1 SSL Certificate Verification 2 ============================ 3 4 SSL is TLS 5 ---------- 6 7 SSL is the old name. It is called TLS these days. 8 9 10 Native SSL 11 ---------- 12 13 If libcurl was built with Schannel or Secure Transport support (the native SSL 14 libraries included in Windows and Mac OS X), then this does not apply to 15 you. Scroll down for details on how the OS-native engines handle SSL 16 certificates. If you're not sure, then run "curl -V" and read the results. If 17 the version string says "WinSSL" in it, then it was built with Schannel 18 support. 19 20 It is about trust 21 ----------------- 22 23 This system is about trust. In your local CA certificate store you have certs 24 from *trusted* Certificate Authorities that you then can use to verify that the 25 server certificates you see are valid. They're signed by one of the CAs you 26 trust. 27 28 Which CAs do you trust? You can decide to trust the same set of companies your 29 operating system trusts, or the set one of the known browsers trust. That's 30 basically trust via someone else you trust. You should just be aware that 31 modern operating systems and browsers are setup to trust *hundreds* of 32 companies and recent years several such CAs have been found untrustworthy. 33 34 Certificate Verification 35 ------------------------ 36 37 libcurl performs peer SSL certificate verification by default. This is done 38 by using a CA certificate store that the SSL library can use to make sure the 39 peer's server certificate is valid. 40 41 If you communicate with HTTPS, FTPS or other TLS-using servers using 42 certificates that are signed by CAs present in the store, you can be sure 43 that the remote server really is the one it claims to be. 44 45 If the remote server uses a self-signed certificate, if you don't install a CA 46 cert store, if the server uses a certificate signed by a CA that isn't 47 included in the store you use or if the remote host is an impostor 48 impersonating your favorite site, and you want to transfer files from this 49 server, do one of the following: 50 51 1. Tell libcurl to *not* verify the peer. With libcurl you disable this with 52 `curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, FALSE);` 53 54 With the curl command line tool, you disable this with -k/--insecure. 55 56 2. Get a CA certificate that can verify the remote server and use the proper 57 option to point out this CA cert for verification when connecting. For 58 libcurl hackers: `curl_easy_setopt(curl, CURLOPT_CAPATH, capath);` 59 60 With the curl command line tool: --cacert [file] 61 62 3. Add the CA cert for your server to the existing default CA certificate 63 store. The default CA certificate store can changed at compile time with the 64 following configure options: 65 66 --with-ca-bundle=FILE: use the specified file as CA certificate store. CA 67 certificates need to be concatenated in PEM format into this file. 68 69 --with-ca-path=PATH: use the specified path as CA certificate store. CA 70 certificates need to be stored as individual PEM files in this directory. 71 You may need to run c_rehash after adding files there. 72 73 If neither of the two options is specified, configure will try to auto-detect 74 a setting. It's also possible to explicitly not hardcode any default store 75 but rely on the built in default the crypto library may provide instead. 76 You can achieve that by passing both --without-ca-bundle and 77 --without-ca-path to the configure script. 78 79 If you use Internet Explorer, this is one way to get extract the CA cert 80 for a particular server: 81 82 - View the certificate by double-clicking the padlock 83 - Find out where the CA certificate is kept (Certificate> 84 Authority Information Access>URL) 85 - Get a copy of the crt file using curl 86 - Convert it from crt to PEM using the openssl tool: 87 openssl x509 -inform DES -in yourdownloaded.crt \ 88 -out outcert.pem -text 89 - Add the 'outcert.pem' to the CA certificate store or use it stand-alone 90 as described below. 91 92 If you use the 'openssl' tool, this is one way to get extract the CA cert 93 for a particular server: 94 95 - `openssl s_client -connect xxxxx.com:443 |tee logfile` 96 - type "QUIT", followed by the "ENTER" key 97 - The certificate will have "BEGIN CERTIFICATE" and "END CERTIFICATE" 98 markers. 99 - If you want to see the data in the certificate, you can do: "openssl 100 x509 -inform PEM -in certfile -text -out certdata" where certfile is 101 the cert you extracted from logfile. Look in certdata. 102 - If you want to trust the certificate, you can add it to your CA 103 certificate store or use it stand-alone as described. Just remember that 104 the security is no better than the way you obtained the certificate. 105 106 4. If you're using the curl command line tool, you can specify your own CA 107 cert path by setting the environment variable `CURL_CA_BUNDLE` to the path 108 of your choice. 109 110 If you're using the curl command line tool on Windows, curl will search 111 for a CA cert file named "curl-ca-bundle.crt" in these directories and in 112 this order: 113 1. application's directory 114 2. current working directory 115 3. Windows System directory (e.g. C:\windows\system32) 116 4. Windows Directory (e.g. C:\windows) 117 5. all directories along %PATH% 118 119 5. Get a better/different/newer CA cert bundle! One option is to extract the 120 one a recent Firefox browser uses by running 'make ca-bundle' in the curl 121 build tree root, or possibly download a version that was generated this 122 way for you: [CA Extract](https://curl.haxx.se/docs/caextract.html) 123 124 Neglecting to use one of the above methods when dealing with a server using a 125 certificate that isn't signed by one of the certificates in the installed CA 126 certificate store, will cause SSL to report an error ("certificate verify 127 failed") during the handshake and SSL will then refuse further communication 128 with that server. 129 130 Certificate Verification with NSS 131 --------------------------------- 132 133 If libcurl was built with NSS support, then depending on the OS distribution, 134 it is probably required to take some additional steps to use the system-wide 135 CA cert db. RedHat ships with an additional module, libnsspem.so, which 136 enables NSS to read the OpenSSL PEM CA bundle. On openSUSE you can install 137 p11-kit-nss-trust which makes NSS use the system wide CA certificate store. NSS 138 also has a new [database format](https://wiki.mozilla.org/NSS_Shared_DB). 139 140 Starting with version 7.19.7, libcurl automatically adds the 'sql:' prefix to 141 the certdb directory (either the hardcoded default /etc/pki/nssdb or the 142 directory configured with SSL_DIR environment variable). To check which certdb 143 format your distribution provides, examine the default certdb location: 144 /etc/pki/nssdb; the new certdb format can be identified by the filenames 145 cert9.db, key4.db, pkcs11.txt; filenames of older versions are cert8.db, 146 key3.db, secmod.db. 147 148 Certificate Verification with Schannel and Secure Transport 149 ----------------------------------------------------------- 150 151 If libcurl was built with Schannel (Microsoft's native TLS engine) or Secure 152 Transport (Apple's native TLS engine) support, then libcurl will still perform 153 peer certificate verification, but instead of using a CA cert bundle, it will 154 use the certificates that are built into the OS. These are the same 155 certificates that appear in the Internet Options control panel (under Windows) 156 or Keychain Access application (under OS X). Any custom security rules for 157 certificates will be honored. 158 159 Schannel will run CRL checks on certificates unless peer verification is 160 disabled. Secure Transport on iOS will run OCSP checks on certificates unless 161 peer verification is disabled. Secure Transport on OS X will run either OCSP 162 or CRL checks on certificates if those features are enabled, and this behavior 163 can be adjusted in the preferences of Keychain Access. 164
