diff --git a/ChangeLog.d/x509-crt-parse-basicConstraints.txt b/ChangeLog.d/x509-crt-parse-basicConstraints.txt
new file mode 100644
index 0000000000..251dc8f0fd
--- /dev/null
+++ b/ChangeLog.d/x509-crt-parse-basicConstraints.txt
@@ -0,0 +1,15 @@
+Security
+   * Fix two bugs 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), Pablo Ruiz García and NVIDIA Project Vanessa. CVE-2026-49300
+
+Changes
+   * X.509 certificates with a basicConstraints extension starting with an
+     INTEGER field are no longer accepted. According to RFC 5280, such
+     certificates are syntactically valid and the first field is the
+     pathLenConstraint value, but certificate authorities must not emit such
+     certificates. Mbed TLS interpreted it as a cA value, which was nonstandard
+     and dangerous.
diff --git a/ChangeLog.d/x509-csr-extension-request-bounds.txt b/ChangeLog.d/x509-csr-extension-request-bounds.txt
new file mode 100644
index 0000000000..dfc706703c
--- /dev/null
+++ b/ChangeLog.d/x509-csr-extension-request-bounds.txt
@@ -0,0 +1,3 @@
+Bugfix
+   * Reject malformed X.509 CSRs where the extensionRequest attribute
+     wrappers do not exactly contain the requested extensions.
diff --git a/framework b/framework
index e92a819966..e0f6275eff 160000
--- a/framework
+++ b/framework
@@ -1 +1 @@
-Subproject commit e92a81996656d82941a9afce843453c57e1d51f5
+Subproject commit e0f6275eff2e45e295fdd6b6877de4cbdaddae83
diff --git a/library/x509_crt.c b/library/x509_crt.c
index 3232760ea2..093d733c1c 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -512,23 +512,34 @@ static int x509_get_basic_constraints(unsigned char **p,
     }
 
     if (*p == end) {
+        /* Empty basicConstraints: valid, not a CA. */
         return 0;
     }
 
-    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);
-        }
-
-        if (ret != 0) {
-            return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret);
-        }
-
-        if (*ca_istrue != 0) {
-            *ca_istrue = 1;
-        }
+    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 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.
+         *
+         * 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.
+         */
+        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
+     * always accepted it, so we continue to accept it. */
+
     if (*p == end) {
         return 0;
     }
diff --git a/library/x509_csr.c b/library/x509_csr.c
index 3e8e407b26..28a4560c43 100644
--- a/library/x509_csr.c
+++ b/library/x509_csr.c
@@ -227,25 +227,34 @@ static int x509_csr_parse_attributes(mbedtls_x509_csr *csr,
 
         /* Check that this is an extension-request attribute */
         if (MBEDTLS_OID_CMP(MBEDTLS_OID_PKCS9_CSR_EXT_REQ, &attr_oid) == 0) {
-            if ((ret = mbedtls_asn1_get_tag(p, end, &len,
+            if ((ret = mbedtls_asn1_get_tag(p, end_attr_data, &len,
                                             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SET)) != 0) {
                 return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret);
             }
 
-            if ((ret = mbedtls_asn1_get_tag(p, end, &len,
+            const unsigned char *end_set = *p + len;
+            if (end_set != end_attr_data) {
+                return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
+                                         MBEDTLS_ERR_ASN1_LENGTH_MISMATCH);
+            }
+
+            if ((ret = mbedtls_asn1_get_tag(p, end_set, &len,
                                             MBEDTLS_ASN1_CONSTRUCTED | MBEDTLS_ASN1_SEQUENCE)) !=
                 0) {
                 return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, ret);
             }
 
-            if ((ret = x509_csr_parse_extensions(csr, p, *p + len, cb, p_ctx)) != 0) {
-                return ret;
-            }
-
-            if (*p != end_attr_data) {
+            const unsigned char *end_exts = *p + len;
+            if (end_exts != end_set) {
                 return MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS,
                                          MBEDTLS_ERR_ASN1_LENGTH_MISMATCH);
             }
+
+            if ((ret = x509_csr_parse_extensions(csr, p, end_exts, cb, p_ctx)) != 0) {
+                return ret;
+            }
+            /* x509_csr_parse_extensions() guarantees *p == end_exts
+             * on success */
         }
 
         *p = end_attr_data;
diff --git a/tests/suites/test_suite_x509parse.data b/tests/suites/test_suite_x509parse.data
index 5e91df9107..63f4c0daa9 100644
--- a/tests/suites/test_suite_x509parse.data
+++ b/tests/suites/test_suite_x509parse.data
@@ -2052,9 +2052,25 @@ X509 CRT ASN1 (TBSCertificate v3, inv CertificatePolicies, policy qualifier leng
 depends_on:PSA_HAVE_ALG_SOME_RSA_VERIFY:PSA_WANT_ALG_SHA_256
 x509parse_crt:"3081b030819aa0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092a864886f70d01010105000319003016021100ffffffffffffffffffffffffffffffff020103a100a200a314301230100603551d200409300730050601003001300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_OUT_OF_DATA)
 
