# CVE-2026-66033: libssh2 integer underflow in ssh2_cipher_crypt

Source: https://junglewise.ai/threats/cve-2026-66033-libssh2-integer-underflow-in-ssh2-cipher-crypt  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-24. Updated: 2026-07-24  
License: CC BY 4.0 with a link back

CVE-2026-66033 · Severity: high · CVSS 7.5 · Published 2026-07-24

libssh2 is a widely used library that allows applications to communicate securely using the SSH protocol. A vulnerability in this library allows a malicious server to crash any application that attempts to connect to it. This occurs before the user even logs in, potentially leading to a widespread denial-of-service for tools and services that rely on libssh2 for automated data transfers or remote management.

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

## Quick answers

**Is CVE-2026-66033 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-66033 being exploited in the wild.

**How severe is CVE-2026-66033?** High severity, with a CVSS score of 7.5, published 24 July 2026.

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

**Is there a fix for CVE-2026-66033?** A patch was published on 2026-07-23: Fix merged in pull request 2401 and commit a2ed82d Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-66033 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-66033?** 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 integer underflow vulnerability exists in the `ssh2_cipher_crypt()` function within `src/openssl.c` of libssh2. The flaw is triggered during the SSH handshake when a malicious server negotiates the use of AES-GCM ciphers. The root cause is an unchecked expression calculating the encryption length (`blocksize - aadlen - authenticationtag`), which can underflow to a large positive value. This leads to an out-of-bounds read and a subsequent `memcpy` call with a length argument near `SIZE_MAX`, resulting in an immediate process crash. The vulnerability is pre-authentication and requires no user interaction beyond the initial connection attempt. A fix is available in commit a2ed82d which introduces runtime bounds checks.

## Affected products

- libssh2 libssh2 through 1.11.1

## Timeline

- 2026-07-23: patched: Fix merged in pull request 2401 and commit a2ed82d
- 2026-07-24: disclosed: CVE-2026-66033 published

## References

- [https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6](https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6)
- [https://github.com/libssh2/libssh2/pull/2401](https://github.com/libssh2/libssh2/pull/2401)
- [https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation](https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation)

## 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 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)
- [libssh2 heap buffer overflow in publickey-subsystem attribute allocation](https://junglewise.ai/threats/cve-2026-58050-libssh2-heap-buffer-overflow-in-publickey-subsystem-attribute)