CVE-2026-128087
Description
A flaw was found in russh. During the initial key exchange, the library invokes client completion callbacks before verifying the server's host key or certificate. A malicious SSH server can exploit this issue to trigger early data processing or secondary-channel communication before establishing trust, potentially leading to unauthorized information disclosure.
Statement
This vulnerability is rated as having Low severity because exploitation requires an application to implement a custom key exchange completion handler rather than relying on default behavior, alongside connecting to a rogue server. Standard client implementations that retain default handlers or defer secondary actions until identity validation completes remain unaffected. Furthermore, the potential impact is limited to premature data exchange prior to connection termination, without enabling arbitrary code execution or privilege escalation.
Mitigation
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.
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 | 3.7 | N/A | 3.7 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | High | N/A | High |
| Privileges Required | None | N/A | None |
| User Interaction | None | N/A | None |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | Low | N/A | Low |
| Integrity Impact | None | N/A | None |
| Availability Impact | None | N/A | None |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N
Understanding the Weakness (CWE)
Access Control
Technical Impact: Bypass Protection Mechanism
No authentication takes place in the key exchange process itself, bypassing an assumed protection of encryption. This may enable an attacker to impersonate an actor by modifying traffic between the two entities. Typically, this involves a victim client that contacts a malicious server that is impersonating a trusted server. If the client skips authentication or ignores an authentication failure, the malicious server may request authentication information from the user. The malicious server can then use this authentication information to log in to the trusted server using the victim's credentials, sniff traffic between the victim and trusted server, etc.
Confidentiality
Technical Impact: Read Application Data
The encrypted communication between a user and a trusted host may be subject to sniffing by any actor in the communication path.
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