openssl: CURLSSLOPT_NO_PARTIALCHAIN can disable partial cert chains

Closes #4655

docs/libcurl/opts/CURLOPT_SSL_OPTIONS.3

@@ -28,23 +28,27 @@ CURLOPT_SSL_OPTIONS \- set SSL behavior options
 
 CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSL_OPTIONS, long bitmask);
 .SH DESCRIPTION
-Pass a long with a bitmask to tell libcurl about specific SSL behaviors.
-
-\fICURLSSLOPT_ALLOW_BEAST\fP tells libcurl to not attempt to use any
-workarounds for a security flaw in the SSL3 and TLS1.0 protocols.  If this
-option isn't used or this bit is set to 0, the SSL layer libcurl uses may use a
-work-around for this flaw although it might cause interoperability problems
-with some (older) SSL implementations. WARNING: avoiding this work-around
-lessens the security, and by setting this option to 1 you ask for exactly that.
-This option is only supported for DarwinSSL, NSS and OpenSSL.
-
-Added in 7.44.0:
-
-\fICURLSSLOPT_NO_REVOKE\fP tells libcurl to disable certificate revocation
-checks for those SSL backends where such behavior is present. This option is
-only supported for Schannel (the native Windows SSL library), with an
-exception in the case of Windows' Untrusted Publishers blacklist which it
-seems can't be bypassed.
+Pass a long with a bitmask to tell libcurl about specific SSL
+behaviors. Available bits:
+.IP CURLSSLOPT_ALLOW_BEAST
+Tells libcurl to not attempt to use any workarounds for a security flaw in the
+SSL3 and TLS1.0 protocols.  If this option isn't used or this bit is set to 0,
+the SSL layer libcurl uses may use a work-around for this flaw although it
+might cause interoperability problems with some (older) SSL
+implementations. WARNING: avoiding this work-around lessens the security, and
+by setting this option to 1 you ask for exactly that.  This option is only
+supported for DarwinSSL, NSS and OpenSSL.
+.IP CURLSSLOPT_NO_REVOKE
+Tells libcurl to disable certificate revocation checks for those SSL backends
+where such behavior is present. This option is only supported for Schannel
+(the native Windows SSL library), with an exception in the case of Windows'
+Untrusted Publishers blacklist which it seems can't be bypassed. (Added in
+7.44.0)
+.IP CURLSSLOPT_NO_PARTIALCHAIN
+Tells libcurl to not accept "partial" certificate chains, which it otherwise
+does by default. This option is only supported for OpenSSL and will fail the
+certificate verification if the chain ends with an intermediate certificate
+and not with a root cert. (Added in 7.68.0)
 .SH DEFAULT
 0
 .SH PROTOCOLS

docs/libcurl/symbols-in-versions

@@ -738,6 +738,7 @@ CURLSSLBACKEND_SCHANNEL         7.34.0
 CURLSSLBACKEND_SECURETRANSPORT  7.64.1
 CURLSSLBACKEND_WOLFSSL          7.49.0
 CURLSSLOPT_ALLOW_BEAST          7.25.0
+CURLSSLOPT_NO_PARTIALCHAIN      7.68.0
 CURLSSLOPT_NO_REVOKE            7.44.0
 CURLSSLSET_NO_BACKENDS          7.56.0
 CURLSSLSET_OK                   7.56.0

include/curl/curl.h

@@ -828,6 +828,10 @@ typedef enum {
    SSL backends where such behavior is present. */
 #define CURLSSLOPT_NO_REVOKE (1<<1)
 
+/* - NO_PARTIALCHAIN tells libcurl to *NOT* accept a partial certificate chain
+   if possible. The OpenSSL backend has this ability. */
+#define CURLSSLOPT_NO_PARTIALCHAIN (1<<2)
+
 /* The default connection attempt delay in milliseconds for happy eyeballs.
    CURLOPT_HAPPY_EYEBALLS_TIMEOUT_MS.3 and happy-eyeballs-timeout-ms.d document
    this value, keep them in sync. */

lib/setopt.c

@@ -2133,6 +2133,7 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
     data->set.ssl.enable_beast =
       (bool)((arg&CURLSSLOPT_ALLOW_BEAST) ? TRUE : FALSE);
     data->set.ssl.no_revoke = !!(arg & CURLSSLOPT_NO_REVOKE);
+    data->set.ssl.no_partialchain = !!(arg & CURLSSLOPT_NO_PARTIALCHAIN);
     break;
 
 #ifndef CURL_DISABLE_PROXY

lib/urldata.h

@@ -257,6 +257,7 @@ struct ssl_config_data {
   BIT(falsestart);
   BIT(enable_beast); /* allow this flaw for interoperability's sake*/
   BIT(no_revoke);    /* disable SSL certificate revocation checks */
+  BIT(no_partialchain); /* don't accept partial certificate chains */
 };
 
 struct ssl_general_config {

lib/vtls/openssl.c

@@ -2786,12 +2786,14 @@ static CURLcode ossl_connect_step1(struct connectdata *conn, int sockindex)
                          X509_V_FLAG_TRUSTED_FIRST);
 #endif
 #ifdef X509_V_FLAG_PARTIAL_CHAIN
-    /* Have intermediate certificates in the trust store be treated as
-       trust-anchors, in the same way as self-signed root CA certificates
-       are. This allows users to verify servers using the intermediate cert
-       only, instead of needing the whole chain. */
-    X509_STORE_set_flags(SSL_CTX_get_cert_store(BACKEND->ctx),
-                         X509_V_FLAG_PARTIAL_CHAIN);
+    if(!SSL_SET_OPTION(no_partialchain)) {
+      /* Have intermediate certificates in the trust store be treated as
+         trust-anchors, in the same way as self-signed root CA certificates
+         are. This allows users to verify servers using the intermediate cert
+         only, instead of needing the whole chain. */
+      X509_STORE_set_flags(SSL_CTX_get_cert_store(BACKEND->ctx),
+                           X509_V_FLAG_PARTIAL_CHAIN);
+    }
 #endif
   }