+X509 CRT ASN1: basicConstraints overflow would make CA=1 (bad signature)
+depends_on:PSA_HAVE_ALG_ECDSA_VERIFY:PSA_WANT_ECC_SECP_R1_256:PSA_WANT_ALG_SHA_256
+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:PSA_HAVE_ALG_ECDSA_VERIFY:PSA_WANT_ECC_SECP_R1_256:PSA_WANT_ALG_SHA_256
+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: basicConstraints: no cA but pathLenConstraint (bad signature)
+depends_on:PSA_HAVE_ALG_ECDSA_VERIFY:PSA_WANT_ECC_SECP_R1_256:PSA_WANT_ALG_SHA_256
+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
+
+X509 CRT ASN1: basicConstraints: no cA but pathLenConstraint (good self signature)
+depends_on:PSA_HAVE_ALG_ECDSA_VERIFY:PSA_WANT_ECC_SECP_R1_256:PSA_WANT_ALG_SHA_256
+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
+
 X509 CRT ASN1 (TBS, inv extBasicConstraint, no pathlen length)
 depends_on:PSA_HAVE_ALG_SOME_RSA_VERIFY:PSA_WANT_ALG_SHA_256
-x509parse_crt:"3081b030819aa0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D01010105000319003016021100ffffffffffffffffffffffffffffffff020103a100a200a314301230100603551d130101010406300402010102300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_OUT_OF_DATA)
+x509parse_crt:"3081b030819aa0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D01010105000319003016021100ffffffffffffffffffffffffffffffff020103a100a200a314301230100603551d13010101040630040101ff02300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_OUT_OF_DATA)
 
 X509 CRT ASN1 (inv extBasicConstraint, pathlen is INT_MAX)
 depends_on:PSA_HAVE_ALG_SOME_RSA_VERIFY:PSA_WANT_ALG_SHA_256:PSA_WANT_ALG_SHA_1
@@ -2066,19 +2082,19 @@ mbedtls_x509_crt_parse_file:"../framework/data_files/parse_input/server1_pathlen
 
 X509 CRT ASN1 (TBS, inv extBasicConstraint, pathlen inv length encoding)
 depends_on:PSA_HAVE_ALG_SOME_RSA_VERIFY:PSA_WANT_ALG_SHA_256
-x509parse_crt:"3081b130819ba0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D01010105000319003016021100ffffffffffffffffffffffffffffffff020103a100a200a315301330110603551d13010101040730050201010285300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_INVALID_LENGTH)
+x509parse_crt:"3081b130819ba0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D01010105000319003016021100ffffffffffffffffffffffffffffffff020103a100a200a315301330110603551d13010101040730050101ff0285300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_INVALID_LENGTH)
 
 X509 CRT ASN1 (TBS, inv extBasicConstraint, pathlen length out of bounds)
 depends_on:PSA_HAVE_ALG_SOME_RSA_VERIFY:PSA_WANT_ALG_SHA_256
-x509parse_crt:"3081b130819ba0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D01010105000319003016021100ffffffffffffffffffffffffffffffff020103a100a200a315301330110603551d13010101040730050201010201300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_OUT_OF_DATA)
+x509parse_crt:"3081b130819ba0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D01010105000319003016021100ffffffffffffffffffffffffffffffff020103a100a200a315301330110603551d13010101040730050101ff0201300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_OUT_OF_DATA)
 
 X509 CRT ASN1 (TBS, inv extBasicConstraint, pathlen empty)
 depends_on:PSA_HAVE_ALG_SOME_RSA_VERIFY:PSA_WANT_ALG_SHA_256
-x509parse_crt:"3081b130819ba0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D01010105000319003016021100ffffffffffffffffffffffffffffffff020103a100a200a315301330110603551d13010101040730050201010200300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_INVALID_LENGTH)
+x509parse_crt:"3081b130819ba0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D01010105000319003016021100ffffffffffffffffffffffffffffffff020103a100a200a315301330110603551d13010101040730050101ff0200300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_INVALID_LENGTH)
 
 X509 CRT ASN1 (TBS, inv extBasicConstraint, pathlen length mismatch)
 depends_on:PSA_HAVE_ALG_SOME_RSA_VERIFY:PSA_WANT_ALG_SHA_256
-x509parse_crt:"3081b430819ea0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D01010105000319003016021100ffffffffffffffffffffffffffffffff020103a100a200a318301630140603551d13010101040a30080201010201010500300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_LENGTH_MISMATCH)
+x509parse_crt:"3081b430819ea0030201028204deadbeef300d06092a864886f70d01010b0500300c310a30080600130454657374301c170c303930313031303030303030170c303931323331323335393539300c310a30080600130454657374302a300d06092A864886F70D01010105000319003016021100ffffffffffffffffffffffffffffffff020103a100a200a318301630140603551d13010101040a30080101ff0201010500300d06092a864886f70d01010b0500030200ff":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_LENGTH_MISMATCH)
 
