MediumCVE-2026-59843

Libssh: libssh: denial of service via zero advertised channel packet size (CVE-2026-59843)

Published 21.07.2026Last modified 01.09.2026Sources: BSI, Msrc Update Guide, CVE.org, Ubuntu, Red Hat, Debian, Debian LTS, SUSE

What is affected, what to do

A flaw was found in libssh. A remote authenticated peer can advertise a zero maximum packet size in SSH_MSG_CHANNEL_OPEN, causing later channel writes to loop indefinitely and consume CPU, leading to denial of service.

Update Red Hat Enterprise Linux 10 to version 0:0.12.0-3.el10_2, Red Hat Enterprise Linux 8 to version 0:0.9.6-17.el8_10, Red Hat Enterprise Linux 9 to version 0:0.10.4-19.el9_8.

Affected: Red Hat Enterprise Linux 10 lt *..0:0.12.0-3.el10_2
Red Hat Enterprise Linux 8 lt *..0:0.9.6-17.el8_10
Red Hat Enterprise Linux 9 lt *..0:0.10.4-19.el9_8
Red Hat Hardened Images lt *..0.12.1-4.hum1
Fixed in: Red Hat Enterprise Linux 10 0:0.12.0-3.el10_2
Red Hat Enterprise Linux 8 0:0.9.6-17.el8_10
Red Hat Enterprise Linux 9 0:0.10.4-19.el9_8
Red Hat Hardened Images 0.12.1-4.hum1

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Metrics

6.5

Priority

XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

Source: NVD, CVSS v3

6.5

CVSS v3

Severity Medium on the scale from 0 to 10.

Source: BSI, Msrc Update Guide, CVE.org, Ubuntu, Red Hat, Debian, Debian LTS, SUSE

0.7 %

EPSS score

Low — model estimates < 1% exploitation likelihood.

Source: FIRST EPSS, percentile 53.9

Show all metrics

Severity: Medium
Priority (0–325) The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first? Read in glossary: 77.03 Methodology
Exploit maturity An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack. Read in glossary: no public PoC known
XIEM Effective CVSS XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command. Read in glossary: 6.5 Methodology
CVSS v3 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 6.5
EPSS score The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“. Read in glossary: 0.7 %; Low — model estimates < 1% exploitation likelihood.
EPSS percentile The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking. Read in glossary: 53.9; High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
KEV status The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: —
Published: 21.07.2026
CWE CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect. Read in glossary: CWE-835

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2 NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management. Read in glossary: Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates. Read in glossary: From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001 ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security. Read in glossary: Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

Source: BSI WID Portal, BSI CSAF, Microsoft Security Update Guide, cvelistv5, NVD (NIST), Ubuntu Security Notices, redhat_csaf, Debian Security Advisories, CVE.org (MITRE), Debian LTS Advisories, suse_csaf

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
Red Hat	Red Hat Enterprise Linux 10	`< 0:0.12.0-3.el10_2`	`0:0.12.0-3.el10_2`
Red Hat	Red Hat Enterprise Linux 8	`< 0:0.9.6-17.el8_10`	`0:0.9.6-17.el8_10`
Red Hat	Red Hat Enterprise Linux 9	`< 0:0.10.4-19.el9_8`	`0:0.10.4-19.el9_8`
Red Hat	Red Hat Hardened Images	`< 0.12.1-4.hum1`	`0.12.1-4.hum1`
libssh	libssh	`—`	—
redhat	hardened_images	`—`	—
IBM	QRadar SIEM	`< 7.6.0.3` `<7.5.0 UP16 IF01`	`7.6.0.3` —
Open Source	libssh	`< 0.12.1`	`0.12.1`

Vendors and products

Affected operating systems

Affected operating systems

Linux	redhat	enterprise_linux	`10.0`
Linux	redhat	enterprise_linux	`8.0`
Linux	redhat	enterprise_linux	`9.0`
Linux	debian	libssh	`bookworm`
Linux	debian	libssh	`trixie`

