Bug 2507410

Summary: CVE-2026-66035 libssh2: libssh2: Arbitrary code execution via heap buffer overflow during SSH negotiation [epel-all]
Product: [Fedora] Fedora EPEL Reporter: Srikanth Balasubramanian <sbalasub>
Component: libssh2Assignee: Paul Howarth <paul>
Status: NEW --- QA Contact: Fedora Extras Quality Assurance <extras-qa>
Severity: high Docs Contact:
Priority: high    
Version: epel10CC: kdudka, paul
Target Milestone: ---Keywords: Security, SecurityTracking
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Hardware: Unspecified   
OS: Unspecified   
Whiteboard: {"flaws": ["cc924207-24ff-493a-9b2e-ea56df41cee3"]}
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Bug Blocks: 2506851    

Description Srikanth Balasubramanian 2026-07-27 08:52:05 UTC 
Disclaimer: Community trackers are created by Red Hat Product Security team on a best effort basis. Package maintainers are required to ascertain if the flaw indeed affects their package, before starting the update process.

libssh2 through 1.11.1, fixed in commit 42e33d8, contains a pre-authentication heap buffer overflow vulnerability that allows a malicious SSH server to corrupt heap metadata in any connecting client by sending a packet with a packet_length smaller than the cipher's block size during Encrypt-then-MAC cipher negotiation. In the fullpacket() function in src/transport.c, the ETM path allocates a buffer of packet_length bytes but copies blocksize minus one bytes via memcpy, causing an overflow that on 32-bit glibc writes attacker-controlled bytes into an adjacent chunk's SIZE field, enabling tcache bin confusion, overlapping live objects, and function pointer overwrite during the session handshake before authentication.