 X509 CRT ASN1 (TBS, inv v3Ext, ExtKeyUsage bad second tag)
 depends_on:PSA_HAVE_ALG_SOME_RSA_VERIFY:PSA_WANT_ALG_SHA_256
@@ -2947,6 +2963,10 @@ X509 CSR ASN.1 (OK)
 depends_on:PSA_HAVE_ALG_SOME_ECDSA:PSA_WANT_ECC_SECP_R1_256:PSA_WANT_ALG_SHA_1:!MBEDTLS_X509_REMOVE_INFO
 mbedtls_x509_csr_parse:"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":"CSR version   \: 1\nsubject name  \: C=NL, O=PolarSSL, CN=localhost\nsigned using  \: ECDSA with SHA1\nEC key size   \: 256 bits\n\nkey usage         \: Digital Signature, Non Repudiation, Key Encipherment\n":0
 
+X509 CSR ASN.1 (extensionRequest SET length too short)
+depends_on:PSA_HAVE_ALG_ECDSA_VERIFY:PSA_WANT_ECC_SECP_R1_256
+mbedtls_x509_csr_parse:"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":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_LENGTH_MISMATCH)
+
 X509 CSR ASN.1 (Unsupported critical extension, critical=true)
 depends_on:PSA_HAVE_ALG_SOME_ECDSA:PSA_WANT_ECC_SECP_R1_256:PSA_WANT_ALG_SHA_256:!MBEDTLS_X509_REMOVE_INFO
 mbedtls_x509_csr_parse:"308201233081cb02010030413119301706035504030c1053656c66207369676e65642074657374310b300906035504061302444531173015060355040a0c0e41757468437274444220546573743059301306072a8648ce3d020106082a8648ce3d03010703420004c11ebb9951848a436ca2c8a73382f24bbb6c28a92e401d4889b0c361f377b92a8b0497ff2f5a5f6057ae85f704ab1850bef075914f68ed3aeb15a1ff1ebc0dc6a028302606092a864886f70d01090e311930173015060b2b0601040183890c8622020101ff0403010101300a06082a8648ce3d040302034700304402200c4108fd098525993d3fd5b113f0a1ead8750852baf55a2f8e670a22cabc0ba1022034db93a0fcb993912adcf2ea8cb4b66389af30e264d43c0daea03255e45d2ccc":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_UNEXPECTED_TAG)
@@ -3082,17 +3102,17 @@ X509 CSR ASN.1 (attributes: invalid len (len > data))
 depends_on:PSA_WANT_ALG_SHA_256:PSA_HAVE_ALG_SOME_RSA_VERIFY
 mbedtls_x509_csr_parse_file:"../framework/data_files/parse_input/test_csr_v3_all_malformed_attributes_len1.csr.der":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_OUT_OF_DATA)
 
-X509 CSR ASN.1 (attributes: invalid len (len < data))
+X509 CSR ASN.1 (attributes: extensionRequest SET extends past attribute)
 depends_on:PSA_WANT_ALG_SHA_256:PSA_HAVE_ALG_SOME_RSA_VERIFY
-mbedtls_x509_csr_parse_file:"../framework/data_files/parse_input/test_csr_v3_all_malformed_attributes_len2.csr.der":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_LENGTH_MISMATCH)
+mbedtls_x509_csr_parse_file:"../framework/data_files/parse_input/test_csr_v3_all_malformed_attributes_len2.csr.der":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_OUT_OF_DATA)
 
 X509 CSR ASN.1 (attributes: extension request invalid len (len > data))
 depends_on:PSA_WANT_ALG_SHA_256:PSA_HAVE_ALG_SOME_RSA_VERIFY
 mbedtls_x509_csr_parse_file:"../framework/data_files/parse_input/test_csr_v3_all_malformed_attributes_extension_request_sequence_len1.csr.der":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_OUT_OF_DATA)
 
-X509 CSR ASN.1 (attributes: extension request invalid len (len < data))
+X509 CSR ASN.1 (attributes: extensionRequest sequence shorter than SET)
 depends_on:PSA_WANT_ALG_SHA_256:PSA_HAVE_ALG_SOME_RSA_VERIFY
-mbedtls_x509_csr_parse_file:"../framework/data_files/parse_input/test_csr_v3_all_malformed_attributes_extension_request_sequence_len2.csr.der":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_OUT_OF_DATA)
+mbedtls_x509_csr_parse_file:"../framework/data_files/parse_input/test_csr_v3_all_malformed_attributes_extension_request_sequence_len2.csr.der":"":MBEDTLS_ERROR_ADD(MBEDTLS_ERR_X509_INVALID_EXTENSIONS, MBEDTLS_ERR_ASN1_LENGTH_MISMATCH)
 
 X509 CSR ASN.1 (extensions: invalid sequence tag)
 depends_on:PSA_WANT_ALG_SHA_256:PSA_HAVE_ALG_SOME_RSA_VERIFY