libssh2 ssh2_transport_read() packet_length Out-of-Bounds Write RCE
CVE-2026-55200 is a critical memory corruption vulnerability in libssh2 affecting versions through 1.11.1. The flaw is in ssh2_transport_read() in src/transport.c, where the library accepts an attacker-controlled SSH packet_length value from a remote peer without enforcing the libssh2 maximum packet-size boundary before arithmetic and allocation decisions are made. In the vulnerable full-packet decryption path, an excessively large packet_length can cause integer wraparound in the size calculation used for heap allocation, resulting in an undersized buffer being allocated while subsequent processing still uses the original oversized packet length. This creates an out-of-bounds heap write condition. The issue is reachable pre-authentication during SSH transport negotiation, so a malicious or impersonated SSH server can trigger it as soon as a libssh2-based client connects.
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Exploits
1 valid exploit after Mallory filtered fakes, detection scripts, and README-only repos.
This repository is a small standalone proof-of-concept exploit consisting of one C source file and a README. The main file, CVE-2026-55200.c, implements a multithreaded malicious SSH server that listens on a configurable TCP port (default 2222), accepts inbound client connections, performs a minimal SSH-like handshake, and then sends a crafted packet designed to trigger an out-of-bounds write in vulnerable libssh2 clients. The exploit flow is: send attacker-controlled SSH banner, receive client banner, send a fake SSH_MSG_KEXINIT structure, receive client key-exchange data, then transmit a malicious packet with packet_length set to 0xFFFFFFFF and a body filled largely with 0x41 bytes. The code uses pthreads to handle multiple clients concurrently and basic socket APIs for bind/listen/accept/send/recv. The README documents the claimed target as libssh2 <= 1.11.1 and describes the vulnerability as a network-reachable packet length validation flaw in SSH transport processing. There is no post-exploitation logic, shell payload, callback infrastructure, or framework integration; the repository is focused purely on triggering memory corruption/DoS and potentially enabling further exploitation research.
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Recent activity
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A critical pre-authentication remote code execution vulnerability in libssh2 caused by an out-of-bounds heap write in ssh2_transport_read() when packet_length is not properly bounded during SSH transport negotiation.
A critical pre-authentication remote code execution vulnerability in libssh2 caused by an out-of-bounds heap write in ssh2_transport_read() when processing attacker-controlled packet_length values during SSH transport negotiation.
A critical remote code execution vulnerability in the libssh2 library caused by improper validation of the packet_length field in incoming SSH packets, leading to integer overflow, out-of-bounds heap write, and potential full system compromise.
An out-of-bounds write vulnerability in libssh2's ssh2_transport_read() caused by unchecked packet_length values, allowing remote attackers to corrupt heap memory and potentially achieve remote code execution.
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