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.TH libcurl-security 3 "13 Feb 2018" "libcurl" "libcurl security"
.SH NAME
libcurl-security \- security considerations when using libcurl
.SH "Security"
The libcurl project takes security seriously.  The library is written with
caution and precautions are taken to mitigate many kinds of risks encountered
while operating with potentially malicious servers on the Internet. It is a
powerful library, however, which allows application writers to make trade-offs
between ease of writing and exposure to potential risky operations. If used
the right way, you can use libcurl to transfer data pretty safely.

Many applications are used in closed networks where users and servers can
(possibly) be trusted, but many others are used on arbitrary servers and are
fed input from potentially untrusted users.  Following is a discussion about
some risks in the ways in which applications commonly use libcurl and
potential mitigations of those risks. It is by no means comprehensive, but
shows classes of attacks that robust applications should consider. The Common
Weakness Enumeration project at https://cwe.mitre.org/ is a good reference for
many of these and similar types of weaknesses of which application writers
should be aware.
.SH "Command Lines"
If you use a command line tool (such as curl) that uses libcurl, and you give
options to the tool on the command line those options can very likely get read
by other users of your system when they use 'ps' or other tools to list
currently running processes.

To avoid these problems, never feed sensitive things to programs using command
line options. Write them to a protected file and use the \-K option to avoid
this.
.SH ".netrc"
\&.netrc is a pretty handy file/feature that allows you to login quickly and
automatically to frequently visited sites. The file contains passwords in
clear text and is a real security risk. In some cases, your .netrc is also
stored in a home directory that is NFS mounted or used on another network
based file system, so the clear text password will fly through your network
every time anyone reads that file!

For applications that enable .netrc use, a user who manage to set the right
URL might then be possible to pass on passwords.

To avoid these problems, don't use .netrc files and never store passwords in
plain text anywhere.
.SH "Clear Text Passwords"
Many of the protocols libcurl supports send name and password unencrypted as
clear text (HTTP Basic authentication, FTP, TELNET etc). It is very easy for
anyone on your network or a network nearby yours to just fire up a network
analyzer tool and eavesdrop on your passwords. Don't let the fact that HTTP
Basic uses base64 encoded passwords fool you. They may not look readable at a
first glance, but they very easily "deciphered" by anyone within seconds.

To avoid this problem, use an authentication mechanism or other protocol that
doesn't let snoopers see your password: Digest, CRAM-MD5, Kerberos, SPNEGO or
NTLM authentication. Or even better: use authenticated protocols that protect
the entire connection and everything sent over it.
.SH "Un-authenticated Connections"
Protocols that don't have any form of cryptographic authentication cannot
with any certainty know that they communicate with the right remote server.

If your application is using a fixed scheme or fixed host name, it is not safe
as long as the connection is un-authenticated. There can be a
man-in-the-middle or in fact the whole server might have been replaced by an
evil actor.

Un-authenticated protocols are unsafe. The data that comes back to curl may
have been injected by an attacker. The data that curl sends might be modified
before it reaches the intended server. If it even reaches the intended server
at all.

Remedies:
.IP "Restrict operations to authenticated transfers"
Ie use authenticated protocols protected with HTTPS or SSH.
.IP "Make sure the server's certificate etc is verified"
Never ever switch off certificate verification.
.SH "Redirects"
The \fICURLOPT_FOLLOWLOCATION(3)\fP option automatically follows HTTP
redirects sent by a remote server.  These redirects can refer to any kind of
URL, not just HTTP. libcurl restricts the protocols allowed to be used in
redirects for security reasons: only HTTP, HTTPS, FTP and FTPS are
enabled by default. Applications may opt to restrict that set further.

A redirect to a file: URL would cause the libcurl to read (or write) arbitrary
files from the local filesystem.  If the application returns the data back to
the user (as would happen in some kinds of CGI scripts), an attacker could
leverage this to read otherwise forbidden data (e.g.
file://localhost/etc/passwd).

If authentication credentials are stored in the ~/.netrc file, or Kerberos
is in use, any other URL type (not just file:) that requires
authentication is also at risk.  A redirect such as
ftp://some-internal-server/private-file would then return data even when
the server is password protected.

