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CVE-2026-66035— libssh2 Heap Buffer Overflow via ETM Cipher Negotiation

CVSS 7.5 · High EPSS 0.32% · P25

Possible ATT&CK Techniques 1AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 2

VendorProductVersion RangeStatus
libssh2libssh2≤ 1.11.1affected
42e33d81577ed4b95d4b4f6f845e5ee8efe5eeb4unaffected
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I. Basic Information for CVE-2026-66035

Vulnerability Information
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Vulnerability Title
libssh2 Heap Buffer Overflow via ETM Cipher Negotiation
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
堆缓冲区溢出
Source: CVE Program / CVE List V5
Vulnerability Title
libssh2 缓冲区错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
libssh2是libssh2团队开源的一款实现SSH2协议的客户端C库。 libssh2 1.11.1及之前版本存在缓冲区错误漏洞，该漏洞源于在Encrypt-then-MAC密码协商过程中，src/transport.c文件中的fullpacket()函数分配一个packet_length字节的缓冲区但复制blocksize-1字节，导致堆缓冲区溢出，可能允许恶意SSH服务器在连接客户端中破坏堆元数据，导致tcache bin混淆、对象重叠和认证前的函数指针覆盖。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

VendorProductAffected VersionsCPESubscribe
libssh2libssh2 0 ~ 1.11.1 -

II. Public POCs for CVE-2026-66035

#POC DescriptionSource LinkShenlong Link
AI-Generated POCPremium
Qwen3.6-35B-A3B · 11259 chars
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III. Intelligence Information for CVE-2026-66035

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Patches & Fixes for CVE-2026-66035 (2)

Vendor Advisories for CVE-2026-66035 (1)

Same Patch Batch · libssh2 · 2026-07-24 · 4 CVEs total

CVE-2026-660328.8 HIGHlibssh2 Double-Free Heap Corruption via sftp_open()
CVE-2026-660337.5 HIGHlibssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation
CVE-2026-660347.5 HIGHlibssh2 Heap Out-of-Bounds Read via publickey subsystem

IV. Related Vulnerabilities

Same product: libssh2
Same vendor: libssh2
Same weakness: CWE-122

V. Comments for CVE-2026-66035

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