OpenSSH 10.4, released on July 6, 2026, introduces a series of security enhancements, protocol refinements, and preliminary support for post-quantum cryptography. This update is now accessible through official OpenSSH mirrors.
Addressing Security Vulnerabilities
The latest version rectifies several critical vulnerabilities across its core utilities. In sftp(1), a flaw permitted malicious servers to redirect downloaded files to unintended locations during command-line operations such as “sftp host:/path .”. Similarly, scp(1) has been updated to prevent malicious servers from writing files outside the designated target directory during remote-to-remote copies.
Additional fixes in sshd(8) include resolving a silent truncation issue in “internal-sftp” that could discard security-relevant options beyond the ninth command-line argument. Moreover, the update enforces minimum authentication delays that were previously bypassed, as identified by the Orange Cyberdefense Vulnerability Team.
Other notable patches address a pre-authentication denial-of-service vulnerability related to GSSAPIAuthentication and a client-side use-after-free issue in ssh(1) triggered when a server changes its host key mid-session, reported by researcher Zhenpeng (Leo) Lin.
Protocol and Compatibility Changes
OpenSSH 10.4 introduces changes aimed at enhancing security posture, which may affect compatibility. The transport protocol now disconnects peers that send non-key exchange messages during post-authentication key re-exchange, mitigating potential memory exhaustion attacks from malicious peers.
On Linux systems, seccomp sandbox failures are now treated as fatal errors rather than merely being logged. This change necessitates that systems lacking these kernel features disable sandboxing explicitly at build time. Additionally, the sshd -G configuration dump mode now outputs directives in mixed-case, a modification that may impact automation scripts relying on previous formatting.
Post-Quantum Cryptography Support
A significant addition in this release is experimental support for a composite post-quantum signature scheme combining ML-DSA 44 and Ed25519, following the draft-miller-sshm-mldsa44-ed25519-composite-sigs specification. This feature is not enabled by default and requires explicit configuration, along with keys generated via “ssh-keygen -t mldsa44-ed25519”.
Furthermore, ssh(1) and sshd(8) now utilize a non-deterministic finite automaton (NFA)-based wildcard pattern matcher, addressing previous performance issues in the implementation.
Additional Bug Fixes and Updates
The release includes numerous bug fixes, such as corrections to FIDO token key downloads, out-of-bounds read fixes in sftp(1), a major refactor of sshd_config parsing for cleaner privilege-separation serialization, and improved bounds checking in cryptographic signing code.
Portability updates synchronize fmt_scaled.c and getrrsetbyname.c with OpenBSD upstream and patch several memory leaks on error paths.
Source packages are available as openssh-10.4.tar.gz and openssh-10.4p1.tar.gz, both verifiable via SHA1 and base64-encoded SHA256 checksums, with signing keys distributed through official mirror sites.
OpenSSH 10.4’s comprehensive security enhancements and forward-looking cryptographic support underscore the project’s commitment to maintaining a secure and robust toolset for remote communication. Users are encouraged to upgrade promptly to benefit from these improvements and ensure the integrity of their systems.
