CVE-2024-30156
Description
A flaw was found in the Varnish cache server, with HTTP/2 support enabled, that may allow a Denial of Service type of attack. A malicious actor can cause the server to run out of credits during the HTTP/2 connection control flow. As a consequence, the server will stop to properly process the active HTTP streams, retaining the already allocated resources, leading to resource starvation.
Statement
CVE-2024-30156 represents a important severity issue due to its potential to cause widespread denial of service (DoS) across Varnish Cacher servers with HTTP/2 protocol enabled. By exploiting this vulnerability, attackers can exhaust the server's HTTP/2 connection control flow window credits, thereby halting the processing of streams and indefinitely retaining associated resources. This can lead to a complete service outage, impacting the availability and performance of web services relying on Varnish Cache. Given the prevalence of HTTP/2 adoption for its performance benefits, the vulnerability poses a significant risk to web infrastructure, necessitating immediate mitigation measures to prevent exploitation and mitigate the impact on affected systems.
Mitigation
A possible mitigation for this issue is to disable http2 support until the package can be updated.
This can be performed by running the following command:
varnishadm param.set feature -http2
Note: you must remove `h2` from the list of protocols if your TLS terminator is advertising it with ALPN.
It's also possible to use the `MAIN.sc_bankrupt` counter to monitor possible on-going attacks to the varnish server.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 7.5 | N/A | 7.5 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | Low | N/A | Low |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | None |
| Integrity Impact | None | N/A | None |
| Availability Impact | High | N/A | High |
Vector
Red Hat: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
cve.org: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory); DoS: Resource Consumption (Other)
If an attacker can trigger the allocation of the limited resources, but the number or size of the resources is not controlled, then the most common result is denial of service. This would prevent valid users from accessing the product, and it could potentially have an impact on the surrounding environment, i.e., the product may slow down, crash due to unhandled errors, or lock out legitimate users. For example, a memory exhaustion attack against an application could slow down the application as well as its host operating system.
Access Control,Other
Technical Impact: Bypass Protection Mechanism; Other
In some cases it may be possible to force the product to "fail open" in the event of resource exhaustion. The state of the product -- and possibly the security functionality - may then be compromised.
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