An attacker, opening a HTTP/2 connection with an initial window size of 0, was able to block handling of that connection indefinitely in Apache HTTP Server. This could be used to exhaust worker resources in the server, similar to the well known slow loris attack pattern. This has been fixed in version 2.4.58, so that such connection are terminated properly after the configured connection timeout.
This issue affects Apache HTTP Server: from 2.4.55 through 2.4.57.
Users are recommended to upgrade to version 2.4.58, which fixes the issue.
Weakness
The product does not properly control the allocation and maintenance of a limited resource.
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Http_server | Apache | 2.4.55 (including) | 2.4.58 (excluding) |
| Red Hat Enterprise Linux 9 | RedHat | mod_http2-0:2.0.26-1.el9 | * |
| Apache2 | Ubuntu | bionic | * |
| Apache2 | Ubuntu | devel | * |
| Apache2 | Ubuntu | lunar | * |
| Apache2 | Ubuntu | mantic | * |
| Apache2 | Ubuntu | trusty | * |
| Apache2 | Ubuntu | upstream | * |
| Apache2 | Ubuntu | xenial | * |
Potential Mitigations
Mitigation of resource exhaustion attacks requires that the target system either:
The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question.
The second solution is simply difficult to effectively institute – and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.
Related Attack Patterns
- https://cwe.mitre.org/data/definitions/147.html
- https://cwe.mitre.org/data/definitions/227.html
- https://cwe.mitre.org/data/definitions/492.html