1SSL Certificate Verification 2============================ 3 4SSL is TLS 5---------- 6 7SSL is the old name. It is called TLS these days. 8 9 10Native SSL 11---------- 12 13If libcurl was built with Schannel or Secure Transport support (the native SSL 14libraries included in Windows and Mac OS X), then this does not apply to 15you. Scroll down for details on how the OS-native engines handle SSL 16certificates. If you're not sure, then run "curl -V" and read the results. If 17the version string says "WinSSL" in it, then it was built with Schannel 18support. 19 20It is about trust 21----------------- 22 23This system is about trust. In your local CA certificate store you have certs 24from *trusted* Certificate Authorities that you then can use to verify that the 25server certificates you see are valid. They're signed by one of the CAs you 26trust. 27 28Which CAs do you trust? You can decide to trust the same set of companies your 29operating system trusts, or the set one of the known browsers trust. That's 30basically trust via someone else you trust. You should just be aware that 31modern operating systems and browsers are setup to trust *hundreds* of 32companies and recent years several such CAs have been found untrustworthy. 33 34Certificate Verification 35------------------------ 36 37libcurl performs peer SSL certificate verification by default. This is done 38by using a CA certificate store that the SSL library can use to make sure the 39peer's server certificate is valid. 40 41If you communicate with HTTPS, FTPS or other TLS-using servers using 42certificates that are signed by CAs present in the store, you can be sure 43that the remote server really is the one it claims to be. 44 45If the remote server uses a self-signed certificate, if you don't install a CA 46cert store, if the server uses a certificate signed by a CA that isn't 47included in the store you use or if the remote host is an impostor 48impersonating your favorite site, and you want to transfer files from this 49server, 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 124Neglecting to use one of the above methods when dealing with a server using a 125certificate that isn't signed by one of the certificates in the installed CA 126certificate store, will cause SSL to report an error ("certificate verify 127failed") during the handshake and SSL will then refuse further communication 128with that server. 129 130Certificate Verification with NSS 131--------------------------------- 132 133If libcurl was built with NSS support, then depending on the OS distribution, 134it is probably required to take some additional steps to use the system-wide 135CA cert db. RedHat ships with an additional module, libnsspem.so, which 136enables NSS to read the OpenSSL PEM CA bundle. On openSUSE you can install 137p11-kit-nss-trust which makes NSS use the system wide CA certificate store. NSS 138also has a new [database format](https://wiki.mozilla.org/NSS_Shared_DB). 139 140Starting with version 7.19.7, libcurl automatically adds the 'sql:' prefix to 141the certdb directory (either the hardcoded default /etc/pki/nssdb or the 142directory configured with SSL_DIR environment variable). To check which certdb 143format your distribution provides, examine the default certdb location: 144/etc/pki/nssdb; the new certdb format can be identified by the filenames 145cert9.db, key4.db, pkcs11.txt; filenames of older versions are cert8.db, 146key3.db, secmod.db. 147 148Certificate Verification with Schannel and Secure Transport 149----------------------------------------------------------- 150 151If libcurl was built with Schannel (Microsoft's native TLS engine) or Secure 152Transport (Apple's native TLS engine) support, then libcurl will still perform 153peer certificate verification, but instead of using a CA cert bundle, it will 154use the certificates that are built into the OS. These are the same 155certificates that appear in the Internet Options control panel (under Windows) 156or Keychain Access application (under OS X). Any custom security rules for 157certificates will be honored. 158 159Schannel will run CRL checks on certificates unless peer verification is 160disabled. Secure Transport on iOS will run OCSP checks on certificates unless 161peer verification is disabled. Secure Transport on OS X will run either OCSP 162or CRL checks on certificates if those features are enabled, and this behavior 163can be adjusted in the preferences of Keychain Access. 164