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GHSA-3m6q-h5gj-7mrw

GHSA-3m6q-h5gj-7mrw: Gitea has insecure default SSH settings

Summary

The built-in SSH server currently advertises a number of key exchange, MAC, and host key algorithms that are considered weak or broken. The defaults should be tightened so a fresh installation passes a baseline SSH security audit out of the box.

Details

Running ssh-audit against a default deployment flags the following as fail:

Key exchange
- ecdh-sha2-nistp256
- ecdh-sha2-nistp384
- ecdh-sha2-nistp521
MAC
- hmac-sha1
Host key
- ssh-rsa

Reproduction

docker run -it --rm positronsecurity/ssh-audit -p 2222 gitea.local

Impact

Default deployments expose algorithms that are known-weak or deprecated upstream. The current workaround requires manually setting several GITEA__server__SSH_SERVER_* variables, which most users will never do.

Workaround

[server]
SSH_SERVER_KEY_EXCHANGES = curve25519-sha256, diffie-hellman-group14-sha256
SSH_SERVER_CIPHERS       = chacha20-poly1305@openssh.com, aes128-ctr, aes192-ctr, aes256-ctr, aes128-gcm@openssh.com, aes256-gcm@openssh.com
SSH_SERVER_MACS          = hmac-sha2-256-etm@openssh.com, hmac-sha2-256

There is no exposed option to restrict host key algorithms, so ssh-rsa remains advertised.

Acceptance criteria

[ ] Default SSH_SERVER_KEY_EXCHANGES, SSH_SERVER_CIPHERS, and SSH_SERVER_MACS updated to the secure list above.
[ ] New SSH_SERVER_HOST_KEY_ALGORITHMS option added, with a default that excludes ssh-rsa.
[ ] Documentation updated to reflect the new defaults.
[ ] ssh-audit against a fresh install reports no [fail] entries.

(GitHub Advisory)

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GHSA-3m6q-h5gj-7mrw

GHSA-3m6q-h5gj-7mrw:

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
code.gitea.io/gitea	go	< 1.25.0	1.25.0

Technical Details

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Root Cause Analysis

The vulnerability lies in the default configuration of Gitea's built-in SSH server, which permitted the use of weak cryptographic algorithms. The analysis of the security advisory and the associated patch in pull request #29740 on GitHub revealed that the root cause was in the modules/setting/server.go file. Specifically, the loadServerFrom function was identified as the key function responsible for this vulnerability.

Prior to the patch, this function initialized the SSH server's key exchanges, ciphers, and MACs with empty strings if no explicit configuration was provided. This resulted in the SSH server falling back to a default set of algorithms that included several outdated and insecure options, such as ecdh-sha2-nistp256, hmac-sha1, and ssh-rsa. An attacker could potentially exploit these weak algorithms to compromise the integrity and confidentiality of SSH communications.

The patch addresses this by replacing the empty defaults with a curated list of strong, modern cryptographic algorithms for key exchanges, ciphers, and MACs. It also introduces a new configuration option, SSH_SERVER_HOST_KEY_ALGORITHMS, to allow administrators to explicitly define which host key algorithms are permissible, with a secure default that excludes the deprecated ssh-rsa algorithm. By enforcing secure defaults, the patch ensures that Gitea instances are secure out-of-the-box, without requiring manual administrator intervention.

Vulnerable functions

code.gitea.io/gitea/modules/setting.loadServerFrom

modules/setting/server.go

The function `loadServerFrom` in `modules/setting/server.go` is responsible for loading the SSH server configuration. Before the patch, it used empty strings as default values for `SSH_SERVER_KEY_EXCHANGES`, `SSH_SERVER_CIPHERS`, and `SSH_SERVER_MACS`. This caused the underlying SSH library to use its default set of algorithms, which included weak and insecure ones like `ecdh-sha2-nistp256`, `hmac-sha1`, and `ssh-rsa`. The patch changes these defaults to a more secure set of algorithms, thus mitigating the vulnerability.

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WAF Rule

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Reasoning

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GHSA-3m6q-h5gj-7mrw: Gitea has insecure default SSH settings

Summary

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GHSA-3m6q-h5gj-7mrw:

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Reasoning

GHSA-3m6q-h5gj-7mrw: Gitea has insecure default SSH settings

Summary

Details

Reproduction

Impact

Workaround

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Root Cause Analysis

Vulnerable functions

Technical Details

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