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CVE-2019-20840

An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a...

7.5 HighFix available

An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a crash because of unaligned accesses in hybiReadAndDecode.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
None
Integrity
None
Availability
High

Exploit likelihood

2.73%Up 1.1 points since Apr 14, 2021
90%50%10%1%

What each source reports

NVD Score used 7.5

An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a crash because of unaligned accesses in hybiReadAndDecode.

Record
CVE-2019-20840
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CVSS 2.0
5.0 MediumAV:N/AC:L/Au:N/C:N/I:N/A:P
Weakness
CWE-787
Updated
2026-11-02
CVE List No score

An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a crash because of unaligned accesses in hybiReadAndDecode.

Record
CVE-2019-20840
Updated
2026-11-02
EUVDEUVD-2019-11377 No score

An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a crash because of unaligned accesses in hybiReadAndDecode.

Record
EUVD-2019-11377
Updated
2026-11-03
GitHub AdvisoriesGHSA-42fc-843w-r7rp 7.5

An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a...

An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a crash because of unaligned accesses in hybiReadAndDecode.

Record
GHSA-42fc-843w-r7rp
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Weakness
CWE-119, CWE-787
Updated
2026-11-02
OSVALPINE-CVE-2019-20840 7.5

An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a crash because of unaligned accesses in hybiReadAndDecode.

Record
ALPINE-CVE-2019-20840
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-03
OSV 7.5

An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a crash because of unaligned accesses in hybiReadAndDecode.

Record
CVE-2019-20840
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-02
Show 9 moreShow less
OSVDEBIAN-CVE-2019-20840 7.5

An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a crash because of unaligned accesses in hybiReadAndDecode.

Record
DEBIAN-CVE-2019-20840
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-02
OSVUBUNTU-CVE-2019-20840 7.5

An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a crash because of unaligned accesses in hybiReadAndDecode.

Record
UBUNTU-CVE-2019-20840
CVSS 3.1
7.5 HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Updated
2026-11-02
OSVopenSUSE-SU-2020:0988-1 No score

Security update for LibVNCServer

This update for LibVNCServer fixes the following issues:

  • security update
  • added patches fix CVE-2018-21247 [bsc#1173874], uninitialized memory contents are vulnerable to Information leak
    • LibVNCServer-CVE-2018-21247.patch fix CVE-2019-20839 [bsc#1173875], buffer overflow in ConnectClientToUnixSock()
    • LibVNCServer-CVE-2019-20839.patch fix CVE-2019-20840 [bsc#1173876], unaligned accesses in hybiReadAndDecode can lead to denial of service
    • LibVNCServer-CVE-2019-20840.patch fix CVE-2020-14398 [bsc#1173880], improperly closed TCP connection causes an infinite loop in libvncclient/sockets.c
    • LibVNCServer-CVE-2020-14398.patch fix CVE-2020-14397 [bsc#1173700], NULL pointer dereference in libvncserver/rfbregion.c
    • LibVNCServer-CVE-2020-14397.patch fix CVE-2020-14399 [bsc#1173743], Byte-aligned data is accessed through uint32_t pointers in libvncclient/rfbproto.c.
    • LibVNCServer-CVE-2020-14399.patch fix CVE-2020-14400 [bsc#1173691], Byte-aligned data is accessed through uint16_t pointers in libvncserver/translate.c.
    • LibVNCServer-CVE-2020-14400.patch fix CVE-2020-14401 [bsc#1173694], potential integer overflows in libvncserver/scale.c
    • LibVNCServer-CVE-2020-14401.patch fix CVE-2020-14402 [bsc#1173701], out-of-bounds access via encodings.
    • LibVNCServer-CVE-2020-14402,14403,14404.patch fix CVE-2017-18922 [bsc#1173477], preauth buffer overwrite

This update was imported from the SUSE:SLE-15:Update update project.

