Vulnerability · Published Jul 24, 2026 · Assigned by VulnCheck
CVE-2026-66034libssh2 Heap Out-of-Bounds Read via publickey subsystem
High 7.5
In plain English
A high-severity out-of-bounds read in libssh2 from libssh2. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 35% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
libssh2 through 1.11.1, fixed in commit a13bb6c, contains a missing bounds check vulnerability that allows a malicious SSH server to trigger an arbitrary-length heap out-of-bounds read and a free of an uninitialized pointer via the publickey subsystem. In libssh2_publickey_list_fetch(), the version 1 response parser reads a server-controlled comment_len value and advances the parse pointer without verifying sufficient bytes remain in the buffer, causing the out-of-bounds read to leak heap pointers from adjacent allocations defeating ASLR, followed by heap allocator state corruption when the error cleanup path frees an uninitialized pointer from a non-zeroed realloc() region.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Alpine:v3.21 | libssh2 | 1.11.1-r3 | ALPINE-CVE-2026-66034 |
| Alpine:v3.22 | libssh2 | 1.11.1-r3 | ALPINE-CVE-2026-66034 |
| Alpine:v3.23 | libssh2 | 1.11.1-r4 | ALPINE-CVE-2026-66034 |
| Alpine:v3.24 | libssh2 | 1.11.1-r4 | ALPINE-CVE-2026-66034 |
| Debian:12 | libssh2 | 1.10.0-3+deb12u1 | DEBIAN-CVE-2026-66034 |
| Debian:13 | libssh2 | 1.11.1-1+deb13u2 | DEBIAN-CVE-2026-66034 |
| Debian:14 | libssh2 | 1.11.1-5 | DEBIAN-CVE-2026-66034 |
| Ubuntu:22.04:LTS | libssh2 | 1.10.0-3ubuntu0.1 | USN-8799-1 |
| Ubuntu:22.04:LTS | libssh2 | 1.10.0-3ubuntu0.1 | UBUNTU-CVE-2026-66034 |
| Ubuntu:24.04:LTS | libssh2 | 1.11.0-4.1ubuntu0.24.04.3 | UBUNTU-CVE-2026-66034 |
| Ubuntu:26.04:LTS | libssh2 | 1.11.1-1ubuntu0.26.04.3 | UBUNTU-CVE-2026-66034 |
Affected products (as reported)
Weakness: CWE-125, CWE-908 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
References
- Pull Request issue-tracking, patch
- Patch Commit patch
- https://www.vulncheck.com/advisories/libssh2-heap-out-of-bounds-read-via-publickey-subsystem third-party-advisory