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NVD / CVE Program· CNA VulnCheck

CVE-2026-55199: libssh2 - Pre-Authentication DoS via SSH_MSG_EXT_INFO Handler

Noroxi assessment

Priority: Monitor (action score 32/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.91% (FIRST EPSS); higher than 60.2% of all records.
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: azl3 libssh2 1.11.1-3 on Azure Linux 3.0 1.11.1-4; azl3 libssh2 1.11.1-4 on Azure Linux 3.0 1.11.1-4.

Low priority for now. Upgrade to azl3 libssh2 1.11.1-3 on Azure Linux 3.0 1.11.1-4; azl3 libssh2 1.11.1-4 on Azure Linux 3.0 1.11.1-4 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

libssh2 through 1.11.1, fixed in commit 1762685, contains a pre-authentication denial of service vulnerability in the SSH_MSG_EXT_INFO handler in src/packet.c that allows a malicious SSH server to cause a client CPU exhaustion loop by sending a crafted extension count value. A malicious server can set nr_extensions to 0xFFFFFFFF during key exchange, causing the client to spin in a tight CPU loop for over 60 seconds because return values from _libssh2_get_string() are unchecked and the session timeout does not apply to CPU-bound loops.

HighCVSS 8.2 · v4.0—No exploit Fix available
Published
Jun 17, 2026
Updated
Jul 14, 2026
EPSS
0.9% · 60th percentile

Report tools

JSON

Action score

32

Monitor

Low priority for now.

CVSS
32 / 40 · 8.2 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.9%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · VulnCheck
8.2
NVD has not scored this yet; the score shown is the CNA’s.

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2026-55199

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
libssh2libssh2

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • libssh2 libssh2up to 1.11.1 inclusive

Versions reported by the vendor

Affected version ranges reported by the assigning authority (VulnCheck). Independent of NVD's CPE analysis and usually ahead of it.

  • libssh2 libssh2

    • 17626857d20b3c9a1addfa45979dadcee1cd84a4not affected
    • up to and including 1.11.1affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Alpine:v3.21libssh2before 1.11.1-r21.11.1-r2
Alpine:v3.23libssh2before 1.11.1-r31.11.1-r3
Alpine:v3.24rustbefore 1.96.1-r01.96.1-r0
Debian:13libssh2before 1.11.1-1+deb13u11.11.1-1+deb13u1
Debian:14libssh2before 1.11.1-41.11.1-4
openSUSE:Leap 16.0libssh2_orgbefore 1.11.1-160000.4.11.11.1-160000.4.1
openSUSE:Tumbleweedlibssh2_orgbefore 1.11.1-3.11.11.1-3.1
Red Hat:hummingbird:1libssh2before 0:1.11.1-8.hum10:1.11.1-8.hum1
Red Hat:hummingbird:1libssh2-develbefore 0:1.11.1-8.hum10:1.11.1-8.hum1
Red Hat:hummingbird:1libssh2-docsbefore 0:1.11.1-8.hum10:1.11.1-8.hum1
SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOSlibssh2_orgbefore 1.11.0-150000.4.32.11.11.0-150000.4.32.1
SUSE:Linux Enterprise Micro 5.3libssh2_orgbefore 1.11.0-150200.9.5.11.11.0-150200.9.5.1
SUSE:Linux Enterprise Module for Basesystem 15 SP7libssh2_orgbefore 1.11.0-150600.20.3.11.11.0-150600.20.3.1
SUSE:Linux Enterprise Server 12 SP5-LTSSlibssh2_orgbefore 1.11.0-29.18.11.11.0-29.18.1
SUSE:Linux Micro 6.0libssh2_orgbefore 1.11.0-3.11.11.0-3.1
SUSE:Linux Micro 6.1libssh2_orgbefore 1.11.0-slfo.1.1_2.11.11.0-slfo.1.1_2.1
SUSE:Linux Micro 6.2libssh2_orgbefore 1.11.1-160000.4.11.11.1-160000.4.1

Same product

libssh2: all records

Other highest-scoring records for the same primary product.

  • CVE-2019-3861An out of bounds read flaw was discovered in libssh2 before 1.8.1 in the way SSH packets with a padding length value greater than the packet
    38Monitor
  • CVE-2019-3860An out of bounds read flaw was discovered in libssh2 before 1.8.1 in the way SFTP packets with empty payloads are parsed.
    38Monitor
  • CVE-2019-3858An out of bounds read flaw was discovered in libssh2 before 1.8.1 when a specially crafted SFTP packet is received from the server.
    38Monitor

Published before and after this one in the same product

  • ← EarlierCVE-2020-22218Aug 22, 2023 · An issue was discovered in function _libssh2_packet_add in libssh2 1.10.0 allows attackers to access out of bounds memory.
    30Monitor
  • ← EarlierCVE-2026-7598May 1, 2026 · libssh2 userauth.c userauth_password integer overflow
    27Monitor
  • Later →CVE-2026-55200Jun 17, 2026 · libssh2 - Out-of-Bounds Write via Unchecked packet_length in transport.c
    37Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
azl3 libssh2 1.11.1-3 on Azure Linux 3.01.11.1-4 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 libssh2 1.11.1-4 on Azure Linux 3.01.11.1-4 · CBL-Mariner ReleasesMicrosoft (MSRC)
alpine:libssh21.11.1-r2 · Alpine:v3.21Package registry (OSV)
alpine:rust1.96.1-r0 · Alpine:v3.24Package registry (OSV)
debian:libssh21.11.1-1+deb13u1 · Debian:13Package registry (OSV)
opensuse:libssh2_org1.11.1-3.1 · openSUSE:TumbleweedPackage registry (OSV)
red hat:libssh20:1.11.1-8.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:libssh2-devel0:1.11.1-8.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:libssh2-docs0:1.11.1-8.hum1 · Red Hat:hummingbird:1Package registry (OSV)
suse:libssh2_org1.11.1-160000.4.1 · SUSE:Linux Micro 6.2Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
Integrity
Availability
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

CWE-835 · Loop with Unreachable Exit Condition ('Infinite Loop')

CVSS vector

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (2 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records