# CVE-2026-55200: libssh2 out-of-bounds write in ssh2_transport_read

Source: https://junglewise.ai/threats/cve-2026-55200-libssh2-out-of-bounds-write-in-ssh2-transport-read  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-17. Updated: 2026-06-20  
License: CC BY 4.0 with a link back

CVE-2026-55200 · Severity: high · CVSS 8.1 · Published 2026-06-17

libssh2 is a widely used library that allows applications to communicate securely using the SSH protocol. A vulnerability in how the library handles incoming data packets could allow a remote attacker to crash the application or potentially take control of the system. This could lead to unauthorized access to sensitive data or a complete disruption of services relying on this library.

Technologies: [Libssh2](https://junglewise.ai/threats/technologies/libssh2). Vendors: [Libssh2](https://junglewise.ai/threats/vendors/libssh2).

## Quick answers

**Is CVE-2026-55200 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-55200 being exploited in the wild.

**How severe is CVE-2026-55200?** High severity, with a CVSS score of 8.1, published 17 June 2026.

**Which products and versions are affected?** Libssh2 through 1.11.1.

**Is there a fix for CVE-2026-55200?** A patch was published on 2026-06-12: Fix merged into master branch via pull request 2052 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-55200 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-55200?** Check whether you run Libssh2 at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

An out-of-bounds (OOB) write vulnerability exists in the ssh2_transport_read() function within transport.c of libssh2. The root cause is a failure to enforce upper bounds on the packet_length field received from the network, leading to an integer overflow or buffer overflow (CWE-680) when processing excessively large values. A remote, unauthenticated attacker can exploit this by sending specially crafted SSH packets to corrupt heap memory. This memory corruption can result in a denial of service or remote code execution (RCE). The issue is fixed in commit 97acf3df (often referenced as 7acf3df) by adding boundary checks against LIBSSH2_PACKET_MAXPAYLOAD.

## Affected products

- libssh2 libssh2 through 1.11.1

## Timeline

- 2026-06-12: patched: Fix merged into master branch via pull request 2052
- 2026-06-17: disclosed: CVE published and advisory released by VulnCheck

## References

- [https://github.com/libssh2/libssh2/commit/97acf3dfda80c91c3a8c9f2372546301d4a1a7a8](https://github.com/libssh2/libssh2/commit/97acf3dfda80c91c3a8c9f2372546301d4a1a7a8)
- [https://github.com/libssh2/libssh2/pull/2052](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://www.vulncheck.com/advisories/libssh2-out-of-bounds-write-via-unchecked-packet-length-in-transport-c)

## Related threats

- [libssh2 heap buffer overflow in transport layer during EtM negotiation](https://junglewise.ai/threats/cve-2026-66035-libssh2-heap-buffer-overflow-in-transport-layer-during-etm)
- [libssh2 heap out-of-bounds read in publickey_list_fetch](https://junglewise.ai/threats/cve-2026-66034-libssh2-heap-out-of-bounds-read-in-publickey-list-fetch)
- [libssh2 integer underflow in ssh2_cipher_crypt](https://junglewise.ai/threats/cve-2026-66033-libssh2-integer-underflow-in-ssh2-cipher-crypt)
- [libssh2 double free in sftp_open](https://junglewise.ai/threats/cve-2026-66032-libssh2-double-free-in-sftp-open)
- [libssh2 uninitialized pointer free in publickey list cleanup](https://junglewise.ai/threats/cve-2026-58051-libssh2-uninitialized-pointer-free-in-publickey-list-cleanup)