
(cherry picked from commit 62dd8a556c01cc289d4fb3f19352a78ede90dac2)
This commit is contained in:
v1rtu3x 2026-10-16 13:53:40 +03:00
parent 8a04fc0861
commit 5af05a964a
4 changed files with 52 additions and 7 deletions
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5
ChangeLog.d/x509-basic-constraints-integer-first.txt Normal file
View File

@ -0,0 +1,5 @@
Bugfix
* Accept a basicConstraints extension that omits cA and contains only
pathLenConstraint, interpreting it as cA=FALSE as specified by RFC 5280.
This restores compatibility with certificates emitted by some devices
without accepting them as CA certificates. Fixes #11006.

12
library/x509_crt.c
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@ -519,17 +519,19 @@ static int x509_get_basic_constraints(unsigned char **p,
}
if ((ret = mbedtls_asn1_get_bool(p, end, ca_istrue)) != 0) {
/* If the SEQUENCE starts with an INTEGER and not a BOOLEAN,
* according to RFC 5280, it's syntactically valid, but it's
* something that a CA MUST NOT produce. So we reject it.
/* cA is DEFAULT FALSE. If it is absent, the first element must
* be pathLenConstraint. Keep cA false and let the INTEGER parser
* below validate the element.
*
* Note: historically, from XySSL 0.9 to Mbed TLS 3.6.6/4.1.0,
* `SEQUENCE { INTEGER n }` was interpreted as cA=FALSE if n == 0
* and cA=TRUE if n != 0. This was dangerous since
* `SEQUENCE { INTEGER n }` should be parsed as cA=FALSE
* according to RFC 5280, so we stopped doing it.
* according to RFC 5280.
*/
return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret);
if (ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG) {
return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret);
}
}
/* `SEQUENCE { BOOLEAN FALSE }` is not DER since default-value fields
* must be omitted in DER. But it seems harmless, and Mbed TLS has

20
tests/suites/test_suite_x509parse.data
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@ -2070,11 +2070,27 @@ mbedtls_x509_crt_parse_file:"../framework/data_files/parse_input/server5.basic-c
X509 CRT ASN1: basicConstraints: no cA but pathLenConstraint (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-integer-first.badsign.crt":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_UNEXPECTED_TAG):0
mbedtls_x509_crt_parse_file:"../framework/data_files/parse_input/server5.basic-constraints-integer-first.badsign.crt":0:1
X509 CRT ASN1: basicConstraints: no cA, pathLen only: not a CA
depends_on:MBEDTLS_PK_CAN_ECDSA_SOME:MBEDTLS_ECP_HAVE_SECP256R1:MBEDTLS_MD_CAN_SHA256
mbedtls_x509_get_ca_istrue:"../framework/data_files/parse_input/server5.basic-constraints-integer-first.badsign.crt":0
X509 CRT ASN1: basicConstraints: no cA but pathLenConstraint (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-integer-first.selfsigned.crt":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_UNEXPECTED_TAG):0
mbedtls_x509_crt_parse_file:"../framework/data_files/parse_input/server5.basic-constraints-integer-first.selfsigned.crt":0:1
X509 CRT ASN1: basicConstraints: no cA but zero pathLenConstraint
depends_on:MBEDTLS_PK_CAN_ECDSA_SOME:MBEDTLS_ECP_HAVE_SECP256R1:MBEDTLS_MD_CAN_SHA256
x509_crt_parse_basic_constraints:"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":0:0:0
X509 CRT ASN1: basicConstraints: invalid cA BOOLEAN length
depends_on:MBEDTLS_PK_CAN_ECDSA_SOME:MBEDTLS_ECP_HAVE_SECP256R1:MBEDTLS_MD_CAN_SHA256
x509_crt_parse_basic_constraints:"308201573081fea003020102020c53a2cb4b124ead837da894b2300a06082a8648ce3d040302300e310c300a06035504030c03457665301e170d3236303531323039353330345a170d3336303531323039353330345a300e310c300a06035504030c034576653059301306072a8648ce3d020106082a8648ce3d0301070342000437cc56d976091e5a723ec7592dff206eee7cf9069174d0ad14b5f768225962924ee500d82311ffea2fd2345d5d16bd8a88c26b770d55cd8a2a0efa01c8b4edffa3423040300f0603551d130101ff040530030100ff300e0603551d0f0101ff040403020780301d0603551d0e041604145061a58fd407d9d782010ce5657f8c6346a713be300a06082a8648ce3d0403020348003045022100d67d572af0aa540cd0f9d86d43788c178ca9501d8c0f4f6690c773f4b82c4b9a022011620feee62aca3944b8321636849030014ef80604550031312bb707b7f2bf04":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_INVALID_LENGTH):0:0
X509 CRT ASN1: basicConstraints: neither cA nor pathLenConstraint
depends_on:MBEDTLS_PK_CAN_ECDSA_SOME:MBEDTLS_ECP_HAVE_SECP256R1:MBEDTLS_MD_CAN_SHA256
x509_crt_parse_basic_constraints:"308201573081fea003020102020c53a2cb4b124ead837da894b2300a06082a8648ce3d040302300e310c300a06035504030c03457665301e170d3236303531323039353330345a170d3336303531323039353330345a300e310c300a06035504030c034576653059301306072a8648ce3d020106082a8648ce3d0301070342000437cc56d976091e5a723ec7592dff206eee7cf9069174d0ad14b5f768225962924ee500d82311ffea2fd2345d5d16bd8a88c26b770d55cd8a2a0efa01c8b4edffa3423040300f0603551d130101ff04053003050100300e0603551d0f0101ff040403020780301d0603551d0e041604145061a58fd407d9d782010ce5657f8c6346a713be300a06082a8648ce3d0403020348003045022100d67d572af0aa540cd0f9d86d43788c178ca9501d8c0f4f6690c773f4b82c4b9a022011620feee62aca3944b8321636849030014ef80604550031312bb707b7f2bf04":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_UNEXPECTED_TAG):0:0
X509 CRT ASN1 (TBS, inv extBasicConstraint, no pathlen length)
depends_on:MBEDTLS_RSA_C:MBEDTLS_MD_CAN_SHA256

22
tests/suites/test_suite_x509parse.function
View File

@ -1121,6 +1121,28 @@ exit:
}
/* END_CASE */
/* BEGIN_CASE depends_on:MBEDTLS_X509_CRT_PARSE_C */
void x509_crt_parse_basic_constraints(data_t *buf, int result,
int ca_istrue, int path_len_constraint)
{
mbedtls_x509_crt crt;
mbedtls_x509_crt_init(&crt);
USE_PSA_INIT();
TEST_EQUAL(mbedtls_x509_crt_parse_der(&crt, buf->x, buf->len), result);
if (result == 0) {
TEST_EQUAL(mbedtls_x509_crt_get_ca_istrue(&crt), ca_istrue);
/* max_pathlen is 1 higher than pathLenConstraint. */
TEST_EQUAL(crt.max_pathlen, path_len_constraint + 1);
}
exit:
mbedtls_x509_crt_free(&crt);
USE_PSA_DONE();
}
/* END_CASE */
/* BEGIN_CASE depends_on:MBEDTLS_X509_CRT_PARSE_C */
void x509parse_crt(data_t *buf, char *result_str, int result)
{