*Redis Security

This document provides an introduction to the topic of security from the point of view of Redis: the access control provided by Redis, code security concerns, attacks that can be triggered from the outside by selecting malicious inputs and other similar topics are covered.

For security related contacts please open an issue on GitHub, or when you feel it is really important that the security of the communication is preserved, use the GPG key at the end of this document.

*Redis general security model

Redis is designed to be accessed by trusted clients inside trusted environments. This means that usually it is not a good idea to expose the Redis instance directly to the internet or, in general, to an environment where untrusted clients can directly access the Redis TCP port or UNIX socket.

For instance, in the common context of a web application implemented using Redis as a database, cache, or messaging system, the clients inside the front-end (web side) of the application will query Redis to generate pages or to perform operations requested or triggered by the web application user.

In this case, the web application mediates access between Redis and untrusted clients (the user browsers accessing the web application).

This is a specific example, but, in general, untrusted access to Redis should always be mediated by a layer implementing ACLs, validating user input, and deciding what operations to perform against the Redis instance.

*Network security

Access to the Redis port should be denied to everybody but trusted clients in the network, so the servers running Redis should be directly accessible only by the computers implementing the application using Redis.

In the common case of a single computer directly exposed to the internet, such as a virtualized Linux instance (Linode, EC2, ...), the Redis port should be firewalled to prevent access from the outside. Clients will still be able to access Redis using the loopback interface.

Note that it is possible to bind Redis to a single interface by adding a line like the following to the redis.conf file:

bind 127.0.0.1

Failing to protect the Redis port from the outside can have a big security impact because of the nature of Redis. For instance, a single FLUSHALL command can be used by an external attacker to delete the whole data set.

*Protected mode

Unfortunately many users fail to protect Redis instances from being accessed from external networks. Many instances are simply left exposed on the internet with public IPs. For this reasons since version 3.2.0, when Redis is executed with the default configuration (binding all the interfaces) and without any password in order to access it, it enters a special mode called protected mode. In this mode Redis only replies to queries from the loopback interfaces, and reply to other clients connecting from other addresses with an error, explaining what is happening and how to configure Redis properly.

We expect protected mode to seriously decrease the security issues caused by unprotected Redis instances executed without proper administration, however the system administrator can still ignore the error given by Redis and just disable protected mode or manually bind all the interfaces.

*Authentication feature

While Redis does not try to implement Access Control, it provides a tiny layer of authentication that is optionally turned on editing the redis.conf file.

When the authorization layer is enabled, Redis will refuse any query by unauthenticated clients. A client can authenticate itself by sending the AUTH command followed by the password.

The password is set by the system administrator in clear text inside the redis.conf file. It should be long enough to prevent brute force attacks for two reasons:

  • Redis is very fast at serving queries. Many passwords per second can be tested by an external client.
  • The Redis password is stored inside the redis.conf file and inside the client configuration, so it does not need to be remembered by the system administrator, and thus it can be very long.

The goal of the authentication layer is to optionally provide a layer of redundancy. If firewalling or any other system implemented to protect Redis from external attackers fail, an external client will still not be able to access the Redis instance without knowledge of the authentication password.

The AUTH command, like every other Redis command, is sent unencrypted, so it does not protect against an attacker that has enough access to the network to perform eavesdropping.

*TLS support

Redis has optional support for TLS on all communication channels, including client connections, replication links and the Redis Cluster bus protocol.

*Disabling of specific commands

It is possible to disable commands in Redis or to rename them into an unguessable name, so that normal clients are limited to a specified set of commands.

For instance, a virtualized server provider may offer a managed Redis instance service. In this context, normal users should probably not be able to call the Redis CONFIG command to alter the configuration of the instance, but the systems that provide and remove instances should be able to do so.

In this case, it is possible to either rename or completely shadow commands from the command table. This feature is available as a statement that can be used inside the redis.conf configuration file. For example:

rename-command CONFIG b840fc02d524045429941cc15f59e41cb7be6c52

In the above example, the CONFIG command was renamed into an unguessable name. It is also possible to completely disable it (or any other command) by renaming it to the empty string, like in the following example:

rename-command CONFIG ""

*Attacks triggered by carefully selected inputs from external clients

There is a class of attacks that an attacker can trigger from the outside even without external access to the instance. An example of such attacks are the ability to insert data into Redis that triggers pathological (worst case) algorithm complexity on data structures implemented inside Redis internals.

For instance an attacker could supply, via a web form, a set of strings that are known to hash to the same bucket into a hash table in order to turn the O(1) expected time (the average time) to the O(N) worst case, consuming more CPU than expected, and ultimately causing a Denial of Service.

