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libssh EUVDEUVD-2026-46263

| CVE-2026-59849 HIGH
Loop with Unreachable Exit Condition (Infinite Loop) (CWE-835)
2026-07-21 redhat GHSA-jp6g-7mqj-wj53
Denial Of Service Red Hat Enterprise Linux 10 Red Hat Enterprise Linux 8 Red Hat Enterprise Linux 9 Red Hat Hardened Images Red Hat
High
Disputed · 7.5 NVD
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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

  1. Access
    Client initiates SSH connection to hostile server
  2. Delivery
    Client offers certificate-based public key auth
  3. Exploit
    Server repeatedly rejects offered certificate
  4. Execution
    Client retry logic never terminates
  5. Impact
    Infinite loop hangs client and exhausts CPU

Vulnerability Assessment

Weakness
CWE
CWE-835
ASVS 5.0
V15 - Secure Coding and Architecture Loop termination conditions
MITRE ATT&CK

1 technique across 1 tactic: Impact

The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up.

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Exploitation Conditions

Exploitation requires a libssh client configured for automatic certificate-based public key authentication, plus either the configured certificates being missing or a server that repeatedly rejects the client's certificate during authentication. The limiting factors are significant: the impact falls on the connecting client (not a listening server), certificate-based public key auth must be enabled (not the default in many deployments), and there is no confidentiality or integrity impact - only availability (client hang / CPU exhaustion). Because the trigger explicitly includes 'configured certificates are missing,' the loop can also arise from local misconfiguration without an actively malicious server; the malicious case additionally requires the client to initiate a connection to an attacker-controlled or hostile server.
Impact

The published CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, base 7.5) scores this high due to a pure availability impact with no confidentiality or integrity loss. However, the supporting signals point to modest real-world priority: EPSS is only 0.17% (7th percentile), it is not on CISA KEV, and no POC is referenced. Critically, this is a client-side denial of service - the victim is the connecting libssh client, and exploitation depends on the client being configured for automatic certificate-based public key authentication, a nuance the AC:L/UI:N scoring does not fully capture (a network-vector, no-interaction score arguably overstates ease here). The resulting impact is a hung or CPU-spinning client process, not remote code execution or data exposure, so this is better handled as a stability/normal-cycle patch item than an emergency, with elevated attention only where automated pipelines or services make outbound SSH connections using libssh certificate auth.
Exploit ScenarioDerived

An attacker operates or compromises an SSH server and lures or waits for a libssh-based client configured with automatic certificate-based public key authentication to connect. The server repeatedly rejects the client's offered certificate (or the client has no valid certificate configured), driving the client into an infinite authentication loop that hangs the process and consumes CPU. No public exploit is referenced, and the outcome is denial of service against the client rather than code execution.
This Week Schedule remediation this week. Why: HIGH severity (CVSS 7.0-8.9).
Patch available 0.2% exploitation probability (EPSS)

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AnalysisAIv2 – Jul 30, 2026

Denial of service in libssh affects clients that use automatic certificate-based public key authentication, where logic errors cause the client to loop indefinitely when configured certificates are absent or are repeatedly rejected by the SSH server. Reported by Red Hat and shipped in Red Hat Enterprise Linux 8, 9, and 10 plus Red Hat Hardened Images, the flaw lets a hostile or misconfigured server hang a connecting client and consume CPU. There is no public exploit identified at time of analysis, and EPSS is low at 0.17% (7th percentile), consistent with an availability-only, client-side bug rather than a mass-exploited issue.

Earlier versions of this analysis (1)
v1 Jul 21, 2026 - 15:01 CVSS 3.1

Infinite loop denial of service in libssh's automatic certificate-based public key authentication causes client processes to hang indefinitely when connecting to a server that is missing certificates or repeatedly rejects them. Affected deployments include libssh as packaged in Red Hat Enterprise Linux 8, 9, 10, and Red Hat Hardened Images. No public exploit code and no CISA KEV listing have been identified; with a CVSS score of 3.1 and high attack complexity, real-world impact is limited to client-side availability disruption.

RemediationAI

Patch available per vendor advisory: apply the libssh update delivered in Red Hat errata RHSA-2026:42922 (https://access.redhat.com/errata/RHSA-2026:42922) for the affected RHEL 8, 9, and 10 streams and Red Hat Hardened Images, then restart or rebuild any services and containers that link libssh so the fixed library is loaded. Exact fixed version strings are not provided in the input data - confirm the target package version from RHSA-2026:42922 before deploying. As a temporary compensating control until patching, disable or avoid automatic certificate-based public key authentication in libssh clients and fall back to standard key or password authentication, since removing the certificate-auth trigger prevents the infinite loop; the trade-off is loss of SSH certificate-based authentication for those clients. Where feasible, restrict outbound SSH from libssh-based clients to trusted, known-good servers to limit exposure to hostile servers that could repeatedly reject certificates. Additional context is available in the oss-security advisory https://seclists.org/oss-sec/2026/q3/197 and https://vuldb.com/vuln/380927.

