diff --git a/crypto/asn1/x_bignum.c b/crypto/asn1/x_bignum.c
index efa2ade30f..aff286a563 100644
--- a/crypto/asn1/x_bignum.c
+++ b/crypto/asn1/x_bignum.c
@@ -114,13 +114,21 @@ static int bn_i2c(const ASN1_VALUE **pval, unsigned char *cont, int *putype,
 static int bn_c2i(ASN1_VALUE **pval, const unsigned char *cont, int len,
     int utype, char *free_cont, const ASN1_ITEM *it)
 {
+    int allocated = 0;
     BIGNUM *bn;
 
-    if (*pval == NULL && !bn_new(pval, it))
+    /* Reject encodings that imply a negative number. */
+    if (len == 0 || (*cont & 0x80) != 0) {
+        ERR_raise(ERR_LIB_ASN1, ASN1_R_INVALID_VALUE);
+        return 0;
+    }
+
+    if (*pval == NULL && (allocated = bn_new(pval, it)) == 0)
         return 0;
     bn = (BIGNUM *)*pval;
     if (!BN_bin2bn(cont, len, bn)) {
-        bn_free(pval, it);
+        if (allocated != 0)
+            bn_free(pval, it);
         return 0;
     }
     return 1;
@@ -129,15 +137,19 @@ static int bn_c2i(ASN1_VALUE **pval, const unsigned char *cont, int len,
 static int bn_secure_c2i(ASN1_VALUE **pval, const unsigned char *cont, int len,
     int utype, char *free_cont, const ASN1_ITEM *it)
 {
-    int ret;
+    int ret, allocated = 0;
     BIGNUM *bn;
 
-    if (*pval == NULL && !bn_secure_new(pval, it))
+    if (*pval == NULL && (allocated = bn_secure_new(pval, it)) == 0)
         return 0;
 
     ret = bn_c2i(pval, cont, len, utype, free_cont, it);
-    if (!ret)
+    if (!ret) {
+        if (allocated != 0)
+            bn_free(pval, it);
+
         return 0;
+    }
 
     /* Set constant-time flag for all secure BIGNUMS */
     bn = (BIGNUM *)*pval;
diff --git a/test/crltest.c b/test/crltest.c
index d2bbc1d9b8..0d81ac69db 100644
--- a/test/crltest.c
+++ b/test/crltest.c
@@ -638,27 +638,6 @@ static X509_CRL *crl_clear_err_parse(const char **pem)
     return CRL_from_strings(pem);
 }
 
-/*
- * Checks whether a specific error reason is present in the error stack.
- * This function iterates over the current thread's error queue using
- * ERR_get_error_all(), extracting all pending errors. If any of them match
- * the specified reason code (as returned by ERR_GET_REASON()), the function
- * returns 1 to indicate that the corresponding error was found.
- */
-static int err_chk(int lib, int reason)
-{
-#if defined(OPENSSL_NO_ERR) || defined(OPENSSL_SMALL_FOOTPRINT) || defined(OPENSSL_NO_DEPRECATED_3_0) || defined(OPENSSL_NO_HTTP)
-    return 1;
-#endif
-    unsigned long e;
-
-    while ((e = ERR_get_error_all(NULL, NULL, NULL, NULL, NULL)))
-        if (ERR_GET_LIB(e) == lib && ERR_GET_REASON(e) == reason)
-            return 1;
-
-    return 0;
-}
-
 static int test_crl_date_invalid(void)
 {
     X509_CRL *tmm = NULL, *tss = NULL, *utc = NULL;
diff --git a/test/testutil.h b/test/testutil.h
index f0b6c6ad5e..b79ebd6914 100644
--- a/test/testutil.h
+++ b/test/testutil.h
@@ -663,5 +663,13 @@ X509_CRL *CRL_from_strings(const char **pem);
  * into |*out| so we can free it.
  */
 BIO *glue2bio(const char **pem, char **out);
+/*
+ * Checks whether a specific error reason is present in the error stack.
+ * This function iterates over the current thread's error queue using
+ * ERR_get_error_all(), extracting all pending errors. If any of them match
+ * the specified reason code (as returned by ERR_GET_REASON()), the function
+ * returns 1 to indicate that the corresponding error was found.
+ */
+int err_chk(int lib, int reason);
 