In the same way, if an unencrypted SSH private key has been configured for the
user running the libcurl application, SCP: or SFTP: URLs could access password
or private-key protected resources,
e.g. sftp://user@some-internal-server/etc/passwd

The \fICURLOPT_REDIR_PROTOCOLS(3)\fP and \fICURLOPT_NETRC(3)\fP options can be
used to mitigate against this kind of attack.

A redirect can also specify a location available only on the machine running
libcurl, including servers hidden behind a firewall from the attacker.
e.g. http://127.0.0.1/ or http://intranet/delete-stuff.cgi?delete=all or
tftp://bootp-server/pc-config-data

Applications can mitigate against this by disabling
\fICURLOPT_FOLLOWLOCATION(3)\fP and handling redirects itself, sanitizing URLs
as necessary. Alternately, an app could leave \fICURLOPT_FOLLOWLOCATION(3)\fP
enabled but set \fICURLOPT_REDIR_PROTOCOLS(3)\fP and install a
\fICURLOPT_OPENSOCKETFUNCTION(3)\fP callback function in which addresses are
sanitized before use.
.SH "Local Resources"
A user who can control the DNS server of a domain being passed in within a URL
can change the address of the host to a local, private address which a
server-side libcurl-using application could then use. e.g. the innocuous URL
http://fuzzybunnies.example.com/ could actually resolve to the IP address of a
server behind a firewall, such as 127.0.0.1 or 10.1.2.3.  Applications can
mitigate against this by setting a \fICURLOPT_OPENSOCKETFUNCTION(3)\fP and
checking the address before a connection.

All the malicious scenarios regarding redirected URLs apply just as well to
non-redirected URLs, if the user is allowed to specify an arbitrary URL that
could point to a private resource. For example, a web app providing a
translation service might happily translate file://localhost/etc/passwd and
display the result.  Applications can mitigate against this with the
\fICURLOPT_PROTOCOLS(3)\fP option as well as by similar mitigation techniques
for redirections.

A malicious FTP server could in response to the PASV command return an IP
address and port number for a server local to the app running libcurl but
behind a firewall.  Applications can mitigate against this by using the
\fICURLOPT_FTP_SKIP_PASV_IP(3)\fP option or \fICURLOPT_FTPPORT(3)\fP.

Local servers sometimes assume local access comes from friends and trusted
users. An application that expects https://example.com/file_to_read that and
instead gets http://192.168.0.1/my_router_config might print a file that would
otherwise be protected by the firewall.

Allowing your application to connect to local hosts, be it the same machine
that runs the application or a machine on the same local network, might be
possible to exploit by an attacker who then perhaps can "port-scan" the
particular hosts - depending on how the application and servers acts.
.SH "IPv6 Addresses"
libcurl will normally handle IPv6 addresses transparently and just as easily
as IPv4 addresses. That means that a sanitizing function that filters out
addresses like 127.0.0.1 isn't sufficient--the equivalent IPv6 addresses ::1,
::, 0:00::0:1, ::127.0.0.1 and ::ffff:7f00:1 supplied somehow by an attacker
would all bypass a naive filter and could allow access to undesired local
resources.  IPv6 also has special address blocks like link-local and
site-local that generally shouldn't be accessed by a server-side libcurl-using
application.  A poorly-configured firewall installed in a data center,
organization or server may also be configured to limit IPv4 connections but
leave IPv6 connections wide open.  In some cases, setting
\fICURLOPT_IPRESOLVE(3)\fP to CURL_IPRESOLVE_V4 can be used to limit resolved
addresses to IPv4 only and bypass these issues.
.SH Uploads
When uploading, a redirect can cause a local (or remote) file to be
overwritten. Applications must not allow any unsanitized URL to be passed in
for uploads. Also, \fICURLOPT_FOLLOWLOCATION(3)\fP should not be used on
uploads.  Instead, the applications should consider handling redirects itself,
sanitizing each URL first.
.SH Authentication
Use of \fICURLOPT_UNRESTRICTED_AUTH(3)\fP could cause authentication
information to be sent to an unknown second server.  Applications can mitigate
against this by disabling \fICURLOPT_FOLLOWLOCATION(3)\fP and handling
redirects itself, sanitizing where necessary.

