CVE-2026-66033
libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation
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.
INFO
Published Date :
July 24, 2026, 5:17 p.m.
Last Modified :
July 30, 2026, 3:44 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | HIGH | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 3.1 | HIGH | [email protected] | ||||
| CVSS 4.0 | HIGH | 83251b91-4cc7-4094-a5c7-464a1b83ea10 | ||||
| CVSS 4.0 | HIGH | [email protected] |
Solution
- Update libssh2 to a version that includes commit a2ed82d.
- Ensure AES-GCM cipher negotiation is handled securely.
- Monitor SSH server interactions for unexpected behavior.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-66033.
| URL | Resource |
|---|---|
| https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 | Patch |
| https://github.com/libssh2/libssh2/pull/2401 | Issue Tracking Patch |
| https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation | Patch Third Party Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-66033 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-66033
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-66033 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-66033 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Jul. 30, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:libssh2:libssh2:*:*:*:*:*:*:*:* versions up to (including) 1.11.1 Added Reference Type VulnCheck: https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 Types: Patch Added Reference Type VulnCheck: https://github.com/libssh2/libssh2/pull/2401 Types: Issue Tracking, Patch Added Reference Type VulnCheck: https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation Types: Patch, Third Party Advisory -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jul. 27, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-66033', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'yes'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-07-27T16:15:25.211388Z'} -
New CVE Received by [email protected]
Jul. 24, 2026
Action Type Old Value New Value Added Affected [{'repo': 'https://github.com/libssh2/libssh2', 'vendor': 'libssh2', 'product': 'libssh2', 'versions': [{'status': 'affected', 'version': '0', 'versionType': 'semver', 'lessThanOrEqual': '1.11.1'}, {'status': 'unaffected', 'version': 'a2ed82d40964bbc0d64cd717aa0a5a892117d2e6', 'versionType': 'git'}], 'packageURL': 'pkg:github/libssh2/libssh2', 'defaultStatus': 'affected'}] Added 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. Added CVSS V4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Added CWE CWE-125 Added CWE CWE-191 Added Reference https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 Added Reference https://github.com/libssh2/libssh2/pull/2401 Added Reference https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation