GitHub SSH Security Updates: RSA key size increase, SHA‑1 deprecation, and post‑quantum KEX rollout

GitHub is rolling out a set of SSH security updates that affect how clients authenticate and negotiate encryption with the service. The changes include dropping the ssh-rsa signature type that relies on SHA‑1, removing the diffie-hellman-group-exchange-sha256 key‑exchange, enforcing a minimum RSA key length of 3072 bits for newly uploaded keys, and introducing the post‑quantum mlkem768x25519-sha256 exchange for GitHub.com and GitHub Enterprise Cloud with Data Residency (excluding the U.S. region). Practitioners who automate Git operations, run CI/CD pipelines, or manage large fleets of servers must verify client compatibility, update key material, and monitor the transition windows to avoid service disruption.

Changed SSH Algorithms and Key Requirements

The following algorithmic changes take effect on the dates specified by GitHub:

• Removal of ssh-rsa (SHA‑1) signatures: All RSA keys using SHA‑1 are no longer accepted.
• Removal of diffie-hellman-group-exchange-sha256 key exchange: This Diffie‑Hellman variant is disabled.
• RSA key size increase: New RSA keys uploaded after 2026‑10‑14 must be at least 3072 bits for both signing and authentication.
• New post‑quantum KEX: mlkem768x25519-sha256 is enabled on supported GitHub endpoints (except the U.S. region).

Impact on Toolchains and Client Versions

Clients that do not support RSA‑SHA‑2 signatures or the new key‑exchange will lose connectivity once the brownout periods end. GitHub lists the minimum versions that already provide robust RSA‑SHA‑2 support:

• OpenSSH 7.2p1
• JSch 0.1.66 (fork)
• TeamCity 2021.2.3
• Go SSH 0.16.0
• libssh2 1.11.0
• PuTTY 0.82

If upgrading is not feasible, practitioners can switch to Ed25519 or ECDSA keys, which GitHub continues to accept indefinitely. When generating new RSA keys for compatibility, ensure the key length meets the 3072‑bit threshold.

Operational Considerations for GitHub Enterprise Server

The rollout follows a staged schedule:

• 2026‑10‑14: Enforce the 3072‑bit RSA minimum and enable mlkem768x25519-sha256 on GitHub.com and Enterprise Cloud with Data Residency (non‑U.S.).
• 2026‑11‑04: First brownout of the ssh-rsa signature type and the removed Diffie‑Hellman KEX.
• 2026‑12‑09: Second brownout of the same mechanisms.
• 2026‑01‑13: Full removal of the deprecated algorithms.

GitHub Enterprise Server version 3.24 introduces the new post‑quantum KEX, while version 3.25 completes the removal of the old algorithms. During the brownout windows, both old and new algorithms are accepted, giving operators a window to test and upgrade.

Related CloudNinjas coverage: security.

What This Means For Practitioners

Actionable steps:

1. Audit all SSH clients used to access GitHub repositories and confirm they meet the minimum version list or support RSA‑SHA‑2.
2. Generate new Ed25519 keys where possible; if RSA is required, create keys of at least 3072 bits before the October deadline.
3. Update CI/CD runners, build agents, and automation scripts to reference the new key material and verify that they can negotiate the mlkem768x25519-sha256 exchange where applicable.
4. Monitor the brownout periods (Nov 4 and Dec 9) for any connection failures and be prepared to roll back to supported key types or upgrade clients quickly.
5. Document the change in security baselines and ensure compliance checks reference the new RSA key size and algorithm set.