CVE-2026-55200: libssh2 1.11.1, fixed in commit 97acf3d, contains an out-of-bounds write vulnerability in ssh2_transport_read(

This week

HIGH 8.1REMOTEDOWNGRADE

CVE-2026-55200 is a high-severity flaw (CVSS 8.1) affecting libssh2. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

libssh2 1.11.1, fixed in commit 97acf3d, contains an out-of-bounds write vulnerability in ssh2_transport_read() that fails to enforce upper bounds on packet_length field. Remote attackers can send crafted SSH packets with excessively large packet_length values to corrupt heap memory and achieve remote code execution.

Why this priority

High severity
EPSS top 9%: very likely to be exploited
Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	8.1 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	4.0% (top 9%)
CISA exploited list	No
Weakness	CWE-680
Published	2026-06-17
Last modified	2026-10-22

Publisher (CNA): VulnCheck · Vendor: libssh2

Last updated 2026-10-22 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-06-17Published CVE record published
2026-10-22 19:17 UTCUpdated Record last modified

libssh2 libssh2 DOWNGRADE

Affected: 1.11.1

Fixed in: no fixed release known

Safe to downgrade to: a release before 1.11.1

1. Check installed version

apt-cache policy libssh2 || rpm -q libssh2 || libssh2 --version

2. No fixed release: DOWNGRADE to a release before 1.11.1 (Debian / Ubuntu)

apt-cache madison libssh2 sudo apt install libssh2=<VERSION_BEFORE_1.11.1> sudo apt-mark hold libssh2

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list libssh2 sudo dnf downgrade libssh2-<VERSION_BEFORE_1.11.1> sudo dnf versionlock add libssh2

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora 46 · python-streamlink · FIXED · 8.6.2-1.fc46 · FEDORA-2026-a56fcfdc08 · source

sudo dnf upgrade --advisory=FEDORA-2026-a56fcfdc08

Fedora ELN · valgrind · FIXED · 3.28.0-1.eln159 · FEDORA-2026-c593ef1d15 · source

sudo dnf upgrade --advisory=FEDORA-2026-c593ef1d15

Ubuntu 20.04 · libssh2 · NOT AFFECTED

Ubuntu 22.04 · libssh2 · NOT AFFECTED

Ubuntu 25.10 · libssh2 · FIXED · 1.11.1-1ubuntu0.25.10.2

sudo apt update && sudo apt install --only-upgrade libssh2 dpkg --compare-versions "$(dpkg-query -W -f='${Version}' libssh2)" ge "1.11.1-1ubuntu0.25.10.2" && echo PATCHED || echo STILL-VULNERABLE

Ubuntu 24.04 · libssh2 · NOT AFFECTED

Debian 12 · libssh2 · NOT AFFECTED

Debian 13 · libssh2 · FIXED · 1.11.1-1+deb13u1

sudo apt update && sudo apt install --only-upgrade libssh2 dpkg --compare-versions "$(dpkg-query -W -f='${Version}' libssh2)" ge "1.11.1-1+deb13u1" && echo PATCHED || echo STILL-VULNERABLE

RHEL 6 · libssh2 · DEFERRED

RHEL 7 · libssh2 · NOT AFFECTED

RHEL 8 · virt:rhel/libssh2 · DEFERRED

Alpine 3.21 · libssh2 · FIXED · libssh2-1.11.1-r2

apk add --upgrade libssh2 apk info -v libssh2

Alpine 3.22 · libssh2 · FIXED · libssh2-1.11.1-r2

apk add --upgrade libssh2 apk info -v libssh2

Alpine 3.23 · libssh2 · FIXED · libssh2-1.11.1-r3

apk add --upgrade libssh2 apk info -v libssh2

Alpine 3.24 · libssh2 · FIXED · libssh2-1.11.1-r3

apk add --upgrade libssh2 apk info -v libssh2

Public exploits and detection

Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.

GitHub PoC: 0xBlackash/CVE-2026-55200 (10 stars)
GitHub PoC: xd20111/CVE-2026-55200 (4 stars)
GitHub PoC: kaleth4/CVE-2026-55200 (0 stars)
Write-up: web.archive.org/web/20260623211210/https://github.com/bikini/exploitarium/tree/main/libssh

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-55200?

libssh2: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2026-55200 being exploited?

CVE-2026-55200 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-55200 affect?

CVE-2026-55200 affects libssh2. Severity is high (CVSS 8.1). libssh2 1.11.1, fixed in commit 97acf3d, contains an out-of-bounds write vulnerability in ssh2_transport_read() that fails to enforce upper bounds on packet_length field. Remote attackers can send crafted SSH packets with excessively large

How urgent is CVE-2026-55200?

Priority 61/100. High severity; EPSS top 9%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action

References

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