Record
openSUSE-SU-2020:0988-1
Updated
2026-11-02
OSVopenSUSE-SU-2020:1025-1 No score

Security update for LibVNCServer

This update for LibVNCServer fixes the following issues:

  • security update
  • added patches fix CVE-2018-21247 [bsc#1173874], uninitialized memory contents are vulnerable to Information leak
    • LibVNCServer-CVE-2018-21247.patch fix CVE-2019-20839 [bsc#1173875], buffer overflow in ConnectClientToUnixSock()
    • LibVNCServer-CVE-2019-20839.patch fix CVE-2019-20840 [bsc#1173876], unaligned accesses in hybiReadAndDecode can lead to denial of service
    • LibVNCServer-CVE-2019-20840.patch fix CVE-2020-14398 [bsc#1173880], improperly closed TCP connection causes an infinite loop in libvncclient/sockets.c
    • LibVNCServer-CVE-2020-14398.patch fix CVE-2020-14397 [bsc#1173700], NULL pointer dereference in libvncserver/rfbregion.c
    • LibVNCServer-CVE-2020-14397.patch fix CVE-2020-14399 [bsc#1173743], Byte-aligned data is accessed through uint32_t pointers in libvncclient/rfbproto.c.
    • LibVNCServer-CVE-2020-14399.patch fix CVE-2020-14400 [bsc#1173691], Byte-aligned data is accessed through uint16_t pointers in libvncserver/translate.c.
    • LibVNCServer-CVE-2020-14400.patch fix CVE-2020-14401 [bsc#1173694], potential integer overflows in libvncserver/scale.c
    • LibVNCServer-CVE-2020-14401.patch fix CVE-2020-14402 [bsc#1173701], out-of-bounds access via encodings.
    • LibVNCServer-CVE-2020-14402,14403,14404.patch fix CVE-2017-18922 [bsc#1173477], preauth buffer overwrite

This update was imported from the SUSE:SLE-15:Update update project.

Record
openSUSE-SU-2020:1025-1
Updated
2026-11-02
OSVopenSUSE-SU-2020:1056-1 No score

Security update for LibVNCServer

This update for LibVNCServer fixes the following issues:

  • security update
  • added patches fix CVE-2018-21247 [bsc#1173874], uninitialized memory contents are vulnerable to Information leak
    • LibVNCServer-CVE-2018-21247.patch fix CVE-2019-20839 [bsc#1173875], buffer overflow in ConnectClientToUnixSock()
    • LibVNCServer-CVE-2019-20839.patch fix CVE-2019-20840 [bsc#1173876], unaligned accesses in hybiReadAndDecode can lead to denial of service
    • LibVNCServer-CVE-2019-20840.patch fix CVE-2020-14398 [bsc#1173880], improperly closed TCP connection causes an infinite loop in libvncclient/sockets.c
    • LibVNCServer-CVE-2020-14398.patch fix CVE-2020-14397 [bsc#1173700], NULL pointer dereference in libvncserver/rfbregion.c
    • LibVNCServer-CVE-2020-14397.patch fix CVE-2020-14399 [bsc#1173743], Byte-aligned data is accessed through uint32_t pointers in libvncclient/rfbproto.c.
    • LibVNCServer-CVE-2020-14399.patch fix CVE-2020-14400 [bsc#1173691], Byte-aligned data is accessed through uint16_t pointers in libvncserver/translate.c.
    • LibVNCServer-CVE-2020-14400.patch fix CVE-2020-14401 [bsc#1173694], potential integer overflows in libvncserver/scale.c
    • LibVNCServer-CVE-2020-14401.patch fix CVE-2020-14402 [bsc#1173701], out-of-bounds access via encodings.
    • LibVNCServer-CVE-2020-14402,14403,14404.patch fix CVE-2017-18922 [bsc#1173477], preauth buffer overwrite

This update was imported from the SUSE:SLE-15:Update update project.

Record
openSUSE-SU-2020:1056-1
Updated
2026-11-02
OSVopenSUSE-SU-2024:10598-1 No score

LibVNCServer-devel-0.9.13-3.1 on GA media

These are all security issues fixed in the LibVNCServer-devel-0.9.13-3.1 package on the GA media of openSUSE Tumbleweed.