 #endif /* OSSL_TESTUTIL_H */
diff --git a/test/testutil/load.c b/test/testutil/load.c
index 3af2ddd53d..d75e14983e 100644
--- a/test/testutil/load.c
+++ b/test/testutil/load.c
@@ -157,3 +157,22 @@ X509 *X509_from_strings(const char **pem)
     BIO_free(b);
     return x;
 }
+
+int err_chk(int lib, int reason)
+{
+#if defined(OPENSSL_NO_ERR) || defined(OPENSSL_SMALL_FOOTPRINT)
+    return 1;
+#endif
+#if defined(OPENSSL_NO_DEPRECATED_3_0) || defined(OPENSSL_NO_HTTP)
+    return 1;
+#endif
+
+    unsigned long e;
+
+    while ((e = ERR_get_error_all(NULL, NULL, NULL, NULL, NULL))) {
+        if (ERR_GET_LIB(e) == lib && ERR_GET_REASON(e) == reason)
+            return 1;
+    }
+
+    return 0;
+}
diff --git a/test/x509_internal_test.c b/test/x509_internal_test.c
index ec99024716..8482e6b47d 100644
--- a/test/x509_internal_test.c
+++ b/test/x509_internal_test.c
@@ -495,11 +495,43 @@ static int tests_X509_check_time(void)
     return do_x509_time_tests(cert_test_data, sizeof(cert_test_data) / sizeof(CERT_TEST_DATA));
 }
 
+static const char *kRSAModulusNeg[] = {
+    "-----BEGIN CERTIFICATE-----\n",
+    "MIIByjCCAXSgAwIBAgIQBjdsAKoAZIoRz7jUqlw19DANBgkqhkiG9w0BAQQFADAW\n",
+    "MRQwEgYDVQQDEwtSb290IEFnZW5jeTAeFw05NjA1MjgyMjAyNTlaFw0zOTEyMzEy\n",
+    "MzU5NTlaMBYxFDASBgNVBAMTC1Jvb3QgQWdlbmN5MFswDQYJKoZIhvcNAQEBBQAD\n",
+    "SgAwRwJAgVUiuYqkb+3W59lmD1W8183VvE5AAiGisfeHMIVe0vJEudybdbb7Rl9C\n",
+    "tp0jNgveVA/NvR+ZKhBYEctAy7WnQQIDAQABo4GeMIGbMFAGA1UEAwRJE0dGb3Ig\n",
+    "VGVzdGluZyBQdXJwb3NlcyBPbmx5IFNhbXBsZSBTb2Z0d2FyZSBQdWJsaXNoaW5n\n",
+    "IENyZWRlbnRpYWxzIEFnZW5jeTBHBgNVHQEEQDA+gBAS5AktBh0dTwCNYSHcFmRj\n",
+    "oRgwFjEUMBIGA1UEAxMLUm9vdCBBZ2VuY3mCEAY3bACqAGSKEc+41KpcNfQwDQYJ\n",
+    "KoZIhvcNAQEEBQADQQAtLj57iUKJP6ghF/rw9cOV22JpW8ncwbP68MRvb2Savecb\n",
+    "JWhyg2e9VrCNAb0q98xLvYeluocgTEIRQa0QFzuM\n",
+    "-----END CERTIFICATE-----\n",
+    NULL
+};
+
+static int tests_X509_check_crypto(void)
+{
+    X509 *rsa_n_neg = NULL;
+    EVP_PKEY *pub = NULL;
+    int test;
+
+    test = TEST_ptr((rsa_n_neg = X509_from_strings(kRSAModulusNeg)))
+        && TEST_ptr_null((pub = X509_get_pubkey(rsa_n_neg)))
+        && TEST_true(err_chk(ERR_LIB_EVP, EVP_R_DECODE_ERROR));
+
+    EVP_PKEY_free(pub);
+    X509_free(rsa_n_neg);
+    return test;
+}
+
 int setup_tests(void)
 {
     ADD_TEST(test_standard_exts);
     ADD_ALL_TESTS(test_a2i_ipaddress, OSSL_NELEM(a2i_ipaddress_tests));
     ADD_TEST(tests_X509_PURPOSE);
     ADD_TEST(tests_X509_check_time);
+    ADD_TEST(tests_X509_check_crypto);
     return 1;
 }