CVE-2021-3634 [Medium] | Out-of-bounds Write in Libssh

A flaw has been found in libssh in versions prior to 0.9.6. The SSH protocol keeps track of two shared secrets during the lifetime of the session. One of them is called secret_hash and the other session_id. Initially, both of them are the same, but after key re-exchange, previous session_id is kept and used as an input to new secret_hash. Historically, both of these buffers had shared length variable, which worked as long as these buffers were same. But the key re-exchange operation can also change the key exchange method, which can be based on hash of different size, eventually creating "secret_hash" of different size than the session_id has. This becomes an issue when the session_id memory is zeroed or when it is used again during second key re-exchange.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-04-08	0.31%	0.12%	-0.20%
2	2026-03-03	0.12%	0.31%	+0.20%
3	2025-11-21	—	0.12%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-04-08

0.31%

0.12%

-0.20%

2026-03-03

0.12%

0.31%

+0.20%

2025-11-21

—

0.12%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
6.5	3.1	MEDIUM	`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H` Click to expand Attack vector (AV:N) Could be attacked over the internet or any normal routed network—not just someone sitting at the machine. Attack complexity (AC:L) Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup. Privileges required (PR:L) A normal user session is enough; they don’t have to be admin. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. Scope (S:U) Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems. Confidentiality (C:N) Doesn’t really leak secrets in a meaningful way. Integrity (I:N) Data isn’t meaningfully altered or forged. Availability (A:H) Could take the service down hard or make it unusable for people who depend on it.	2.8	3.6	[email protected]
4.0	2.0	MEDIUM	`AV:N/AC:L/Au:S/C:N/I:N/A:P` Click to expand Access vector (AV:N) Can be exploited remotely over network reachability. Access complexity (AC:L) Exploitation conditions are straightforward and predictable. Authentication (AU:S) A single authentication is required. Confidentiality impact (C:N) No confidentiality impact. Integrity impact (I:N) No integrity impact. Availability impact (A:P) Partial availability impact.	8.0	2.9	[email protected]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

6.5

3.1

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H Click to expand

Attack vector (AV:N): Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L): Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L): A normal user session is enough; they don’t have to be admin.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U): Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N): Doesn’t really leak secrets in a meaningful way.
Integrity (I:N): Data isn’t meaningfully altered or forged.
Availability (A:H): Could take the service down hard or make it unusable for people who depend on it.

2.8

3.6

[email protected]

4.0

2.0

MEDIUM

AV:N/AC:L/Au:S/C:N/I:N/A:P Click to expand

Access vector (AV:N): Can be exploited remotely over network reachability.
Access complexity (AC:L): Exploitation conditions are straightforward and predictable.
Authentication (AU:S): A single authentication is required.
Confidentiality impact (C:N): No confidentiality impact.
Integrity impact (I:N): No integrity impact.
Availability impact (A:P): Partial availability impact.

8.0

2.9

[email protected]

OS Trackers for CVE-2021-3634

vendor	priority	summary	link
`alpine`	—	CVE-2021-3634: 1 source package rows (libssh); 16 state rows across 8 repos (3.11-main, 3.17-community, 3.18-community, 3.19-community, 3.20-community, 3.21-community, 3.22-community, edge-community); fixed 7, open 9.	https://security.alpinelinux.org/vuln/CVE-2021-3634
`debian`	not yet assigned	CVE-2021-3634 not yet assigned priority: Debian including 1 source packages (libssh), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5.	https://security-tracker.debian.org/tracker/CVE-2021-3634
`gentoo`	high	CVE-2021-3634: 1 GLSA(s) (202312-05), 1 atom(s) (net-libs/libssh); latest impact high.	https://bugs.gentoo.org/buglist.cgi?quicksearch=CVE-2021-3634
`redhat`	low	—	https://access.redhat.com/security/cve/CVE-2021-3634
`suse`	low	CVE-2021-3634 severity low: SUSE including 278 source package names (13.2-3.8.1:libssh-config-0.9.8-150200.13.3.1, 13.2-3.8.1:libssh4-0.9.8-150200.13.3.1, …), 500 product×package rows across 101 product lines (Container suse/ltss/sle12.5/sles12sp5, Container suse/sle-micro-rancher/5.2, … (101 product lines)): Known Affected 225, Fixed 186, Known Not Affected 89.	https://www.suse.com/security/cve/CVE-2021-3634/
`ubuntu`	medium	CVE-2021-3634 medium priority: Ubuntu including 1 source packages (libssh), 8 status rows across 8 suites (bionic, focal, hirsute, impish, jammy, trusty, upstream, xenial): released 5, not-affected 2, DNE 1.	https://ubuntu.com/security/CVE-2021-3634

Affected software / configurations for CVE-2021-3634

Vendor	Product	Version	Raw CPE
libssh	libssh	>= 0.9.1, < 0.9.6	cpe:2.3:a:libssh:libssh::::::::
redhat	virtualization	4.0	cpe:2.3:a:redhat:virtualization:4.0:::::::*
redhat	enterprise_linux	8.0	cpe:2.3:o:redhat:enterprise_linux:8.0::::-:::
debian	debian_linux	10.0	cpe:2.3:o:debian:debian_linux:10.0:::::::*
debian	debian_linux	11.0	cpe:2.3:o:debian:debian_linux:11.0:::::::*
fedoraproject	fedora	33	cpe:2.3:o:fedoraproject:fedora:33:::::::*
fedoraproject	fedora	34	cpe:2.3:o:fedoraproject:fedora:34:::::::*
fedoraproject	fedora	35	cpe:2.3:o:fedoraproject:fedora:35:::::::*
oracle	mysql_workbench	<= 8.0.27	cpe:2.3:a:oracle:mysql_workbench::::::::
netapp	cloud_backup	—	cpe:2.3:a:netapp:cloud_backup:-:::::::*

References for CVE-2021-3634

URL	Tags
https://bugzilla.redhat.com/show_bug.cgi?id=1978810	Issue Tracking Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/DRK67AJCWYYVAGF5SGAHNZXCX3PN3ZFP/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/JKYD3ZRAMDAQX3ZW6THHUF3GXN7FF6B4/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/SVWAAB2XMKEUMPMDALINKAA4U2QM4LNG/
https://security.gentoo.org/glsa/202312-05
https://security.netapp.com/advisory/ntap-20211004-0003/	Third Party Advisory
https://www.debian.org/security/2021/dsa-4965	Third Party Advisory
https://www.oracle.com/security-alerts/cpujan2022.html	Patch Third Party Advisory

URL

CVE-2021-3634

Exploit prediction scoring system (EPSS) score for CVE-2021-3634

Common vulnerability scoring system (CVSS) metrics for CVE-2021-3634

Weakness enumeration for CVE-2021-3634

OS Trackers for CVE-2021-3634

Affected software / configurations for CVE-2021-3634

References for CVE-2021-3634