OpenSSH bugs allows Man-in-the
Vulnerabilities mentionedAll →
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2024-6387 | A security regression (CVE-2006-5051) was discovered in OpenSSH's server (sshd). A security regression (CVE-2006-5051) was discovered in OpenSSH's server (sshd). There is a race condition which can lead sshd to handle some signals in an unsafe manner. An unauthenticated, remote attacker may be able to trigger it by failing to authenticate within a set time period. NVD description · AI analysis pending | 8.1 | 100% | PoC ×7 |
| — | |
| CVE-2024-6409 | A race condition vulnerability was discovered in how signals are handled by OpenSSH's server (sshd). A race condition vulnerability was discovered in how signals are handled by OpenSSH's server (sshd). If a remote attacker does not authenticate within a set time period, then sshd's SIGALRM handler is called asynchronously. However, this signal handler calls various functions that are not async-signal-safe, for example, syslog(). As a consequence of a successful attack, in the worst case scenario, an attacker may be able to perform a remote code execution (RCE) as an unprivileged user running the sshd server. NVD description · AI analysis pending | 7.0 | 28% | — | — | ||
| CVE-2025-26465 | A vulnerability was found in OpenSSH when the VerifyHostKeyDNS option is enabled. A vulnerability was found in OpenSSH when the VerifyHostKeyDNS option is enabled. A machine-in-the-middle attack can be performed by a malicious machine impersonating a legit server. This issue occurs due to how OpenSSH mishandles error codes in specific conditions when verifying the host key. For an attack to be considered successful, the attacker needs to manage to exhaust the client's memory resource first, turning the attack complexity high. NVD description · AI analysis pending | 6.8 | 8% |
| — | ||
| CVE-2025-26466 | Unauthenticated Memory-Exhaustion DoS in OpenSSH CVE-2025-26466 is a denial-of-service flaw in the OpenSSH server: for every ping packet the SSH server receives, it allocates a corresponding pong packet in a memory buffer and adds it to a queue, and that memory is only freed once the server/client key exchange completes. A remote, unauthenticated attacker can hold a connection open and keep flooding it with ping packets faster than the key exchange finishes, driving uncontrolled memory growth until the SSH server becomes unavailable. The impact is availability-only, with no code execution or data exposure (CVSS 3.1 score 5.9, network vector with high attack complexity). Any system running an affected OpenSSH build is exposed, notably as packaged in Ubuntu Linux and Debian Linux, with internet-facing SSH endpoints the most attractive targets. No public PoC or confirmed in-the-wild exploitation is known and it is not in CISA KEV, but EPSS of ~40% (99th percentile) signals an elevated probability of exploitation within 30 days; the related headlines describing critical OpenSSH flaws also cover companion issues (including a man-in-the-middle bug) from the same disclosure. Do: Install the OpenSSH security updates released by Canonical and Debian in February 2025 (upstream fix in OpenSSH 9.9p2) and verify the running version with 'ssh -V' or your package manager. Until patched, reduce exposure by restricting SSH to trusted sources via firewall allowlists and rate-limiting unauthenticated connections (e.g., MaxStartups or fail2ban), and monitor sshd memory usage for abnormal growth. | 5.9 | 40% |
| mass~10-20 million internet-exposed SSH endpoints, with potentially millions of unpatched Ubuntu/Debian servers among them |
Full article483 words · extracted from securityaffairs.com · click to collapse
Pierluigi Paganini
February 19, 2025
Two OpenSSH vulnerabilities could allow machine-in-the-middle (MitM) and denial-of-service (DoS) attacks under certain conditions.
The Qualys Threat Research Unit (TRU) has discovered two vulnerabilities in OpenSSH. The first, tracked as CVE-2025-26465 (CVSS score: 6.8) can be exploited by an attacker to conduct an active machine-in-the-middle attack on the OpenSSH client when the VerifyHostKeyDNS option is enabled. The second vulnerability, tracked CVE-2025-26466 (CVSS score: 5.9), affects both the OpenSSH client and server, allowing a pre-authentication denial-of-service attack.
The OpenSSH client vulnerability (CVE-2025-26465) allows an attack to succeed regardless of the VerifyHostKeyDNS setting, without user interaction or reliance on SSHFP DNS records. Introduced in December 2014 (OpenSSH 6.8p1), this flaw remained active, with FreeBSD enabling VerifyHostKeyDNS by default from 2013 to 2023, increasing exposure.
“If an attacker can perform a man-in-the-middle attack via CVE-2025-26465, the client may accept the attacker’s key instead of the legitimate server’s key. This would break the integrity of the SSH connection, enabling potential interception or tampering with the session before the user even realizes it. SSH sessions can be a prime target for attackers aiming to intercept credentials or hijack sessions.” reads the report published by Qualys. “If compromised, hackers could view or manipulate sensitive data, move across multiple critical servers laterally, and exfiltrate valuable information such as database credentials.”
The OpenSSH client and server are vulnerable (CVE-2025-26466) to a pre-authentication denial-of-service (DoS) attack. Successful exploitation of the issue can cause high memory and CPU consumption. Introduced in August 2023 (before OpenSSH 9.5p1), the attack can be mitigated on servers using LoginGraceTime, MaxStartups, and PerSourcePenalties settings.
“SSH is a critical service for remote system administration. If attackers can repeatedly exploit the flaw CVE-2025-26466, they may cause prolonged outages or prevent administrators from managing servers, effectively locking legitimate users out.” continues the report. “An enterprise facing this vulnerability could see critical servers become unreachable, interrupting routine operations and stalling essential maintenance tasks.”
Both vulnerabilities have been fixed in OpenSSH 9.9p2, released today by OpenSSH maintainers.
In July, OpenSSH fixed another vulnerability, tracked as CVE-2024-6409 (CVSS score: 7.0), that impacts select versions of the OpenSSH secure networking suite. The flaw can be exploited to achieve remote code execution (RCE).
The issue is a possible race condition in cleanup_exit() in openssh’s privsep child that impacts openssh versions 8.7p1 and 8.8p1. `cleanup_exit()` gets called from the privsep child, which appears to call the non-asynchronous safe `do_cleanup()`, but possibly only post authentication (`the_authctxt != NULL`).
The vulnerability CVE-2024-6409 is distinct from CVE-2024-6387 (aka RegreSSHion) because in the former the race condition and RCE potential are triggered in the privsep child process, which runs with reduced privileges compared to the parent server process.
The CVE-2024-6409 vulnerability affects only the sshd server shipped in RHEL 9, while the upstream versions of sshd are not impacted.
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