To prevent this specific attack, Redis uses a per-execution pseudo-random seed to the hash function.

Redis implements the SORT command using the qsort algorithm. Currently, the algorithm is not randomized, so it is possible to trigger a quadratic worst-case behavior by carefully selecting the right set of inputs.

*String escaping and NoSQL injection

The Redis protocol has no concept of string escaping, so injection is impossible under normal circumstances using a normal client library. The protocol uses prefixed-length strings and is completely binary safe.

Lua scripts executed by the EVAL and EVALSHA commands follow the same rules, and thus those commands are also safe.

While it would be a very strange use case, the application should avoid composing the body of the Lua script using strings obtained from untrusted sources.

*Code security

In a classical Redis setup, clients are allowed full access to the command set, but accessing the instance should never result in the ability to control the system where Redis is running.

Internally, Redis uses all the well known practices for writing secure code, to prevent buffer overflows, format bugs and other memory corruption issues. However, the ability to control the server configuration using the CONFIG command makes the client able to change the working dir of the program and the name of the dump file. This allows clients to write RDB Redis files at random paths, that is a security issue that may easily lead to the ability to compromise the system and/or run untrusted code as the same user as Redis is running.

Redis does not requires root privileges to run. It is recommended to run it as an unprivileged redis user that is only used for this purpose. The Redis authors are currently investigating the possibility of adding a new configuration parameter to prevent CONFIG SET/GET dir and other similar run-time configuration directives. This would prevent clients from forcing the server to write Redis dump files at arbitrary locations.

