Medium [CVE-2026-66033] Denial of Service via integer underflow in AES-GCM cipher negotiation
This medium-severity Red Hat Linux advisory covers CVE-2026-66033 affecting Red Hat Hardened Images, Red Hat Enterprise Linux 6, Red Hat package: libssh2.
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Summary
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. A flaw was found in libssh2.
A malicious SSH server can exploit this to trigger an out-of-bounds read and a memory copy operation with an extremely large length argument, leading to an immediate client process crash. This vulnerability results in a Denial of Service (DoS) for any connecting client.
This Moderate impact flaw in libssh2 allows a malicious SSH server to trigger a client-side denial of service. Exploitation requires the client to connect to a specially crafted, untrusted SSH server.
Red Hat severity: Moderate — CVSS 6.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H). Weakness: CWE-125.
Affected Red Hat products: Red Hat Hardened Images; Red Hat Enterprise Linux 6. Red Hat lists Red Hat Enterprise Linux 7 as not affected.
Will not fix / out of support: Red Hat Enterprise Linux 6.
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative. Check it before change work.
Official advisory · high-confidence parse· fetched 2 days ago·verify at source
- 1.11.1
- libssh2-main-1.11.1-10.2.hum1
- RHSA-2026:46927
Official advisory · high-confidence parse· fetched 2 days ago·verify at source
Mitigation checklist
- To mitigate the vulnerability, developers or administrators must configure the specific applications utilizing libssh2 to explicitly exclude AES-GCM from their preferred cipher lists.
Official advisory · high-confidence parse· fetched 2 days ago·verify at source
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