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

Severity: high (CVSS 8.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 3.9% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-06-17 · updated 2026-10-22
Weakness: CWE-680

Description

libssh2 1.11.1, fixed in commit 97acf3d, 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.

Affected products

libssh2: 1.11.1

References

https://github.com/libssh2/libssh2/commit/97acf3dfda80c91c3a8c9f2372546301d4a1a7a8
https://github.com/libssh2/libssh2/pull/2052
https://www.vulncheck.com/advisories/libssh2-out-of-bounds-write-via-unchecked-packet-length-in-transport-c
https://web.archive.org/web/20260623211210/https://github.com/bikini/exploitarium/tree/main/libssh2-cve-2026-55200-poc

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