*GPG key

-----BEGIN PGP PUBLIC KEY BLOCK-----

mQINBF9FWioBEADfBiOE/iKpj2EF/cJ/KzFX+jSBKa8SKrE/9RE0faVF6OYnqstL
S5ox/o+yT45FdfFiRNDflKenjFbOmCbAdIys9Ta0iq6I9hs4sKfkNfNVlKZWtSVG
W4lI6zO2Zyc2wLZonI+Q32dDiXWNcCEsmajFcddukPevj9vKMTJZtF79P2SylEPq
mUuhMy/jOt7q1ibJCj5srtaureBH9662t4IJMFjsEe+hiZ5v071UiQA6Tp7rxLqZ
O6ZRzuamFP3xfy2Lz5NQ7QwnBH1ROabhJPoBOKCATCbfgFcM1Rj+9AOGfoDCOJKH
7yiEezMqr9VbDrEmYSmCO4KheqwC0T06lOLIQC4nnwKopNO/PN21mirCLHvfo01O
H/NUG1LZifOwAURbiFNF8Z3+L0csdhD8JnO+1nphjDHr0Xn9Vff2Vej030pRI/9C
SJ2s5fZUq8jK4n06sKCbqA4pekpbKyhRy3iuITKv7Nxesl4T/uhkc9ccpAvbuD1E
NczN1IH05jiMUMM3lC1A9TSvxSqflqI46TZU3qWLa9yg45kDC8Ryr39TY37LscQk
9x3WwLLkuHeUurnwAk46fSj7+FCKTGTdPVw8v7XbvNOTDf8vJ3o2PxX1uh2P2BHs
9L+E1P96oMkiEy1ug7gu8V+mKu5PAuD3QFzU3XCB93DpDakgtznRRXCkAQARAQAB
tBtSZWRpcyBMYWJzIDxyZWRpc0ByZWRpcy5pbz6JAk4EEwEKADgWIQR5sNCo1OBf
WO913l22qvOUq0evbgUCX0VaKgIbAwULCQgHAgYVCgkICwIEFgIDAQIeAQIXgAAK
CRC2qvOUq0evbpZaD/4rN7xesDcAG4ec895Fqzk3w74W1/K9lzRKZDwRsAqI+sAz
ZXvQMtWSxLfF2BITxLnHJXK5P+2Y6XlNgrn1GYwC1MsARyM9e1AzwDJHcXFkHU82
2aALIMXGtiZs/ejFh9ZSs5cgRlxBSqot/uxXm9AvKEByhmIeHPZse/Rc6e3qa57v
OhCkVZB4ETx5iZrgA+gdmS8N7MXG0cEu5gJLacG57MHi+2WMOCU9Xfj6+Pqhw3qc
E6lBinKcA/LdgUJ1onK0JCnOG1YVHjuFtaisfPXvEmUBGaSGE6lM4J7lass/OWps
Dd+oHCGI+VOGNx6AiBDZG8mZacu0/7goRnOTdljJ93rKkj31I+6+j4xzkAC0IXW8
LAP9Mmo9TGx0L5CaljykhW6z/RK3qd7dAYE+i7e8J9PuQaGG5pjFzuW4vY45j0V/
9JUMKDaGbU5choGqsCpAVtAMFfIBj3UQ5LCt5zKyescKCUb9uifOLeeQ1vay3R9o
eRSD52YpRBpor0AyYxcLur/pkHB0sSvXEfRZENQTohpY71rHSaFd3q1Hkk7lZl95
m24NRlrJnjFmeSPKP22vqUYIwoGNUF/D38UzvqHD8ltTPgkZc+Y+RRbVNqkQYiwW
GH/DigNB8r2sdkt+1EUu+YkYosxtzxpxxpYGKXYXx0uf+EZmRqRt/OSHKnf2GLkC
DQRfRVoqARAApffsrDNo4JWjX3r6wHJJ8IpwnGEJ2IzGkg8f1Ofk2uKrjkII/oIx
sXC3EeauC1Plhs+m9GP/SPY0LXmZ0OzGD/S1yMpmBeBuXJ0gONDo+xCg1pKGshPs
75XzpbggSOtEYR5S8Z46yCu7TGJRXBMGBhDgCfPVFBBNsnG5B0EeHXM4trqqlN6d
PAcwtLnKPz/Z+lloKR6bFXvYGuN5vjRXjcVYZLLCEwdV9iY5/Opqk9sCluasb3t/
c2gcsLWWFnNz2desvb/Y4ADJzxY+Um848DSR8IcdoArSsqmcCTiYvYC/UU7XPVNk
Jrx/HwgTVYiLGbtMB3u3fUpHW8SabdHc4xG3sx0LeIvl+JwHgx7yVhNYJEyOQfnE
mfS97x6surXgTVLbWVjXKIJhoWnWbLP4NkBc27H4qo8wM/IWH4SSXYNzFLlCDPnw
vQZSel21qxdqAWaSxkKcymfMS4nVDhVj0jhlcTY3aZcHMjqoUB07p5+laJr9CCGv
0Y0j0qT2aUO22A3kbv6H9c1Yjv8EI7eNz07aoH1oYU6ShsiaLfIqPfGYb7LwOFWi
PSl0dCY7WJg2H6UHsV/y2DwRr/3oH0a9hv/cvcMneMi3tpIkRwYFBPXEsIcoD9xr
RI5dp8BBdO/Nt+puoQq9oyialWnQK5+AY7ErW1yxjgie4PQ+XtN+85UAEQEAAYkC
NgQYAQoAIBYhBHmw0KjU4F9Y73XeXbaq85SrR69uBQJfRVoqAhsMAAoJELaq85Sr
R69uoV0QAIvlxAHYTjvH1lt5KbpVGs5gwIAnCMPxmaOXcaZ8V0Z1GEU+/IztwV+N
MYCBv1tYa7OppNs1pn75DhzoNAi+XQOVvU0OZgVJutthZe0fNDFGG9B4i/cxRscI
Ld8TPQQNiZPBZ4ubcxbZyBinE9HsYUM49otHjsyFZ0GqTpyne+zBf1GAQoekxlKo
tWSkkmW0x4qW6eiAmyo5lPS1bBjvaSc67i+6Bv5QkZa0UIkRqAzKN4zVvc2FyILz
+7wVLCzWcXrJt8dOeS6Y/Fjbhb6m7dtapUSETAKu6wJvSd9ndDUjFHD33NQIZ/nL
WaPbn01+e/PHtUDmyZ2W2KbcdlIT9nb2uHrruqdCN04sXkID8E2m2gYMA+TjhC0Q
JBJ9WPmdBeKH91R6wWDq6+HwOpgc/9na+BHZXMG+qyEcvNHB5RJdiu2r1Haf6gHi
Fd6rJ6VzaVwnmKmUSKA2wHUuUJ6oxVJ1nFb7Aaschq8F79TAfee0iaGe9cP+xUHL
zBDKwZ9PtyGfdBp1qNOb94sfEasWPftT26rLgKPFcroCSR2QCK5qHsMNCZL+u71w
NnTtq9YZDRaQ2JAc6VDZCcgu+dLiFxVIi1PFcJQ31rVe16+AQ9zsafiNsxkPdZcY
U9XKndQE028dGZv1E3S5BwpnikrUkWdxcYrVZ4fiNIy5I3My2yCe
=J9BD
-----END PGP PUBLIC KEY BLOCK-----