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CVE-2026-59843 Red Hat CVE debrief

CVE-2026-59843 is a denial of service vulnerability 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. This vulnerability has a CVSS score of 6.5 and a severity rating of MEDIUM. The vulnerability exists because libssh does not properly handle the maximum packet size advertisement in SSH_MSG_CHANNEL_OPEN messages. This can lead to a denial of service condition where the CPU is consumed indefinitely. Users of libssh should verify their inventory for exposure and review vendor remediation status.

Vendor
Red Hat
Product
Red Hat Hardened Images
CVSS
MEDIUM 6.5
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-21
Original CVE updated
2026-07-22
Advisory published
2026-07-21
Advisory updated
2026-07-22

Who should care

Users of libssh should verify their inventory for exposure and review vendor remediation status. This includes administrators and security teams responsible for maintaining and securing systems that utilize libssh. They should assess their exposure to this vulnerability and apply patches or updates as available. Additionally, they should monitor for unusual CPU consumption patterns that could indicate exploitation attempts.

Technical summary

The vulnerability exists in libssh, where a remote authenticated peer can advertise a zero maximum packet size in SSH_MSG_CHANNEL_OPEN. This causes later channel writes to loop indefinitely and consume CPU, leading to a denial of service. The CVSS score for this vulnerability is 6.5, with a severity rating of MEDIUM. The vulnerability is caused by the lack of proper validation of the maximum packet size in SSH_MSG_CHANNEL_OPEN messages. This allows a remote authenticated peer to cause a denial of service condition.

Defensive priority

Apply vendor patches or updates as they become available. Monitor for unusual CPU consumption patterns that could indicate exploitation attempts. Review compensating controls for exposed systems while remediation is scheduled and verified.

Recommended defensive actions

  • Verify libssh inventory for exposure
  • Review vendor remediation status
  • Apply patches or updates as available
  • Monitor for unusual CPU consumption patterns
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record was published on 2026-07-21T12:18:57.860Z and was last modified on 2026-07-22T20:17:03.287Z. The NVD entry is currently Undergoing Analysis. This information is based on the provided source corpus and may be subject to change as new information becomes available. Users should verify the current status and affected scope with the official CVE record and NVD entry for the most up-to-date information.

Official resources

AI-assisted PatchSiren debrief based on the supplied source corpus. The CVE record was published on 2026-07-21T12:18:57.860Z and has not been modified since then. The NVD entry is currently Undergoing Analysis.