Bug 2506851 (CVE-2026-66035)

Summary: CVE-2026-66035 libssh2: libssh2: Arbitrary code execution via heap buffer overflow during SSH negotiation
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: kshier, rhel-process-autobot, stcannon, teagle, watson-tool-maintainers, yguenane
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in libssh2. A malicious SSH server can exploit a pre-authentication heap buffer overflow vulnerability by sending a specially crafted packet during the Encrypt-then-MAC (ETM) cipher negotiation. This allows the server to corrupt heap memory, potentially leading to arbitrary code execution on the connecting client before authentication is complete.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Bug Depends On: 2507410, 2507411, 2507412    
Bug Blocks:    

Description OSIDB Bzimport 2026-07-24 17:01:36 UTC 
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.