CVE-2026-66035
Description
A flaw was found in libssh2. A malicious SSH server can exploit a pre-authentication heap buffer overflow vulnerability by sending a specially crafted packet during the Encrypt-then-MAC (ETM) cipher negotiation. This allows the server to corrupt heap memory, potentially leading to arbitrary code execution on the connecting client before authentication is complete.
Statement
This Moderate impact flaw in libssh2 allows a malicious SSH server to trigger a pre-authentication heap buffer overflow on a connecting client. During Encrypt-then-MAC (ETM) cipher negotiation, a specially crafted packet can corrupt heap memory, potentially leading to arbitrary code execution. Exploitation requires a client to connect to a compromised server, limiting the attack surface.
Mitigation
To reduce exposure, restrict SSH client connections to only trusted and verified SSH servers. Avoid initiating SSH connections to unknown or unverified hosts, as successful exploitation requires the client to connect to a malicious 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 | 5.3 | N/A | 7.5 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | High | N/A | High |
| Privileges Required | None | N/A | None |
| User Interaction | Required | N/A | Required |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | High |
| Integrity Impact | None | N/A | High |
| Availability Impact | High | N/A | High |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:H
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
Understanding the Weakness (CWE)
Integrity,Confidentiality,Availability
Technical Impact: Modify Memory; Execute Unauthorized Code or Commands
Buffer overflows often can be used to execute arbitrary code, which is usually outside the scope of the product's implicit security policy. This can often be used to subvert any other security service.
Availability
Technical Impact: Modify Memory; DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU)
Buffer overflows generally lead to crashes. Other attacks leading to lack of availability are possible, including putting the product into an infinite loop.
Frequently Asked Questions
Not sure what something means? Check out our Security Glossary.
Want to get errata notifications? Sign up here.
