Redis security

This document provides an introduction to the topic of security from the point of view of Redis. It covers 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.

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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).

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. Since version 3.2.0, Redis enters a special mode called protected mode when it is executed with the default configuration (binding all the interfaces) and without any password in order to access it. In this mode, Redis only replies to queries from the loopback interfaces, and replies to clients connecting from other addresses with an error that explains the problem 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 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 optional authentication that is turned on by 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 in the redis.conf file and inside the client configuration. Since the system administrator does not need to remember it, the password 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.

Since the AUTH command, like every other Redis command, is sent unencrypted, 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.

Disallow specific commands

It is possible to disallow commands in Redis or to rename them as 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 disallow it (or any other command) by renaming it to the empty string, like in the following example:

rename-command CONFIG ""

Attacks triggered by malicious 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. For example, an attacker might insert data into Redis that triggers pathological (worst case) algorithm complexity on data structures implemented inside Redis internals.

An attacker could supply, via a web form, a set of strings that are known to hash to the same bucket in a hash table in order to turn the O(1) expected time (the average time) to the O(N) worst case. This can consume more CPU than expected and ultimately cause 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.

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

While it would be a strange use case, the application should avoid composing the body of the Lua script from 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 allows the client to change the working directory of the program and the name of the dump file. This allows clients to write RDB Redis files to random paths. This is a security issue that may lead to the ability to compromise the system and/or run untrusted code as the same user as Redis is running.

Redis does not require root privileges to run. It is recommended to run it as an unprivileged redis user that is only used for this purpose.

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