Bug 2553376

Summary: CVE-2026-110926 openssh: openssh: shared LZ77 compression dictionary leaks plaintext across channels [fedora-all]
Product: [Fedora] Fedora Reporter: Ganesh <gnaik>
Component: opensshAssignee: Dmitry Belyavskiy <dbelyavs>
Status: CLOSED DUPLICATE QA Contact: Fedora Extras Quality Assurance <extras-qa>
Severity: medium Docs Contact:
Priority: medium    
Version: rawhideCC: crypto-team, dbelyavs, dwalsh, jjelen, mattias.ellert, tm
Target Milestone: ---Keywords: Security, SecurityTracking
Target Release: ---Flags: fedora-admin-xmlrpc: mirror+
Hardware: Unspecified   
OS: Unspecified   
Whiteboard: {"flaws": ["b0d1cc7b-7cff-48b9-aca0-4735d9f31c64"]}
Fixed In Version: Doc Type: ---
Doc Text:
Story Points: ---
Clone Of: Environment:
Last Closed: 2026-10-19 15:02:07 UTC Type: ---
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Embargoed:
Bug Depends On:    
Bug Blocks: 2553035    

Description Ganesh 2026-10-15 11:28:22 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.

A flaw was found in OpenSSH. Outbound compression used deflateInit with the default level, so every channel on a connection shared one LZ77 search buffer. A peer who can write on one channel can recover plaintext from another channel by observing ciphertext lengths, as described in "Crossing the Streams" (Fabian Bäumer and Marcus Brinkmann, 2026). The fix switches outbound compression to Z_HUFFMAN_ONLY, which removes LZ77 matching and lowers the compression ratio. The RHEL maintainer will take that fix. The OpenSSH client does not offer compression by default. The server zlib default is delayed compression, which applies only if the client offers it.
Comment 1 Ganesh 2026-10-19 15:02:07 UTC 

*** This bug has been marked as a duplicate of bug 2555582 ***