Record
openSUSE-SU-2024:10598-1
Updated
2026-11-02
OSVSUSE-SU-2020:1922-1 No score

Security update for LibVNCServer

This update for LibVNCServer fixes the following issues:

  • security update
  • added patches fix CVE-2018-21247 [bsc#1173874], uninitialized memory contents are vulnerable to Information leak
    • LibVNCServer-CVE-2018-21247.patch fix CVE-2019-20839 [bsc#1173875], buffer overflow in ConnectClientToUnixSock()
    • LibVNCServer-CVE-2019-20839.patch fix CVE-2019-20840 [bsc#1173876], unaligned accesses in hybiReadAndDecode can lead to denial of service
    • LibVNCServer-CVE-2019-20840.patch fix CVE-2020-14398 [bsc#1173880], improperly closed TCP connection causes an infinite loop in libvncclient/sockets.c
    • LibVNCServer-CVE-2020-14398.patch fix CVE-2020-14397 [bsc#1173700], NULL pointer dereference in libvncserver/rfbregion.c
    • LibVNCServer-CVE-2020-14397.patch fix CVE-2020-14399 [bsc#1173743], Byte-aligned data is accessed through uint32_t pointers in libvncclient/rfbproto.c.
    • LibVNCServer-CVE-2020-14399.patch fix CVE-2020-14400 [bsc#1173691], Byte-aligned data is accessed through uint16_t pointers in libvncserver/translate.c.
    • LibVNCServer-CVE-2020-14400.patch fix CVE-2020-14401 [bsc#1173694], potential integer overflows in libvncserver/scale.c
    • LibVNCServer-CVE-2020-14401.patch fix CVE-2020-14402 [bsc#1173701], out-of-bounds access via encodings.
    • LibVNCServer-CVE-2020-14402,14403,14404.patch fix CVE-2017-18922 [bsc#1173477], preauth buffer overwrite
Record
SUSE-SU-2020:1922-1
Updated
2026-11-02
OSVSUSE-SU-2020:2167-1 No score

Security update for LibVNCServer

This update for LibVNCServer fixes the following issues:

  • security update fix CVE-2018-21247 [bsc#1173874], uninitialized memory contents are vulnerable to Information leak fix CVE-2019-20839 [bsc#1173875], buffer overflow in ConnectClientToUnixSock() fix CVE-2019-20840 [bsc#1173876], unaligned accesses in hybiReadAndDecode can lead to denial of service fix CVE-2020-14398 [bsc#1173880], improperly closed TCP connection causes an infinite loop in libvncclient/sockets.c fix CVE-2020-14397 [bsc#1173700], NULL pointer dereference in libvncserver/rfbregion.c fix CVE-2020-14399 [bsc#1173743], Byte-aligned data is accessed through uint32_t pointers in libvncclient/rfbproto.c. fix CVE-2020-14400 [bsc#1173691], Byte-aligned data is accessed through uint16_t pointers in libvncserver/translate.c. fix CVE-2020-14401 [bsc#1173694], potential integer overflows in libvncserver/scale.c fix CVE-2020-14402 [bsc#1173701], out-of-bounds access via encodings. fix CVE-2020-14403 [bsc#1173701], out-of-bounds access via encodings. fix CVE-2020-14404 [bsc#1173701], out-of-bounds access via encodings. fix CVE-2017-18922 [bsc#1173477], preauth buffer overwrite
Record
SUSE-SU-2020:2167-1
Updated
2026-11-02
OSVUSN-4434-1 No score

libvncserver vulnerabilities

Ramin Farajpour Cami discovered that LibVNCServer incorrectly handled certain malformed unix socket names. A remote attacker could exploit this with a crafted socket name, leading to a denial of service, or possibly execute arbitrary code. (CVE-2019-20839)

It was discovered that LibVNCServer did not properly access byte-aligned data. A remote attacker could possibly use this issue to cause LibVNCServer to crash, resulting in a denial of service. This issue only affected Ubuntu 18.04 LTS and Ubuntu 16.04 LTS. (CVE-2019-20840)

Christian Beier discovered that LibVNCServer incorrectly handled anonymous TLS connections. A remote attacker could possibly use this issue to cause LibVNCServer to crash, resulting in a denial of service. This issue only affected Ubuntu 20.04 LTS. (CVE-2020-14396)

