CVE-2025-26466
Description
A flaw was found in the OpenSSH package. For each ping packet the SSH server receives, a pong packet is allocated in a memory buffer and stored in a queue of packages. It is only freed when the server/client key exchange has finished. A malicious client may keep sending such packages, leading to an uncontrolled increase in memory consumption on the server side. Consequently, the server may become unavailable, resulting in a denial of service attack.
Statement
The versions of OpenSSH as shipped with Red Hat Enterprise Linux 9 and older, and Red Hat Enterprise Linux CoreOS are not affected by this vulnerability as the OpenSSH versions distributed with those products are based on previous versions of OpenSSH's version that introduced the issue (openssh-9.5p1 upstream version).
Also this issue marked as a moderate severity vulnerability rather than important because, while it enables a pre-authentication denial-of-service (DoS) attack, it does not allow remote code execution, privilege escalation, or data exfiltration.
The attack is asymmetric in resource consumption, meaning an attacker can force excessive CPU and memory usage, but it still requires repeated connection attempts to sustain the impact. Moreover, OpenSSH provides built-in mitigation mechanisms, such as LoginGraceTime, MaxStartups, and PerSourcePenalties, which can significantly limit the attack’s effectiveness if properly configured.
Mitigation
This issue can be mitigated by setting the following three different options in the sshd configuration file located at: /etc/ssh/sshd_config
MaxStartups: Set to a reasonable value, this option controls the maximum number of concurrent unauthenticated connections the SSH server accepts;
PerSourcePenalties: Set its suboptions to a reasonable value, this option is used to help sshd to detect and drop connections that are potentially malicious for the SSH server;
LoginGraceTime: Set to a resonable value, this option controls how much time the SSH server will wait the client to authenticate before dropping its connection;
All the three option above needs to be set to implement a full mitigation for this vulnerability.
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).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 5.9 | 5.9 | 5.9 |
| Attack Vector | Network | Network | Network |
| Attack Complexity | High | High | High |
| Privileges Required | None | None | None |
| User Interaction | None | None | None |
| Scope | Unchanged | Unchanged | Unchanged |
| Confidentiality | None | None | None |
| Integrity Impact | None | None | None |
| Availability Impact | High | High | High |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory); DoS: Resource Consumption (Other)
When allocating resources without limits, an attacker could prevent other systems, applications, or processes from accessing the same type of resource. It can be easy for an attacker to consume many resources by rapidly making many requests or causing larger resources to be used than is needed.
Frequently Asked Questions
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