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Technical ContextAI · show details

libssh is a widely used C library implementing the SSHv2 protocol for both client and server roles, and is embedded in numerous applications and Linux distributions. This issue lives in its client-side automatic certificate-based public key authentication path, where SSH certificates (signed public keys) are offered to the server during the auth handshake. The root cause is CWE-835 (Loop with Unreachable Exit Condition, i.e. an infinite loop): the certificate-selection/retry logic lacks a correct termination condition, so when no valid certificate is available or the server keeps rejecting the offered certificate, the client re-enters the authentication attempt loop without progressing and spins indefinitely.

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Severity by source

Sources disagree (Low–High)
Vendor (redhat) PRIMARY LOW
NVD 7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
vuln.today AI 5.3 MEDIUM

Client must be configured for certificate auth and must connect to a hostile/misconfigured server, so AC:H and UI:R; impact is availability-only (A:H) with no confidentiality or integrity effect.

3.1 AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:H
4.0 AV:N/AC:H/AT:N/PR:N/UI:A/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
SUSE 3.1 LOW
AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L
Red Hat 3.1 LOW

vuln.today treats the vendor’s rating as authoritative. A higher third-party CVSS (e.g. CISA-ADP) is shown for transparency but does not drive the headline severity.

CVSS VectorNVD

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

Lifecycle Timeline

4
Analysis Updated
Jul 30, 2026 - 16:45 vuln.today
v2 - CVSS changed
Severity Changed
Jul 30, 2026 - 16:37 NVD
LOW HIGH
CVSS changed
Jul 30, 2026 - 16:37 NVD
3.1 (LOW) 7.5 (HIGH)
CVE Published
Jul 21, 2026 - 14:08 cve.org
LOW 3.1

DescriptionNVD

A flaw was found in libssh. Logic errors in automatic certificate-based public key authentication can cause libssh clients to loop indefinitely when configured certificates are missing or repeatedly rejected by a server, leading to denial of service.

Executive ViewAI

Why It Matters

libssh in Red Hat Enterprise Linux 8, 9, and 10 contains a denial-of-service vulnerability that hangs SSH client processes and exhausts CPU when connecting to hostile or misconfigured SSH servers, disrupting certificate-based authentication workflows. Organizations using automated SSH client connections for CI/CD, system administration, or remote access face operational disruption risk. Immediate asset inventory and interim controls are necessary while awaiting vendor remediation.

Business Impact: High
Compensating Controls

No vendor-released patch identified at time of analysis. Interim mitigations: deploy host-level process monitoring to detect SSH client hangs with CPU spikes; enforce network egress controls restricting SSH client connections to internal/verified servers; implement connection timeout enforcement in libssh configuration; disable automatic certificate-based authentication where operationally feasible and substitute explicit server verification; segment systems running untrusted SSH workflows to limit blast radius.

Threat Intelligence Overview

EPSS Score
0.2%
Top 93% exploitable
Prevalence
High
widely-used SSH library bundled across major Linux distributions and applications
Data Quality
6 sources
Confidence: High
EUVD NVD CVE.org EPSS Vendor VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity HIGH
CISA KEV No
Public POC No
EPSS 0.2%
Patch available Yes
Active threat No
Deciding rule
HIGH severity (CVSS 7.0-8.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision· derived
Scheduled
Exploitation· derived
None
System Exposure· derived
Open
Automatable· derived
Yes
Human Impact
High

Priority Score

20
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +20
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Denial of Service MEDIUM

Denial of Service attacks render applications or systems unavailable by overwhelming resources or triggering failure conditions.

Affected Products (CPE)NVD

cpe:2.3:a:red_hat:red_hat_enterprise_linux_10:*:*:*:*:*:*:*:*
cpe:2.3:a:red_hat:red_hat_enterprise_linux_8:*:*:*:*:*:*:*:*
cpe:2.3:a:red_hat:red_hat_enterprise_linux_9:*:*:*:*:*:*:*:*
cpe:2.3:a:red_hat:red_hat_hardened_images:*:*:*:*:*:*:*:*

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Same weakness CWE-835 – Loop with Unreachable Exit Condition (Infinite Loop)

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Same technique Denial Of Service

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CVE-2015-5119 CRITICAL POC
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CVE-2011-2462 CRITICAL POC
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CVE-2011-0611 HIGH POC
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CVE-2012-1889 HIGH POC
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CVE-2012-0507 CRITICAL POC
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CVE-2015-5122 CRITICAL POC
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CVE-2011-0609 HIGH POC
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Vendor StatusVendor

SUSE

Severity: Low
Product Status
SUSE Linux Enterprise Server 16.0 SUSE Linux Enterprise Server for SAP applications 16.0 Fixed
SUSE Linux Micro 6.2 Fixed
openSUSE Leap 16.0 Fixed
openSUSE Tumbleweed Fixed
SUSE Linux Enterprise Desktop 15 SP7 Not-Affected

References

Advisories & patches
ADVISORY https://seclists.org/oss-sec/2026/q3/197 ADVISORY https://ubuntu.com/security/notices/USN-8699-1 ADVISORY https://vuldb.com/vuln/380927 ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:22940/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:22948/ ADVISORY https://www.suse.com/support/update/SUSE-SU-2026:22956/ REF https://access.redhat.com/errata/RHSA-2026:42922 REF https://access.redhat.com/errata/RHSA-2026:55855 REF https://access.redhat.com/security/cve/CVE-2026-59849 REF https://bugzilla.redhat.com/show_bug.cgi?id=2498182

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