CVE-2021-29509 Keepalive Connections Causing Denial Of Service in puma
An attacker can exhaust server threads with keep-alive connections, denying service to legitimate users in Puma HTTP server.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2021-29509?
CVE-2021-29509 is a high-severity denial of service vulnerability in Debian Debian Linux, published on 11 May 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Puma is a concurrent HTTP 1.1 server for Ruby/Rack applications. The fix for CVE-2019-16770 was incomplete. The original fix only protected existing connections that had already been accepted from having their requests starved by greedy persistent-connections saturating all threads in the same process. However, new connections may still be starved by greedy persistent-connections saturating all threads in all processes in the cluster. A `puma` server which received more concurrent `keep-alive` connections than the server had threads in its threadpool would service only a subset of connections, denying service to the unserved connections. This problem has been fixed in `puma` 4.3.8 and 5.3.1. Setting `queue_requests false` also fixes the issue. This is not advised when using `puma` without a reverse proxy, such as `nginx` or `apache`, because you will open yourself to slow client attacks (e.g. slowloris). The fix is very small and a git patch is available for those using unsupported versions of Puma.
Which versions are affected by CVE-2021-29509?
4 open source packages ship an affected version, including puma. It affects 2 products.
| Package | Affected | Fixed in |
|---|---|---|
puma RubyGems | All versions | 4.3.8 |
puma RubyGems | All versions | 4.3.8 |
puma RubyGems | From 5.0.0 | 5.3.1 |
puma RubyGems | From 5.0.0 | 5.3.1 |
Linux distribution packages (2)
rubygem-puma SUSE:OpenStack Cloud Crowbar 8 | All versions | 2.16.0-3.12.1 |
rubygem-puma SUSE:OpenStack Cloud Crowbar 9 | All versions | 2.16.0-4.12.1 |
Products include Debian Debian Linux, Puma Puma.
How do I fix CVE-2021-29509?
Upgrade puma to a fixed version for your release line: 4.3.8 or 5.3.1.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2021-29509 being exploited?
There is no known exploitation of CVE-2021-29509 in the wild and no public exploit code.
Other vulnerabilities in Debian Debian Linux
Sources
Also known as DEBIAN-CVE-2021-29509, GHSA-q28m-8xjw-8vr5, OESA-2021-1203, SUSE-SU-2021:2761-1, SUSE-SU-2021:2914-1.