Use of the CURLAUTH_ANY option to \fICURLOPT_HTTPAUTH(3)\fP could result in
user name and password being sent in clear text to an HTTP server.  Instead,
use CURLAUTH_ANYSAFE which ensures that the password is encrypted over the
network, or else fail the request.

Use of the CURLUSESSL_TRY option to \fICURLOPT_USE_SSL(3)\fP could result in
user name and password being sent in clear text to an FTP server.  Instead,
use CURLUSESSL_CONTROL to ensure that an encrypted connection is used or else
fail the request.
.SH Cookies
If cookies are enabled and cached, then a user could craft a URL which
performs some malicious action to a site whose authentication is already
stored in a cookie. e.g. http://mail.example.com/delete-stuff.cgi?delete=all
Applications can mitigate against this by disabling cookies or clearing them
between requests.
.SH "Dangerous SCP URLs"
SCP URLs can contain raw commands within the scp: URL, which is a side effect
of how the SCP protocol is designed. e.g.

  scp://user:pass@host/a;date >/tmp/test;

Applications must not allow unsanitized SCP: URLs to be passed in for
downloads.
.SH "file://"
By default curl and libcurl support file:// URLs. Such a URL is always an
access, or attempted access, to a local resource. If your application wants to
avoid that, keep control of what URLs to use and/or prevent curl/libcurl from
using the protocol.

By default, libcurl prohibits redirects to file:// URLs.

.SH "Warning: file:// on Windows"
The Windows operating system will automatically, and without any way for
applications to disable it, try to establish a connection to another host over
the network and access it (over SMB or other protocols), if only the correct
file path is accessed.

When first realizing this, the curl team tried to filter out such attempts in
order to protect applications for inadvertent probes of for example internal
networks etc. This resulted in CVE-2019-15601 and the associated security fix.

However, we've since been made aware of the fact that the previous fix was far
from adequate as there are several other ways to accomplish more or less the
same thing: accessing a remote host over the network instead of the local file
system.

The conclusion we have come to is that this is a weakness or feature in the
Windows operating system itself, that we as an application cannot safely
protect users against. It would just be a whack-a-mole race we don't want to
participate in. There are too many ways to do it and there's no knob we can
use to turn off the practice.

If you use curl or libcurl on Windows (any version), disable the use of the
FILE protocol in curl or be prepared that accesses to a range of "magic paths"
will potentially make your system try to access other hosts on your
network. curl cannot protect you against this.
.SH "What if the user can set the URL"
Applications may find it tempting to let users set the URL that it can work
on. That's probably fine, but opens up for mischief and trickery that you as
an application author may want to address or take precautions against.

If your curl-using script allow a custom URL do you also, perhaps
unintentionally, allow the user to pass other options to the curl command line
if creative use of special characters are applied?

If the user can set the URL, the user can also specify the scheme part to
other protocols that you didn't intend for users to use and perhaps didn't
consider. curl supports over 20 different URL schemes. "http://" might be what
you thought, "ftp://" or "imap://" might be what the user gives your
application. Also, cross-protocol operations might be done by using a
particular scheme in the URL but point to a server doing a different protocol
on a non-standard port.

Remedies:
.IP "Use --proto"
curl command lines can use \fI--proto\fP to limit what URL schemes it accepts
.IP "Use CURLOPT_PROTOCOLS"
libcurl programs can use \fICURLOPT_PROTOCOLS(3)\fP to limit what URL schemes it accepts
.IP "consider not allowing the user to set the full URL"
Maybe just let the user provide data for parts of it? Or maybe filter input to
only allow specific choices?
.SH "RFC 3986 vs WHATWG URL"
curl supports URLs mostly according to how they are defined in RFC 3986, and
has done so since the beginning.

Web browsers mostly adhere to the WHATWG URL Specification.

This deviance makes some URLs copied between browsers (or returned over HTTP
for redirection) and curl not work the same way. This can mislead users into
getting the wrong thing, connecting to the wrong host or otherwise not work
identically.
.SH "FTP uses two connections"
When performing an FTP transfer, two TCP connections are used: one for setting
up the transfer and one for the actual data.

