This advisory details 5 new security vulnerabilities discovered in the Prosody.im XMPP server software. All issues are fixed in the 0.11.9 release default configuration.
References
Description
It was discovered that default settings leave Prosody susceptible to remote unauthenticated denial-of-service (DoS) attacks via memory exhaustion when running under Lua 5.2 or Lua 5.3. Lua 5.2 is the default and recommended Lua version for Prosody 0.11.x series.
Affected configurations
The default configuration is susceptible to this issue.
Configurations with stricter settings for stanza size limits, rate limits and garbage collection parameters are at decreased risk from this attack. For more details please review the ‘Mitigation’ section for recommended values.
Mitigation
Mitigation is possible through configuration changes (on 0.11.7+). All the configuration changes described in this section are applied by default in Prosody 0.11.9.
Enable more aggressive garbage collection
On Lua 5.2 and 5.3, the garbage collector does not free unused memory fast enough by default. This allowed Prosody’s memory usage to grow excessively during certain traffic patterns.
It is recommended to set a garbage collection speed of at least 500 in the global section of your configuration file:
gc = {
speed = 500;
}Be aware that this setting may increase CPU usage if the other mitigations in this section are not applied.
Enable stricter stanza size limits
By default Prosody ships with extremely permissive stanza size limits (up to 10MB). This value was introduced as a way to place a limit on memory usage without affecting legitimate use of the server. However testing demonstrates that the default limit is too high for most deployments.
Our recommendation (and the default in 0.11.9) is to adopt the same default size limits that are already enforced by ejabberd, one of the other major XMPP servers on the network.
To enable the new limits explicitly, add to the global section of your configuration file the following options:
c2s_stanza_size_limit = 256 * 1024
s2s_stanza_size_limit = 512 * 1024Be aware that reducing limits has the potential to introduce interoperability issues with deployments that do not enforce the same size limits. For example, remote contacts with large avatars.
Enable rate limits
By default Prosody does not enable any rate limits. However we recommend enabling them for all production and public deployments to ensure fair consumption of resources across all connections.
First, ensure that mod_limits is enabled by adding “limits” to your global modules_enabled configuration option:
modules_enabled = {
...
"limits";
...
}Next, configure the limits:
limits = {
c2s = {
rate = "10kb/s";
};
s2sin = {
rate = "30kb/s";
}
}Advice
All public deployments should upgrade to 0.11.9 or apply the above configuration changes.
Deployments using nightly builds should upgrade to the latest available builds.
Credits
Many thanks to Travis Burtrum (moparisthebest) for discovering and reporting this issue, and providing a test case.
Commits
Description
It was discovered that Prosody does not disable SSL/TLS renegotiation, even though this is not used in XMPP. A malicious client may flood a connection with renegotiation requests to consume excessive CPU resources on the server.
Support for disabling renegotiation depends on OpenSSL 1.1.1+ and LuaSec 0.7+.
Affected configurations
The default configuration is susceptible to this issue.
Temporary mitigation
Ensure you have OpenSSL 1.1.1 or higher and LuaSec 0.7 or higher, and set the following ssl option (or add to your existing one if you have one):
ssl = {
options = {
no_renegotiation = true;
}
}This configuration is applied by default in 0.11.9.
Advice
All public deployments should upgrade to 0.11.9 or apply the above configuration changes.
Deployments using nightly builds should upgrade to the latest available builds.
Credits
This flaw was discovered by Kim Alvefur, a member of the Prosody team.
Commits
Description
It was discovered that Prosody does not use a constant-time algorithm for comparing certain secret strings when running under Lua 5.2 or later. This can potentially be used in a timing attack to reveal the contents of secret strings to an attacker.
Lua 5.1 utilizes a technique called “string interning”, which protected string comparisons from timing attacks. In Lua 5.2 and later versions, strings over 40 bytes in length are excluded from interning.
With Prosody running under Lua 5.2, this makes any secret string over 40 bytes in length vulnerable to potential discovery via timing attacks.
