libssh2 Heap Buffer Overflow via ETM Cipher Negotiation

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-66035
StatePUBLISHED
AssignerVulnCheck
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-07-24 17:17:35 UTC
Updated2026-07-24 19:17:10 UTC
Descriptionlibssh2 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.

Risk And Classification

Primary CVSS: v4.0 7.7 HIGH from [email protected]

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

Problem Types: CWE-122 | CWE-122 Heap-based Buffer Overflow

VersionSourceTypeScoreSeverityVector
4.0[email protected]Secondary7.7HIGHCVSS: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/C...
4.0CNACVSS7.7HIGHCVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
3.1[email protected]Secondary7.5HIGHCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
3.1CNACVSS7.5HIGHCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H

CVSS v4.0 Breakdown

Attack Vector
Network
Attack Complexity
High
Attack Requirements
None
Privileges Required
None
User Interaction
Passive
Confidentiality
High
Integrity
High
Availability
High
Sub Conf.
None
Sub Integrity
None
Sub Availability
None

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

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
High
Privileges Required
None
User Interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Libssh2 Libssh2 affected 1.11.1 semver Not specified
CNA Libssh2 Libssh2 unaffected 42e33d81577ed4b95d4b4f6f845e5ee8efe5eeb4 git Not specified

References

ReferenceSourceLinkTags
www.vulncheck.com/advisories/libssh2-heap-buffer-overflow-via-etm-cipher-negoti... [email protected] www.vulncheck.com
github.com/libssh2/libssh2/commit/42e33d81577ed4b95d4b4f6f845e5ee8efe5eeb4 [email protected] github.com
github.com/libssh2/libssh2/pull/2198 [email protected] github.com
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: VladimirEliTokarev (en)

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