1 SSL Certificate Verification
2 ============================
7 SSL is the old name. It is called TLS these days.
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
23 This system is about trust. In your local CA cert bundle you have certs from
24 *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
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.
34 Certificate Verification
35 ------------------------
37 libcurl performs peer SSL certificate verification by default. This is done
38 by using CA cert bundle that the SSL library can use to make sure the peer's
39 server certificate is valid.
41 If you communicate with HTTPS, FTPS or other TLS-using servers using
42 certificates that are signed by CAs present in the bundle, you can be sure
43 that the remote server really is the one it claims to be.
45 If the remote server uses a self-signed certificate, if you don't install a CA
46 cert bundle, if the server uses a certificate signed by a CA that isn't
47 included in the bundle 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:
51 1. Tell libcurl to *not* verify the peer. With libcurl you disable this with
52 `curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, FALSE);`
54 With the curl command line tool, you disable this with -k/--insecure.
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);`
60 With the curl command line tool: --cacert [file]
62 3. Add the CA cert for your server to the existing default CA cert bundle.
63 The default path of the CA bundle used can be changed by running configure
64 with the --with-ca-bundle option pointing out the path of your choice.
66 To do this, you need to get the CA cert for your server in PEM format and
67 then append that to your CA cert bundle.
69 If you use Internet Explorer, this is one way to get extract the CA cert
70 for a particular server:
72 - View the certificate by double-clicking the padlock
73 - Find out where the CA certificate is kept (Certificate>
74 Authority Information Access>URL)
75 - Get a copy of the crt file using curl
76 - Convert it from crt to PEM using the openssl tool:
77 openssl x509 -inform DES -in yourdownloaded.crt \
78 -out outcert.pem -text
79 - Append the 'outcert.pem' to the CA cert bundle or use it stand-alone
82 If you use the 'openssl' tool, this is one way to get extract the CA cert
83 for a particular server:
85 - `openssl s_client -connect xxxxx.com:443 |tee logfile`
86 - type "QUIT", followed by the "ENTER" key
87 - The certificate will have "BEGIN CERTIFICATE" and "END CERTIFICATE"
89 - If you want to see the data in the certificate, you can do: "openssl
90 x509 -inform PEM -in certfile -text -out certdata" where certfile is
91 the cert you extracted from logfile. Look in certdata.
92 - If you want to trust the certificate, you can append it to your
93 cert bundle or use it stand-alone as described. Just remember that the
94 security is no better than the way you obtained the certificate.
96 4. If you're using the curl command line tool, you can specify your own CA
97 cert path by setting the environment variable `CURL_CA_BUNDLE` to the path
100 If you're using the curl command line tool on Windows, curl will search
101 for a CA cert file named "curl-ca-bundle.crt" in these directories and in
103 1. application's directory
104 2. current working directory
105 3. Windows System directory (e.g. C:\windows\system32)
106 4. Windows Directory (e.g. C:\windows)
107 5. all directories along %PATH%
109 5. Get a better/different/newer CA cert bundle! One option is to extract the
110 one a recent Firefox browser uses by running 'make ca-bundle' in the curl
111 build tree root, or possibly download a version that was generated this
112 way for you: [CA Extract](http://curl.haxx.se/docs/caextract.html)
114 Neglecting to use one of the above methods when dealing with a server using a
115 certificate that isn't signed by one of the certificates in the installed CA
116 cert bundle, will cause SSL to report an error ("certificate verify failed")
117 during the handshake and SSL will then refuse further communication with that
120 Certificate Verification with NSS
121 ---------------------------------
123 If libcurl was built with NSS support, then depending on the OS distribution,
124 it is probably required to take some additional steps to use the system-wide
125 CA cert db. RedHat ships with an additional module, libnsspem.so, which
126 enables NSS to read the OpenSSL PEM CA bundle. This library is missing in
127 OpenSuSE, and without it, NSS can only work with its own internal formats. NSS
128 also has a new [database format](https://wiki.mozilla.org/NSS_Shared_DB).
130 Starting with version 7.19.7, libcurl automatically adds the 'sql:' prefix to
131 the certdb directory (either the hardcoded default /etc/pki/nssdb or the
132 directory configured with SSL_DIR environment variable). To check which certdb
133 format your distribution provides, examine the default certdb location:
134 /etc/pki/nssdb; the new certdb format can be identified by the filenames
135 cert9.db, key4.db, pkcs11.txt; filenames of older versions are cert8.db,
138 Certificate Verification with Schannel and Secure Transport
139 -----------------------------------------------------------
141 If libcurl was built with Schannel (Microsoft's native TLS engine) or Secure
142 Transport (Apple's native TLS engine) support, then libcurl will still perform
143 peer certificate verification, but instead of using a CA cert bundle, it will
144 use the certificates that are built into the OS. These are the same
145 certificates that appear in the Internet Options control panel (under Windows)
146 or Keychain Access application (under OS X). Any custom security rules for
147 certificates will be honored.
149 Schannel will run CRL checks on certificates unless peer verification is
150 disabled. Secure Transport on iOS will run OCSP checks on certificates unless
151 peer verification is disabled. Secure Transport on OS X will run either OCSP
152 or CRL checks on certificates if those features are enabled, and this behavior
153 can be adjusted in the preferences of Keychain Access.