Note that if a secret string contains non-ASCII (unicode) characters, it may be longer than 40 bytes when encoded as UTF-8 (Prosody’s internal encoding) even if it is fewer than 40 characters long.
It should be noted that to successfully perform a timing attack, a significant number of failed attempts must typically be made to “guess” at the contents of the secret string.
We are not aware of any attempts to exploit this vulnerability (which would likely be noticeable), and no known proof-of-concept exploit exists.
Affected configurations
This flaw affects the following modules:
mod_auth_internal_plain (disabled by default)
mod_auth_internal_plain performs a timing-dependent comparison to the user’s password if the user’s password is longer than 40 bytes. This may allow an attacker to discover a user’s password via a timing attack.
We do not generally recommend mod_auth_internal_plain for new deployments, and mod_auth_internal_hashed has been the default for Prosody 0.11.x.
mod_muc (disabled by default)
mod_muc supports password-protection of MUCs. The password validity check is performed using a timing-dependent comparison, which may allow an attacker to discover the MUC password via a timing attack if the password is longer than 40 bytes.
We do not generally recommend using password-protected MUCs. Instead use affiliations to directly grant access to specific JIDs whenever possible.
mod_auth_internal_hashed (enabled by default but not typically vulnerable)
mod_auth_internal_hashed has been updated for safety, but it is not vulnerable in the default configuration of Lua 5.2 as the password hashes it compares do not exceed 40 bytes.
mod_dialback (enabled by default but not typically vulnerable)
mod_dialback has been updated for safety, but due to the single-use nature of s2s dialback verification strings a timing attack on this module is not believed to be possible, or to grant an attacker any advantage if it were.
Temporary mitigation
mod_auth_internal_plain: we recommend that people upgrade to mod_auth_internal_hashed due to this and also to benefit from its other security properties.
mod_muc: use affiliations to grant access to a MUC instead of passwords. If passwords must be used, ensure they are shorter than 40 bytes.
Rate limits can greatly lengthen the amount of time required to successfully complete a timing attack. Enable and configure mod_limits.
Advice
All deployments should upgrade to 0.11.9.
Deployments using nightly builds should upgrade to the latest available builds.
Credits
This flaw was discovered by Matthew Wild, a member of the Prosody team. The issue with MUC passwords was also previously identified by Marvin Zerulla and reported by Robert Grösser.
Commits
Description
mod_proxy65 is a file transfer proxy provided with Prosody to facilitate the transfer of files and other data between XMPP clients.
It was discovered that the proxy65 component of Prosody allows open access by default, even if neither of the users have an XMPP account on the local server, allowing unrestricted use of the server’s bandwidth.
Affected configurations
The default configuration does not enable mod_proxy65 and is not affected.
With mod_proxy65 enabled, all configurations without a ‘proxy65_acl’ setting configured are affected.
Temporary mitigation
Configure ‘proxy65_acl’ to a list of XMPP domains that should be allowed to use the file transfer proxy.
Advice
All deployments should upgrade to 0.11.9 and/or configure a ‘proxy65_acl’ as desired.
Deployments using nightly builds should upgrade to the latest available builds.
The default behaviour in 0.11.9 allows all local clients to initiate a data stream through the proxy if proxy65_acl is unconfigured.
Credits
This flaw was discovered by the Prosody team.
Commits
Description
The undocumented option ‘dialback_without_dialback’ enabled an experimental feature for server-to-server authentication. A flaw in this feature meant it did not correctly authenticate remote servers, allowing a remote server to impersonate another server when this option is enabled.
Affected configurations
The default configuration is not affected.
Configurations with the setting ‘dialback_without_dialback’ set to true are affected.
Temporary mitigation
Remove or disable the ‘dialback_without_dialback’ option.
Advice
All deployments should upgrade to 0.11.9 or disable this feature.
Deployments using nightly builds should upgrade to the latest available builds.
The affected feature has been removed in 0.11.9.
Credits
This flaw was discovered by the Prosody team.
Commits