Weakness classes (CWE)

CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
Base
The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.
Entry at MITRE CWE-835

References & sources

access.redhat.comsecurity/cve/CVE-2026-59843Vulnerability database, x_refsource_REDHAT
bugzilla.redhat.comRHBZ#2498176Issue tracking, x_refsource_REDHAT
access.redhat.comRHSA-2026:42922Vendor advisory, x_refsource_REDHAT
access.redhat.comRHSA-2026:55855Vendor advisory, x_refsource_REDHAT
access.redhat.comRHSA-2026:62217Vendor advisory, x_refsource_REDHAT
access.redhat.comRHSA-2026:62218Vendor advisory, x_refsource_REDHAT

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

01.09.2026Record modifiedRed Hat

New references:

https://access.redhat.com/errata/RHSA-2026:62217

Show 2 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Affected versions (publisher)	added	—	Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/59xxx/CVE-2026-59843.json">CVE-2026-59843</a>
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:62217

Source identifier: secalert@redhat.com

01.09.2026Record modifiedRed Hat

New references:

and 1 more reference

Show 13 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Affected versions (publisher)	added	—	Affected value modified. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/59xxx/CVE-2026-59843.json">CVE-2026-59843</a>
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:42922
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:55855
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:62218
Reference	added	—	https://access.redhat.com/security/cve/CVE-2026-59843
Reference	added	—	https://bugzilla.redhat.com/show_bug.cgi?id=2498176
Reference	removed	https://access.redhat.com/errata/RHSA-2026:42922	—
Reference	removed	https://access.redhat.com/errata/RHSA-2026:55855	—
Reference	removed	https://access.redhat.com/security/cve/CVE-2026-59843	—
Reference	removed	https://bugzilla.redhat.com/show_bug.cgi?id=2498176	—
Reference type	removed	https://access.redhat.com/errata/RHSA-2026:42922 Types: Vendor Advisory	—
Reference type	removed	https://access.redhat.com/security/cve/CVE-2026-59843 Types: Vendor Advisory	—
Reference type	removed	https://bugzilla.redhat.com/show_bug.cgi?id=2498176 Types: Issue Tracking, Vendor Advisory	—

Source identifier: secalert@redhat.com

17.08.2026Record modifiedRed Hat

New references:

https://access.redhat.com/errata/RHSA-2026:55855

Show 2 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:55855
Affected versions (publisher)	changed	Red Hat Hardened Images, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8 (+1)	Red Hat Enterprise Linux 10, Red Hat Hardened Images, Red Hat Enterprise Linux 8 (+1)

Source identifier: secalert@redhat.com

22.07.2026Record modifiedRed Hat

New references:

https://access.redhat.com/errata/RHSA-2026:42922

Show 2 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference	added	—	https://access.redhat.com/errata/RHSA-2026:42922
Affected versions (publisher)	changed	Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9 (+1)	Red Hat Hardened Images, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8 (+1)

Source identifier: secalert@redhat.com

21.07.2026ReceivedRed Hat

CVSS V3.1: AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Description added or changed

New references:

Show 5 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Affected versions (publisher)	added	—	Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9 (+1)
Description	added	—	A flaw was found in libssh. A remote authenticated peer can advertise a zero maximum packet size in SSH_MSG_CHANNEL_OPEN, causing later channel writes to loop indefinitely and consume CPU, leading to denial of service.
CVSS	added	—	AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Reference	added	—	https://access.redhat.com/security/cve/CVE-2026-59843
Reference	added	—	https://bugzilla.redhat.com/show_bug.cgi?id=2498176

Source identifier: secalert@redhat.com

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<a href="https://intel.neosec.eu/en/cve/CVE-2026-59843"><img src="https://intel.neosec.eu/badge/cve/CVE-2026-59843.svg?lang=en" alt="CVE-2026-59843 – NEOSEC Intel" height="20"></a>