It was discovered that LibVNCServer incorrectly handled region clipping. A remote attacker could possibly use this issue to cause LibVNCServer to crash, resulting in a denial of service. (CVE-2020-14397)

It was discovered that LibVNCServer did not properly reset incorrectly terminated TCP connections. A remote attacker could possibly use this issue to cause an infinite loop, resulting in a denial of service. (CVE-2020-14398)

It was discovered that LibVNCServer did not properly access byte-aligned data. A remote attacker could possibly use this issue to cause LibVNCServer to crash, resulting in a denial of service. (CVE-2020-14399, CVE-2020-14400)

It was discovered that LibVNCServer incorrectly handled screen scaling on the server side. A remote attacker could use this issue to cause LibVNCServer to crash, resulting in a denial of service, or possibly execute arbitrary code. (CVE-2020-14401)

It was discovered that LibVNCServer incorrectly handled encodings. A remote attacker could use this issue to cause LibVNCServer to crash, resulting in a denial of service, or possibly execute arbitrary code. (CVE-2020-14402, CVE-2020-14403, CVE-2020-14404)

It was discovered that LibVNCServer incorrectly handled TextChat messages. A remote attacker could possibly use this issue to cause LibVNCServer to crash, resulting in a denial of service. (CVE-2020-14405)

Record
USN-4434-1
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
libvncserver
Alpine:v3.12
 before 0.9.13-r0
0.9.13-r0
libvncserver
Alpine:v3.13
 before 0.9.13-r0
0.9.13-r0
libvncserver
Alpine:v3.14
 before 0.9.13-r0
0.9.13-r0
libvncserver
Alpine:v3.15
 before 0.9.13-r0
0.9.13-r0
libvncserver
Alpine:v3.16
 before 0.9.13-r0
0.9.13-r0
libvncserver
Alpine:v3.17
 before 0.9.13-r0
0.9.13-r0
libvncserver
Alpine:v3.18
 before 0.9.13-r0
0.9.13-r0
libvncserver
Alpine:v3.19
 before 0.9.13-r0
0.9.13-r0
libvncserver
Alpine:v3.20
 before 0.9.13-r0
0.9.13-r0
libvncserver
Alpine:v3.21
 before 0.9.13-r0
0.9.13-r0
Show 41 moreShow less
Affected packages
PackageAffectedFixed
libvncserver
Alpine:v3.22
 before 0.9.13-r0
0.9.13-r0
libvncserver
Alpine:v3.23
 before 0.9.13-r0
0.9.13-r0
libvncserver
Alpine:v3.24
 before 0.9.13-r0
0.9.13-r0
libvncserver
Debian:12
 before 0.9.13+dfsg-1
0.9.13+dfsg-1
libvncserver
Debian:13
 before 0.9.13+dfsg-1
0.9.13+dfsg-1
libvncserver
Debian:14
 before 0.9.13+dfsg-1
0.9.13+dfsg-1
LibVNCServer
SUSE:Linux Enterprise Module for Package Hub 15 SP1
 before 0.9.10-4.22.1
0.9.10-4.22.1
LibVNCServer
SUSE:Linux Enterprise Module for Package Hub 15 SP2
 before 0.9.10-4.22.1
0.9.10-4.22.1
LibVNCServer
SUSE:Linux Enterprise Workstation Extension 15 SP1
 before 0.9.10-4.22.1
0.9.10-4.22.1
LibVNCServer
SUSE:Linux Enterprise Workstation Extension 15 SP2
 before 0.9.10-4.22.1
0.9.10-4.22.1
LibVNCServer
SUSE:HPE Helion OpenStack 8
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:OpenStack Cloud 7
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:OpenStack Cloud 8
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:OpenStack Cloud 9
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:OpenStack Cloud Crowbar 8
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:OpenStack Cloud Crowbar 9
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Linux Enterprise Server for SAP Applications 12 SP2
