CVE-2026-56624: Apache MINA SSHD Auth Bypass Vulnerability

CVE-2026-56624 Overview

CVE-2026-56624 is an improper certificate validation vulnerability [CWE-295] in Apache MINA SSHD, a Java library that provides client-side and server-side Secure Shell (SSH) functionality. The server-side OpenSSH user certificate validation logic fails to inspect or reject certificates containing the force-command or verify-required options. An authenticated user presenting a certificate with a force-command restriction can bypass that restriction and execute commands the certificate was intended to prevent. The exact commands available depend on the server implementation built on Apache MINA SSHD. Apache addressed the flaw in MINA SSHD versions 2.19.0 and 3.0.0-M5.

Critical Impact
Authenticated attackers can bypass command restrictions embedded in OpenSSH user certificates, executing commands the certificate issuer intended to block on Apache MINA SSHD servers.

Affected Products

Apache MINA SSHD versions prior to 2.19.0 (2.x branch)
Apache MINA SSHD versions prior to 3.0.0-M5 (3.x branch)
Java applications and servers embedding vulnerable Apache MINA SSHD releases

Discovery Timeline

2026-07-20 - CVE-2026-56624 published to the National Vulnerability Database
2026-07-20 - Apache announces the issue via the Openwall OSS-Security list
2026-07-23 - Last updated in NVD database

Technical Details for CVE-2026-56624

Vulnerability Analysis

Apache MINA SSHD supports OpenSSH user certificates for SSH authentication. OpenSSH certificates can carry critical options such as force-command, which restricts the authenticated session to a single predefined command, and verify-required, which mandates a physical presence check on hardware security keys. The server implementation in Apache MINA SSHD accepts certificates that carry these options but does not enforce or reject them. An attacker who authenticates with a certificate marked with force-command="/usr/bin/backup.sh", for example, is not confined to that command. The user can execute arbitrary commands permitted by the underlying server logic. The scope of exposure depends on the command execution surface exposed by the host application built on Apache MINA SSHD.

Root Cause

The root cause is missing validation of OpenSSH certificate critical options during user authentication. Apache MINA SSHD implements neither force-command nor sk-*-cert-v01@openssh.com certificate types where verify-required is meaningful. The library silently ignored these options instead of rejecting the certificate outright. This gap violates the OpenSSH certificate specification, which requires that unknown or unimplemented critical options cause authentication to fail.

Attack Vector

Exploitation requires a valid OpenSSH user certificate signed by a certificate authority (CA) trusted by the Apache MINA SSHD server. An attacker who obtains such a certificate, or who is issued one under the assumption that force-command restrictions will hold, can connect to the server and request commands outside the certificate's stated scope. Because the server treats the certificate as unrestricted, the session proceeds with the full command execution privileges of the authenticated user account. The attack requires user interaction and low-privilege access, matching the network-based attack profile described in the advisory.

No verified public exploit code is available. See the Apache List Discussion for the vendor's technical description.

Detection Methods for CVE-2026-56624

Indicators of Compromise

SSH authentication events on Apache MINA SSHD servers where certificate-authenticated sessions execute commands that differ from the certificate's force-command value.
Successful shell sessions from principals whose issued certificates were expected to be command-restricted.
Unexpected process executions spawned by service accounts that authenticate exclusively through OpenSSH certificates.

Detection Strategies

Inventory all Java applications that bundle Apache MINA SSHD and cross-reference the shipped version against 2.19.0 and 3.0.0-M5.
Enable verbose SSH authentication logging in the host application and capture the certificate serial, key ID, and requested command per session.
Compare requested commands against the force-command option recorded in the CA's certificate issuance logs to identify mismatches.

Monitoring Recommendations

Forward SSH session logs to a centralized SIEM or data lake and alert on command deviation from certificate policy.
Monitor CA-signed certificate issuance for the presence of force-command or verify-required options and flag connections that should be constrained.
Track privileged process creation on hosts running Apache MINA SSHD to detect lateral command execution following certificate authentication.

How to Mitigate CVE-2026-56624

Immediate Actions Required

Upgrade Apache MINA SSHD to 2.19.0 on the 2.x branch or 3.0.0-M5 on the 3.x branch.
Audit issued OpenSSH user certificates for reliance on force-command restrictions and treat those restrictions as unenforced until patching is complete.
Rotate or revoke CA-signed user certificates whose security model depends on force-command enforcement by the SSHD server.

Patch Information

Apache MINA SSHD 2.19.0 and 3.0.0-M5 reject OpenSSH user certificates that include force-command or verify-required options. Because the library implements neither feature, rejection is the correct behavior per the OpenSSH certificate specification. Details are available in the Apache List Discussion and the Openwall OSS-Security Update.

Workarounds

Restrict trusted certificate authorities to those that do not issue certificates carrying force-command or verify-required options.
Enforce command restrictions at the host application layer by validating requested commands against an out-of-band policy source rather than relying on certificate options.
Limit account privileges on the target system so that any commands executed after certificate authentication have the minimum required scope.

bash

# Verify the Apache MINA SSHD version bundled with your application
find . -name 'sshd-core-*.jar' -exec basename {} \;
# Expected output after patching: sshd-core-2.19.0.jar or sshd-core-3.0.0-M5.jar