libcurl-security.3: split out from libcurl-tutorial.3

  curl_mime_init.3 curl_mime_free.3 curl_mime_addpart.3 curl_mime_name.3 \

  curl_mime_data.3 curl_mime_data_cb.3 curl_mime_filedata.3              \

  curl_mime_filename.3 curl_mime_subparts.3                              \

  curl_mime_type.3 curl_mime_headers.3 curl_mime_encoder.3 libcurl-env.3

  curl_mime_type.3 curl_mime_headers.3 curl_mime_encoder.3 libcurl-env.3 \

  libcurl-security.3

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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 can not

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 include:

 - Restrict operations to authenticated transfers

 - Make sure the server's certificate etc is verified

.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: FILE, SCP, SMB and SMBS are disabled by

default. Applications are encouraged 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.

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 "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:

 - curl command lines can use \fI--proto\fP to limit what schemes it accepts

 - libcurl programs can use \fICURLOPT_PROTOCOLS(3)\fP

 - consider not allowing the user to set the full URL

 - consider strictly filtering input to only allow specific choices

.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 "Resport Security Problems"

Should you detect or just suspect a security problem in libcurl or curl,

contact the project curl security team immediately. See the separate

SECURITY.md document 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.