CVE-2026-66034: libssh2 Out-of-bounds Read

Vulnerability summary

CVE-2026-66034 is a high (CVSS 7.7) Out-of-bounds Read vulnerability in libssh2. EPSS estimates a 0.40% chance of exploitation in the next 30 days, higher than 33.3% of scored vulnerabilities.

Timeline

1. 24-07-2026
   PublishedVulnerability first disclosed.
2. 28-07-2026
   Last modifiedVulnerability information updated.

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.

• CWE-125Out-of-bounds ReadThe product reads data past the end, or before the beginning, of the intended buffer.
• CWE-908Use of Uninitialized ResourceThe product uses or accesses a resource that has not been initialized.