1 Peer SSL Certificate Verification
2 =================================
4 libcurl performs peer SSL certificate verification by default. This is done
5 by using CA cert bundle that the SSL library can use to make sure the peer's
6 server certificate is valid.
8 If you communicate with HTTPS or FTPS servers using certificates that are
9 signed by CAs present in the bundle, you can be sure that the remote server
10 really is the one it claims to be.
12 Until 7.18.0, curl bundled a severely outdated ca bundle file that was
13 installed by default. These days, the curl archives include no ca certs at
14 all. You need to get them elsewhere. See below for example.
16 If the remote server uses a self-signed certificate, if you don't install a CA
17 cert bundle, if the server uses a certificate signed by a CA that isn't
18 included in the bundle you use or if the remote host is an impostor
19 impersonating your favorite site, and you want to transfer files from this
20 server, do one of the following:
22 1. Tell libcurl to *not* verify the peer. With libcurl you disable this with
23 curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, FALSE);
25 With the curl command line tool, you disable this with -k/--insecure.
27 2. Get a CA certificate that can verify the remote server and use the proper
28 option to point out this CA cert for verification when connecting. For
29 libcurl hackers: curl_easy_setopt(curl, CURLOPT_CAPATH, capath);
31 With the curl command line tool: --cacert [file]
33 3. Add the CA cert for your server to the existing default CA cert bundle.
34 The default path of the CA bundle used can be changed by running configure
35 with the --with-ca-bundle option pointing out the path of your choice.
37 To do this, you need to get the CA cert for your server in PEM format and
38 then append that to your CA cert bundle.
40 If you use Internet Explorer, this is one way to get extract the CA cert
41 for a particular server:
43 o View the certificate by double-clicking the padlock
44 o Find out where the CA certificate is kept (Certificate>
45 Authority Information Access>URL)
46 o Get a copy of the crt file using curl
47 o Convert it from crt to PEM using the openssl tool:
48 openssl x509 -inform DES -in yourdownloaded.crt \
49 -out outcert.pem -text
50 o Append the 'outcert.pem' to the CA cert bundle or use it stand-alone
53 If you use the 'openssl' tool, this is one way to get extract the CA cert
54 for a particular server:
56 o openssl s_client -connect xxxxx.com:443 |tee logfile
57 o type "QUIT", followed by the "ENTER" key
58 o The certificate will have "BEGIN CERTIFICATE" and "END CERTIFICATE"
60 o If you want to see the data in the certificate, you can do: "openssl
61 x509 -inform PEM -in certfile -text -out certdata" where certfile is
62 the cert you extracted from logfile. Look in certdata.
63 o If you want to trust the certificate, you can append it to your
64 cert_bundle or use it stand-alone as described. Just remember that the
65 security is no better than the way you obtained the certificate.
67 4. If you're using the curl command line tool, you can specify your own CA
68 cert path by setting the environment variable CURL_CA_BUNDLE to the path
71 If you're using the curl command line tool on Windows, curl will search
72 for a CA cert file named "curl-ca-bundle.crt" in these directories and in
74 1. application's directory
75 2. current working directory
76 3. Windows System directory (e.g. C:\windows\system32)
77 4. Windows Directory (e.g. C:\windows)
78 5. all directories along %PATH%
80 5. Get a better/different/newer CA cert bundle! One option is to extract the
81 one a recent Firefox browser uses by running 'make ca-bundle' in the curl
82 build tree root, or possibly download a version that was generated this
85 http://curl.haxx.se/docs/caextract.html
87 Neglecting to use one of the above methods when dealing with a server using a
88 certificate that isn't signed by one of the certificates in the installed CA
89 cert bundle, will cause SSL to report an error ("certificate verify failed")
90 during the handshake and SSL will then refuse further communication with that
93 Peer SSL Certificate Verification with NSS
94 ==========================================
96 If libcurl is build with NSS support then depending on the OS distribution it
97 is probably required to take some additional steps to use the system-wide CA
98 cert db. RedHat ships with an additional module libnsspem.so which enables NSS
99 to read the OpenSSL PEM CA bundle. With OpenSuSE this lib is missing, and NSS
100 can only work with its own internal formats. Also NSS got a new database
102 https://wiki.mozilla.org/NSS_Shared_DB
103 Starting with version 7.19.7 libcurl will check for the NSS version it runs,
104 and add automatically the 'sql:' prefix to the certdb directory (either the
105 hardcoded default /etc/pki/nssdb or the directory configured with SSL_DIR
106 environment variable) if a version 3.12.0 or later is detected.
107 To check which certdb format your distribution provides examine the default
108 certdb location /etc/pki/nssdb; the new certdb format can be identified by
109 the filenames cert9.db, key4.db, pkcs11.txt; filenames of older versions are
110 cert8.db, key3.db, modsec.db.
111 Usually these cert databases are empty; but NSS also has built-in CAs which are
112 provided through a shared library libnssckbi.so; if you want to use these
113 built-in CAs then create a symlink to libnssckbi.so in /etc/pki/nssdb:
114 ln -s /usr/lib[64]/libnssckbi.so /etc/pki/nssdb/libnssckbi.so