Bug 2498176 (CVE-2026-59843)

Summary:	CVE-2026-59843 libssh: libssh: denial of service via zero advertised channel packet size
Product:	[Other] Security Response	Reporter:	OSIDB Bzimport <bzimport>
Component:	vulnerability	Assignee:	Product Security <prodsec-ir-bot>
Status:	NEW ---	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	rhel-process-autobot, security-response-team, watson-tool-maintainers
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	---
Doc Text:	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.	Story Points:	---
Clone Of:		Environment:
Last Closed:		Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:
Bug Depends On:	2503657
Bug Blocks:

Description OSIDB Bzimport 2026-07-08 18:14:52 UTC

A flaw was found in libssh channel handling. In ssh_packet_channel_open() in src/messages.c and ssh_packet_channel_open_conf() in src/channels.c, the implementation accepts a peer-controlled maximum packet size of 0 in channel open messages. That zero value is stored in channel state and later reaches channel_write_common(), where forward progress depends on the remote maximum packet size being positive; with 0, the remaining length is never reduced and the write path loops indefinitely, consuming CPU and causing denial of service. A remote authenticated peer can trigger this by advertising a zero maximum packet size in SSH_MSG_CHANNEL_OPEN or SSH_MSG_CHANNEL_OPEN_CONFIRMATION.

Comment 3 errata-xmlrpc 2026-08-17 20:53:23 UTC

This issue has been addressed in the following products:

  Red Hat Enterprise Linux 10

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

Comment 4 errata-xmlrpc 2026-09-01 17:53:31 UTC

This issue has been addressed in the following products:

  Red Hat Enterprise Linux 9

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

Comment 5 errata-xmlrpc 2026-09-01 18:33:10 UTC

This issue has been addressed in the following products:

  Red Hat Enterprise Linux 8

Via RHSA-2026:62218 https://access.redhat.com/errata/RHSA-2026:62218