Bug 1896739 (CVE-2020-25708)

Summary:	CVE-2020-25708 libvncserver: libvncserver/rfbserver.c has a divide by zero which could result in DoS
Product:	[Other] Security Response	Reporter:	Michael Kaplan <mkaplan>
Component:	vulnerability	Assignee:	Red Hat Product Security <security-response-team>
Status:	CLOSED ERRATA	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	unspecified	CC:	dcommander, jadahl, negativo17, oholy, rdieter
Target Milestone:	---	Keywords:	Security
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:
Fixed In Version:	libvncserver 0.9.13	Doc Type:	If docs needed, set a value
Doc Text:	A divide by zero flaw was found in libvncserver. This flaw allows a malicious client to send a specially crafted message that, when processed by the VNC server, leads to a floating-point exception, resulting in a denial of service. The highest threat from this vulnerability is to system availability.	Story Points:	---
Clone Of:		Environment:
Last Closed:	2021-05-18 14:36:07 UTC	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:	1896740, 1898077, 1898078
Bug Blocks:	1896743

Description Michael Kaplan 2020-11-11 12:39:52 UTC

An issue was discovered in libvncserver-0.9.12. There is a divide by zero in rfbSendRectEncodingRaw function in libvncserver/rfbserver.c.
Attackers can launch a denial of service attack by sending a special message to the VNC server.

Upstream issue:
https://github.com/LibVNC/libvncserver/issues/409

Upstream commit:
https://github.com/LibVNC/libvncserver/commit/673c07a75ed844d74676f3ccdcfdc706a7052dba

Comment 1 Michael Kaplan 2020-11-11 12:39:58 UTC

Acknowledgments:

Name: Kailong Zhu, Hui Huang, Lu Yu

Comment 2 Michael Kaplan 2020-11-11 12:40:02 UTC

External References:

https://github.com/LibVNC/libvncserver/issues/409
https://github.com/LibVNC/libvncserver/commit/673c07a75ed844d74676f3ccdcfdc706a7052dba

Comment 3 Michael Kaplan 2020-11-11 12:40:40 UTC

Created libvncserver tracking bugs for this issue:

Affects: epel-7 [bug 1896740]

Comment 7 DRC 2021-02-23 21:49:30 UTC

NOTE: at least one commercial firewall has flagged TurboVNC connections as vulnerable to this CVE, but to the best of my knowledge and testing, LibVNCServer is the only TightVNC-compatible code base that is (was) vulnerable.  I was unable to reproduce the vulnerability with TightVNC 1.3.x, TigerVNC, or TurboVNC.  Refer to https://github.com/TurboVNC/turbovnc/pull/273#issuecomment-784498698.

Comment 9 Product Security DevOps Team 2021-05-18 14:36:07 UTC

This bug is now closed. Further updates for individual products will be reflected on the CVE page(s):

https://access.redhat.com/security/cve/cve-2020-25708

Comment 10 errata-xmlrpc 2021-05-18 15:16:21 UTC

This issue has been addressed in the following products:

  Red Hat Enterprise Linux 8

Via RHSA-2021:1811 https://access.redhat.com/errata/RHSA-2021:1811