High Memory corruption exploit reference

CVE-2026-55200 — libssh2 - Out-of-Bounds Write via Unchecked packet_length in transport.c

libssh2 through 1.11.1, fixed in commit 7acf3df 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.

Published Jun 17, 2026 · Updated Oct 13, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: VulnCheck

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

EPSS 0.8% (30-day exploitation probability)+8
Technical severity (high)+6
CISA SSVC: exploitation "poc"+5

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

0.83%

ahead of 56% of scored CVEs · Oct 14, 2026

Affects

Libssh2

CVSS base

8.3

High · 3 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation

Proof of concept

none · proof-of-concept · active

Automatable

can an attacker script the kill-chain steps

Technical impact

Total

partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

Documented patch — github.com (NIST National Vulnerability Database)
Documented patch — github.com (NIST National Vulnerability Database)

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Libssh2 Libssh2).
The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
Apply the fixed version or the patch documented under Remediation.
Exploitation requires user interaction: tighten email/web filtering and warn affected users.
Search available logs for exploit probes matching the vulnerability class (memory corruption), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Libssh2	Libssh2	version 1.11.1	Vulnerable	NIST National Vulnerability Database
Libssh2	Libssh2	≤ 1.11.1	Vulnerable	CVE Program (cve.org) e CISA ADP

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

Low

Privileges required

None

User interaction

Required

Scope

Unchanged

Confidentiality

High

Integrity

High

Availability

Low

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
9.2	CVSS 4.0	Critical	—	—	disclosure@vulncheck.com via NIST National Vulnerability Database
9.2	CVSS 4.0	Critical	—	—	VulnCheck via CVE Program (cve.org) e CISA ADP
8.3	CVSS 3.1	High	2.8	5.5	nvd@nist.gov via NIST National Vulnerability Database
8.1	CVSS 3.1	High	2.2	5.9	disclosure@vulncheck.com via NIST National Vulnerability Database
8.1	CVSS 3.1	High	—	—	VulnCheck via CVE Program (cve.org) e CISA ADP

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L (score shown at the top: nvd@nist.gov)

Scoring entities (NIST, CNA, ADP) score independently; each score is shown with its origin. For prioritization we use 8.3 (NVD (NIST)) as a conservative provisional reference — not a definitive official score.

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-680 · Integer Overflow to Buffer Overflow

Other CVEs with this weakness →

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.83%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
Correlate service crashes/restarts with inbound traffic; repeated crashes may indicate attempts.

web/proxy/WAF logsnetwork IDS/IPSendpoint telemetry (EDR)crash dumps

Timeline

What happened to this CVE, newest first.

2026
Affected products added · libssh2/libssh2
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
SSVC assessment updated
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 13, 2026
Fix documented
Oct 13, 2026 · NIST National Vulnerability Database
CVSS changed · 8.3
Oct 13, 2026 · NIST National Vulnerability Database
Source status changed · → Analyzed
Oct 13, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 13, 2026 · NIST National Vulnerability Database
CVE published
Jun 17, 2026 · VulnCheck

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

4 on the record

github.com/libssh2/libssh2/commit/97acf3dfda80c91c3a8c9f2372546301d4a1a7a8
Patch, patch, NIST National Vulnerability Database
github.com/libssh2/libssh2/pull/2052
Patch, issue-tracking, NIST National Vulnerability Database
web.archive.org/web/20260623211210/https://github.com/bikini/exploitarium/tree/main/l
Exploit, Third Party Advisory, NIST National Vulnerability Database
www.vulncheck.com/advisories/libssh2-out-of-bounds-write-via-unchecked-packet-length-in
Third Party Advisory, third-party-advisory, NIST National Vulnerability Database

Elsewhere on this site

Libssh2
every CVE for this vendor

Memory corruption
same class

CWE-680
other pages naming this weakness

Related CVEs (same products)

High CVE-2026-66032 Double Free in Libssh2 Jul 24, 2026
High CVE-2026-66033 Out-of-bounds Read in Libssh2 Jul 24, 2026
High CVE-2026-66034 Out-of-bounds Read in Libssh2 Jul 24, 2026
High CVE-2026-66035 Heap-based Buffer Overflow in Libssh2 Jul 24, 2026
Medium CVE-2026-58051 libssh2 - Free of Uninitialized Pointer in publickey List Cleanup Jun 27, 2026
High CVE-2026-58050 libssh2 - Integer Overflow in publickey Subsystem Attribute Allocation Jun 27, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-55200 is "libssh2 - Out-of-Bounds Write via Unchecked packet_length in transport.c", rated High. It affects Libssh2 (Libssh2); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 8.3 describes technical severity. EPSS 0.83% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

Comments

Moderated technical discussion. Read the community guidelines.

No comments yet.