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CVE-2026-66035

libssh2 Heap Buffer Overflow via ETM Cipher Negotiation

High7.7EPSS 0.47%
Published
Updated
Reserved
Assigned by
VulnCheck

Description

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.

Timeline

  1. ID reserved

    By VulnCheck

  2. · 1 day later

    Published

  3. · 4 days later

    Record updated

Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.

Scores

CVSS scores of CVE-2026-66035, by who scored it
Scored byCVSSScoreVector
VulnCheckCNA (assigner)4.0High7.7CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
VulnCheckCNA (assigner)3.1High7.5CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
NVDNVD · Analyzed4.0High7.7CVSS: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
EPSS (exploitation in 30 days)
0.47% · top 59% of all CVEs (7 Nov 2026)
CISA SSVC
Exploitation: none · Automatable: no · Technical impact: total
Loading the EPSS history…

The list uses the assigner's score when there is one, then CISA's, then NVD's: High 7.7.

Weakness

Affected products

  • libssh2 · libssh2

    • affected ≤ 1.11.1
    • unaffected 42e33d81577ed4b95d4b4f6f845e5ee8efe5eeb4

References (3)

Sources

The same CVE in the primary databases and aggregators, to check against.

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Record last updated .