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libssh CVE-2026-59845

| EUVDEUVD-2026-46178 MEDIUM
Detection of Error Condition Without Action (CWE-390)
2026-07-21 redhat GHSA-97fp-2vw9-q5jg
Denial Of Service Red Hat Enterprise Linux 10 Red Hat Enterprise Linux 8 Red Hat Enterprise Linux 9 Red Hat Hardened Images
5.9
CVSS 3.1 · NVD
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Severity by source

Vendor (redhat) PRIMARY
MEDIUM
qualitative
NVD
5.9 MEDIUM
AV:L/AC:L/PR:L/UI:R/S:C/C:N/I:N/A:H
vuln.today AI
5.3 MEDIUM

ProxyCommand config and fork() failure both required (AC:H, AV:L); low-privileged local account and active SSH initiation needed (PR:L, UI:R); signal propagation beyond libssh to caller's process tree drives S:C with availability-only impact.

3.1 AV:L/AC:H/PR:L/UI:R/S:C/C:N/I:N/A:H
4.0 AV:L/AC:H/AT:P/PR:L/UI:A/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H
SUSE
6.5 MEDIUM
AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H
Red Hat
5.3 MEDIUM
qualitative

Primary rating from Vendor (redhat).

CVSS VectorNVD

CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:C/C:N/I:N/A:H
Attack Vector
Local
Attack Complexity
Low
Privileges Required
Low
User Interaction
Required
Scope
Changed
Confidentiality
None
Integrity
None
Availability
High

Lifecycle Timeline

3
CVSS changed
Jul 30, 2026 - 13:22 NVD
5.3 (MEDIUM) 5.9 (MEDIUM)
Analysis Generated
Jul 21, 2026 - 12:03 vuln.today
CVE Published
Jul 21, 2026 - 11:26 cve.org
MEDIUM 5.3

DescriptionNVD

A flaw was found in libssh. When ProxyCommand is used, an unchecked fork() failure can be stored as process ID -1; during cleanup, signals may then be sent across the caller's accessible process tree, leading to local denial of service.

AnalysisAI

Signal misdelivery in libssh's ProxyCommand handling allows a low-privileged local user to trigger a local denial of service affecting their entire accessible process tree. When fork() fails during ProxyCommand execution, the error return value of -1 is stored unchecked as a child process ID; at cleanup, libssh signals PID -1, which Linux interprets as a broadcast to all processes the caller has permission to signal. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

Recon
Gain low-privileged local account access
Delivery
Leverage or configure ProxyCommand in SSH client config
Exploit
Exhaust process slots to induce fork() failure
Install
libssh stores return value -1 as child PID unchecked
C2
SSH cleanup issues kill(-1, sig)
Execute
Signal broadcasts to all user-accessible processes
Impact
Local denial of service across process tree
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Vulnerability AssessmentAI

Exploitation Three concurrent conditions are required: (1) ProxyCommand must be active in the SSH client configuration - either in the connecting user's ~/.ssh/config or in the system-wide /etc/ssh/ssh_config - this is a non-default configuration absent in most standard deployments; (2) a fork() failure must occur during ProxyCommand subprocess creation, which requires the system or per-user process count to be at or near its limit (natural exhaustion or attacker-induced via fork-bombing within their own process quota); (3) a low-privileged local account (PR:L) must actively initiate an SSH connection using the ProxyCommand path (UI:R). … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment The CVSS 3.1 base score of 5.3 Medium accurately reflects the constrained exploitation path: AV:L limits impact to users with local system access, AC:H reflects that ProxyCommand must be configured and a fork() failure must coincide with connection handling, PR:L requires a low-privileged account, and UI:R requires the user to actually initiate an SSH connection. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario A low-privileged local user on a multi-tenant RHEL system that has ProxyCommand configured (e.g., for jump-host access) initiates an SSH connection while the system is under process table pressure - either naturally occurring or induced by the attacker exhausting their own RLIMIT_NPROC quota. libssh's fork() call fails and returns -1, which is stored unchecked as the child PID; when the SSH session tears down and cleanup signals the 'child', Linux broadcasts the signal to all processes the attacker's UID can reach, terminating the user's own shells, background jobs, or any co-owned services.
Remediation No specific patched libssh package version is confirmed in the available data; monitor the Red Hat advisory at https://access.redhat.com/security/cve/CVE-2026-59845 and the Bugzilla entry at https://bugzilla.redhat.com/show_bug.cgi?id=2498178 for errata providing fixed packages for RHEL 8, 9, 10, and Hardened Images - apply the errata update as soon as it is released. … Detailed patch versions, workarounds, and compensating controls in full report.
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Vendor StatusVendor

SUSE

Severity: Moderate
Product Status
SUSE Linux Enterprise Module for Basesystem 15 SP7 Affected
SUSE Linux Enterprise Server 15 SP7 Affected
SUSE Linux Enterprise Desktop 15 SP7 Affected
SUSE Linux Enterprise Server for SAP Applications 15 SP7 Affected
SUSE Linux Enterprise High Performance Computing 15 SP7 Affected

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CVE-2026-59845 vulnerability details – vuln.today
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