FTP is not only un-authenticated, but the setting up of the second transfer is
also a weak spot. The second connection to use for data, is either setup with
the PORT/EPRT command that makes the server connect back to the client on the
given IP+PORT, or with PASV/EPSV that makes the server setup a port to listen
to and tells the client to connect to a given IP+PORT.

Again, un-authenticated means that the connection might be meddled with by a
man-in-the-middle or that there's a malicious server pretending to be the
right one.

A malicious FTP server can respond to PASV commands with the IP+PORT of a
totally different machine. Perhaps even a third party host, and when there are
many clients trying to connect to that third party, it could create a
Distributed Denial-Of-Service attack out of it! If the client makes an upload
operation, it can make the client send the data to another site. If the
attacker can affect what data the client uploads, it can be made to work as a
HTTP request and then the client could be made to issue HTTP requests to third
party hosts.

An attacker that manages to control curl's command line options can tell curl
to send an FTP PORT command to ask the server to connect to a third party host
instead of back to curl.

The fact that FTP uses two connections makes it vulnerable in a way that is
hard to avoid.
.SH "Denial of Service"
A malicious server could cause libcurl to effectively hang by sending data
very slowly, or even no data at all but just keeping the TCP connection open.
This could effectively result in a denial-of-service attack. The
\fICURLOPT_TIMEOUT(3)\fP and/or \fICURLOPT_LOW_SPEED_LIMIT(3)\fP options can
be used to mitigate against this.

A malicious server could cause libcurl to download an infinite amount of data,
potentially causing all of memory or disk to be filled. Setting the
\fICURLOPT_MAXFILESIZE_LARGE(3)\fP option is not sufficient to guard against
this.  Instead, applications should monitor the amount of data received within
the write or progress callback and abort once the limit is reached.

A malicious HTTP server could cause an infinite redirection loop, causing a
denial-of-service. This can be mitigated by using the
\fICURLOPT_MAXREDIRS(3)\fP option.
.SH "Arbitrary Headers"
User-supplied data must be sanitized when used in options like
\fICURLOPT_USERAGENT(3)\fP, \fICURLOPT_HTTPHEADER(3)\fP,
\fICURLOPT_POSTFIELDS(3)\fP and others that are used to generate structured
data. Characters like embedded carriage returns or ampersands could allow the
user to create additional headers or fields that could cause malicious
transactions.
.SH "Server-supplied Names"
A server can supply data which the application may, in some cases, use as a
file name. The curl command-line tool does this with
\fI--remote-header-name\fP, using the Content-disposition: header to generate
a file name.  An application could also use \fICURLINFO_EFFECTIVE_URL(3)\fP to
generate a file name from a server-supplied redirect URL. Special care must be
taken to sanitize such names to avoid the possibility of a malicious server
supplying one like "/etc/passwd", "\\autoexec.bat", "prn:" or even ".bashrc".
.SH "Server Certificates"
A secure application should never use the \fICURLOPT_SSL_VERIFYPEER(3)\fP
option to disable certificate validation. There are numerous attacks that are
enabled by applications that fail to properly validate server TLS/SSL
certificates, thus enabling a malicious server to spoof a legitimate
one. HTTPS without validated certificates is potentially as insecure as a
plain HTTP connection.
.SH "Report Security Problems"
Should you detect or just suspect a security problem in libcurl or curl,
contact the project curl security team immediately. See
https://curl.se/dev/secprocess.html for details.
.SH "Showing What You Do"
Relatedly, be aware that in situations when you have problems with libcurl and
ask someone for help, everything you reveal in order to get best possible help
might also impose certain security related risks. Host names, user names,
paths, operating system specifics, etc. (not to mention passwords of course)
may in fact be used by intruders to gain additional information of a potential
target.

Be sure to limit access to application logs if they could hold private or
security-related data.  Besides the obvious candidates like user names and
passwords, things like URLs, cookies or even file names could also hold
sensitive data.

To avoid this problem, you must of course use your common sense. Often, you
can just edit out the sensitive data or just search/replace your true
information with faked data.