Out-of-bounds write in libssh2 ssh2_transport_read() via unchecked packet_length
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 in a full-packet decryption path without enforcing the libssh2 maximum packet size before arithmetic and allocation decisions are made. According to the provided content, packet_length is combined with MAC and authentication lengths to compute a total allocation size; with a maliciously large value such as 0xffffffff, the arithmetic can wrap, producing an undersized heap allocation while later processing still uses the original oversized packet-derived length. This creates an out-of-bounds heap write condition during pre-authentication SSH transport handling. The issue is described as fixed upstream by commit 97acf3dfda80c91c3a8c9f2372546301d4a1a7a8 / 7acf3df, which adds boundary checks rejecting packet_length values greater than LIBSSH2_PACKET_MAXPAYLOAD before the vulnerable addition occurs.
Are you exposed to this one?
Mallory correlates every CVE against your assets, your vendors, and active adversary campaigns. Know which vulnerabilities matter for you, not just which ones are loud.
Impact, mitigation & remediation
What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.
Impact
What an attacker gets, and what they’ve been doing with it.
Mitigation
If you can’t patch tonight, do this now.
Remediation
Patch, then assume compromise.
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.
Affected products & vendors
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
Recent activity
50 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
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 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 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.
The version that knows your environment.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.