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CVE-2026-66033· NVD / CVE Program· CNA VulnCheck

libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation

Noroxi assessment

Priority: Monitor (action score 34/100)

  • 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: 0.58% (FIRST EPSS).
  • 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.
  • According to CISA the attack is automatable: targets can be found by mass scanning.
  • Fixed version: azl3 libssh2 1.11.1-4 on Azure Linux 3.0 1.11.1-5; azl3 rust 1.96.1-1 on Azure Linux 3.0 1.96.1-2.

Low priority for now. Upgrade to azl3 libssh2 1.11.1-4 on Azure Linux 3.0 1.11.1-5; azl3 rust 1.96.1-1 on Azure Linux 3.0 1.96.1-2 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 a2ed82d, contains a pre-authentication integer underflow vulnerability in the ssh2_cipher_crypt() function in src/openssl.c that allows a malicious SSH server to crash any connecting client by negotiating AES-GCM ciphers during handshake. Attackers can exploit the underflow in the expression computing blocksize minus aadlen minus authentication tag length to trigger an out-of-bounds read and a memcpy call with a near-SIZE_MAX length argument, causing immediate process crash before any authentication occurs.

HighCVSS 8.7 · v4.0—No exploit Fix available
Published
Jul 24, 2026
Updated
Jul 30, 2026
EPSS
0.5% · 46th percentile

Report tools

JSON

Action score

34

Monitor

Low priority for now.

CVSS
34 / 40 · 8.7 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.5%

CISA SSVC decision

Exploitation
none
Automatable
yes
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · VulnCheck
8.7
NVD has not scored this yet; the score shown is the CNA’s.

CVSS:4.0/AV:N/AC:L/AT:N/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-2026-66033

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

    • a2ed82d40964bbc0d64cd717aa0a5a892117d2e6not 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:v3.24libssh2before 1.11.1-r41.11.1-r4
Debian:13libssh2before 1.11.1-1+deb13u21.11.1-1+deb13u2
Debian:14libssh2before 1.11.1-51.11.1-5
openSUSE:Leap 16.0libssh2_orgbefore 1.11.1-160000.5.11.11.1-160000.5.1
openSUSE:Tumbleweedlibssh2_orgbefore 1.11.1-4.11.11.1-4.1
Red Hat:hummingbird:1libssh2before 0:1.11.1-10.2.hum10:1.11.1-10.2.hum1
Red Hat:hummingbird:1libssh2-develbefore 0:1.11.1-10.2.hum10:1.11.1-10.2.hum1
Red Hat:hummingbird:1libssh2-docsbefore 0:1.11.1-10.2.hum10:1.11.1-10.2.hum1
SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOSlibssh2_orgbefore 1.11.0-150000.4.35.11.11.0-150000.4.35.1
SUSE:Linux Enterprise Micro 5.3libssh2_orgbefore 1.11.0-150200.9.8.11.11.0-150200.9.8.1
SUSE:Linux Enterprise Module for Basesystem 15 SP7libssh2_orgbefore 1.11.0-150600.20.6.11.11.0-150600.20.6.1
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 1.11.0-4.11.11.0-4.1
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 1.11.1-160000.5.11.11.1-160000.5.1

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-2026-58051Jun 27, 2026 · libssh2 - Free of Uninitialized Pointer in publickey List Cleanup
    33Monitor
  • ← EarlierCVE-2026-66032Jul 24, 2026 · libssh2 Double-Free Heap Corruption via sftp_open()
    34Monitor
  • Later →CVE-2026-66034Jul 24, 2026 · libssh2 Heap Out-of-Bounds Read via publickey subsystem
    30Monitor

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 1.11.1-4 on Azure Linux 3.01.11.1-5 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 rust 1.96.1-1 on Azure Linux 3.01.96.1-2Microsoft (MSRC)
alpine:libssh21.11.1-r4 · Alpine:v3.24Package registry (OSV)
debian:libssh21.11.1-1+deb13u2 · Debian:13Package registry (OSV)
opensuse:libssh2_org1.11.1-4.1 · openSUSE:TumbleweedPackage registry (OSV)
red hat:libssh20:1.11.1-10.2.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:libssh2-devel0:1.11.1-10.2.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:libssh2-docs0:1.11.1-10.2.hum1 · Red Hat:hummingbird:1Package registry (OSV)
suse:libssh2_org1.11.1-160000.5.1 · 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.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

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.

  • CVE-2025-1566136 days apartlibssh2 - Heap Buffer Over-read via sftp_symlink() in sftp.c
    33Monitor
  • CVE-2026-660340 days apartlibssh2 Heap Out-of-Bounds Read via publickey subsystem
    30Monitor

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-125 · Out-of-bounds Read

CVSS vector

CVSS:4.0/AV:N/AC:L/AT:N/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.

ATT&CK techniques

—

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