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Fencer Vulns
high 7.5

CVE-2026-66035 json

libssh2 Heap Buffer Overflow via ETM Cipher Negotiation

published 2026-07-24 3 mo ago modified 2026-11-08 7 h ago also EUVD-2026-48722 , GHSA-v6rf-8q4r-r495 , JLSEC-2026-1088

cvss
7.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 41%
public exploits
0
none known

Summary

libssh2 through 1.11.1, fixed in commit 42e33d8, contains a pre-authentication heap buffer overflow vulnerability that allows a malicious SSH server to corrupt heap metadata in any connecting client by sending a packet with a packet_length smaller than the cipher's block size during Encrypt-then-MAC cipher negotiation. In the fullpacket() function in src/transport.c, the ETM path allocates a buffer of packet_length bytes but copies blocksize minus one bytes via memcpy, causing an overflow that on 32-bit glibc writes attacker-controlled bytes into an adjacent chunk's SIZE field, enabling tcache bin confusion, overlapping live objects, and function pointer overwrite during the session handshake before authentication.

Patch / fix

  • julia LibSSH2_jll 1.11.104+0
  • libssh2 Debian 13 1.11.1-1+deb13u2
  • libssh2 Debian 14 1.11.1-5

debian advisories DEBIAN-CVE-2026-66035

  • libssh2 Ubuntu 24.04 LTS 1.11.0-4.1ubuntu0.24.04.4
  • libssh2 Ubuntu 26.04 LTS 1.11.1-1ubuntu0.26.04.4

ubuntu advisories UBUNTU-CVE-2026-66035

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • libssh2 hummingbird 1 1.11.1-10.3.hum1

red hat advisories RHSA-2026:46955

  • libssh2_org SLES 15 SP4-LTSS, SLE High Performance Computing 15 SP4-LTSS, SLE High Performance Computing 15 SP4-ESPOS 1.11.0-150000.4.35.1
  • libssh2_org Linux Micro 6.0 1.11.0-4.1
  • libssh2_org Linux Micro 6.1 1.11.0-slfo.1.1_2.1
  • libssh2_org SLES 16.0, Linux Micro 6.2, SLES for SAP applications 16.0 1.11.1-160000.5.1
  • libssh2_org SLES for SAP 15 SP6, SLES 15 SP6-LTSS, SLE Module for Basesystem 15 SP7 1.11.0-150600.20.6.1
  • libssh2_org SLES for SAP 15 SP5, SLES for SAP 15 SP4, SLES 15 SP5-LTSS +8 more 1.11.0-150200.9.8.1
  • libssh2_org SLES LTSS Extended Security 12 SP5, SLES 12 SP5-LTSS 1.11.0-29.18.1

suse advisories SUSE-SU-2026:4095-1SUSE-SU-2026:23245-1SUSE-SU-2026:23187-1SUSE-SU-2026:23049-1SUSE-SU-2026:23214-1SUSE-SU-2026:23225-1

  • libssh2_org openSUSE Leap 16.0 1.11.1-160000.5.1
  • libssh2_org openSUSE Tumbleweed 1.11.1-4.1

opensuse advisories openSUSE-SU-2026:21549-1openSUSE-SU-2026:11454-1

  • libssh2 Alpine 3.22, Alpine 3.21 1.11.1-r3
  • libssh2 Alpine 3.24, Alpine 3.23 1.11.1-r4

alpine advisories ALPINE-CVE-2026-66035

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • libssh2 Azure Linux 3.0 1.11.1-5

azure linux advisories AZL-93399

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-66035 updated cvss 5.3

libssh2: libssh2: Arbitrary code execution via heap buffer overflow during SSH negotiation

  • Red Hat Enterprise Linux 6 libssh2
    affected: workaround, no fix planned “Out of support scope”
  • Red Hat Hardened Images libssh2 1.11.1-10.3.hum1
    fixed: vendor fix RHSA-2026:46955
  • Red Hat Enterprise Linux 7 libssh2
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
total
Assessed
3 mo ago

Exploit prediction

0.47% chance of exploitation in the next 30 days, higher than 41% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 7.7 CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
CNA 3.1 7.5 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
CNA 4.0 7.7 CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/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
OSV 3.1 7.5 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H

Affected packages

ecosystem package affected fixed in
julia LibSSH2_jll
<1.11.104+0
1.11.104+0

Affected products

  • libssh2 libssh2

Classification

Weakness
  • CWE-122 Heap-based Buffer Overflow

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.