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Linux Enterprise Server for SAP Applications 12 SP3
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Linux Enterprise Server for SAP Applications 12 SP4
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Linux Enterprise Software Development Kit 12 SP5
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Linux Enterprise Server 12 SP2-LTSS
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Linux Enterprise Server 12 SP2-BCL
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Linux Enterprise Server 12 SP3-LTSS
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Linux Enterprise Server 12 SP3-BCL
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Linux Enterprise Server 12 SP4-LTSS
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Linux Enterprise Server 12 SP5
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Linux Enterprise Server for SAP Applications 12 SP5
 before 0.9.9-17.31.1
0.9.9-17.31.1
LibVNCServer
SUSE:Enterprise Storage 5
 before 0.9.9-17.31.1
0.9.9-17.31.1
x11vnc
Ubuntu:14.04:LTS
 All versions, plus listed versions
0.9.13-1.1
 None known
libvncserver
Ubuntu:16.04:LTS
 before 0.9.10+dfsg-3ubuntu0.16.04.5, plus listed versions
0.9.10+dfsg-30.9.10+dfsg-3build10.9.10+dfsg-3ubuntu0.16.04.10.9.10+dfsg-3ubuntu0.16.04.20.9.10+dfsg-3ubuntu0.16.04.30.9.10+dfsg-3ubuntu0.16.04.4
0.9.10+dfsg-3ubuntu0.16.04.5
libvncserver
Ubuntu:18.04:LTS
 before 0.9.11+dfsg-1ubuntu1.3, plus listed versions
0.9.11+dfsg-10.9.11+dfsg-1ubuntu10.9.11+dfsg-1ubuntu1.10.9.11+dfsg-1ubuntu1.2
0.9.11+dfsg-1ubuntu1.3
veyon
Ubuntu:20.04:LTS
 All versions, plus listed versions
4.2.4+repack1-24.2.5+repack1-14.3.1+repack1-14.3.1+repack1-24.3.1+repack1-2build14.3.1+repack1-2build2
 None known
veyon
Ubuntu:22.04:LTS
 All versions, plus listed versions
4.5.3+repack1-1build14.5.3+repack1-1build2
 None known
veyon
Ubuntu:24.04:LTS
 All versions, plus listed versions
4.7.5+repack1-1build24.7.5+repack1-1ubuntu14.7.5+repack1-1ubuntu34.7.5+repack1-1ubuntu44.7.5+repack1-1ubuntu5
 None known
veyon
Ubuntu:25.10
 All versions, plus listed versions
4.7.5+repack1-1ubuntu64.7.5+repack1-1ubuntu74.9.7+repack1-1
 None known
veyon
Ubuntu:26.10
 All versions, plus listed versions
4.9.7+repack1-1build14.9.7+repack1-1.14.9.7+repack1-1.1build14.9.7+repack1-1.1build2
 None known
veyon
Ubuntu:26.04:LTS
 All versions, plus listed versions
4.9.7+repack1-14.9.7+repack1-1build1
 None known
libvncserver
Ubuntu:20.04:LTS
 before 0.9.12+dfsg-9ubuntu0.2, plus listed versions
0.9.11+dfsg-1.30.9.12+dfsg-3ubuntu30.9.12+dfsg-60.9.12+dfsg-70.9.12+dfsg-80.9.12+dfsg-90.9.12+dfsg-9ubuntu0.1
0.9.12+dfsg-9ubuntu0.2
LibVNCServer
openSUSE:Leap 15.1
 before 0.9.10-lp151.7.9.1
0.9.10-lp151.7.9.1
LibVNCServer
openSUSE:Leap 15.2
 before 0.9.10-lp152.9.8.1
0.9.10-lp152.9.8.1
LibVNCServer
openSUSE:Leap 15.2
 before 0.9.10-lp152.9.4.2
0.9.10-lp152.9.4.2

History

History
RecordedChangedSources
affected, aliases, referencesOSV
aliasesEUVD
first indexedCVE List, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2019-20840 is a high vulnerability (CVSS 7.5) in Libvnc Project Libvncserver, published June 17, 2020. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 2.73%, the 86th percentile. Fixed in libvncserver 0.9.13-r0.

References

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