Proof-of-Concept Exploits Published for 2 New OpenSSH Bugs

Millions of servers are at risk from vulnerabilities in a remote server management and file transfer tool that can be used to launch a man-in-the-middle attack or effect a denial of service.

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The vulnerabilities are present in multiple versions of OpenSSH, an open source implementation of the Secure Shell-based protocol 2.0, used for remote access and server management and file transfers, and which includes SFTP client and server support. The tool is available stand-alone and also built into numerous operating systems, including Microsoft Windows 10 and 11, macOS, and most Linux distributions.

The OpenSSH project has patched the two flaws via the Tuesday release of OpenSSH 9.9p2.

“Both vulnerabilities were discovered and demonstrated to be exploitable by the Qualys Security Advisory team,” OpenSSH said. “We thank them for their detailed review of OpenSSH.”

Internet of Things search engine Shodan on Tuesday reported seeing about 33 million internet-exposed servers that run OpenSSH. Not all would be vulnerable to either flaw.

The man-in-the-middle vulnerability, tracked as CVE-2025-26465, exists in versions 6.8p1 – released in March 2018 – through 9.9p1, and could be used by “an on-path attacker to impersonate any server,” OpenSSH said.

“When a vulnerable client connects to a server, an active machine-in-the-middle can impersonate the server by completely bypassing the client’s checks of the server’s identity,” Qualys said in a security advisory.

If exploited, “this would break the integrity of the SSH connection, enabling potential interception or tampering with the session before the user even realizes it,” Qualys said. “SSH sessions can be a prime target for attackers aiming to intercept credentials or hijack sessions. If compromised, hackers could view or manipulate sensitive data, move across multiple critical servers laterally, and exfiltrate valuable information such as database credentials.”

One mitigating factor is that an OpenSSH client is only vulnerable to CVE-2025-26465 if the VerifyHostKeyDNS option is enabled; it is usually disabled by default. One notable exception is that VerifyHostKeyDNS “was enabled by default on FreeBSD from September 2013 until March 2023,” Qualys said.

If the option isn’t disabled, “this attack against the OpenSSH client succeeds whether VerifyHostKeyDNS is ‘yes’ or ‘ask’ (it is ‘no’ by default), without user interaction, and whether the impersonated server actually has an SSHFP resource record or not (an SSH fingerprint stored in DNS),” it said.

The denial of service vulnerability, tracked as CVE-2025-26466, was introduced with OpenSSH version 9.5p1 – released in October 2023 – and existed through version 9.9p1, and exists in both clients and servers. The vulnerability could be exploited by attackers using SSH2_MSG_PING packets to create “a pre-authentication denial-of-service attack” via “asymmetric resource consumption of both memory and CPU,” Qualys said.

“On the server side, this attack can be easily mitigated by mechanisms that are already built in OpenSSH: LoginGraceTime, MaxStartups, and more recently (OpenSSH 9.8p1 and newer) PerSourcePenalties,” it said.

The risk posed by this flaw is simply one of client and server availability. “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,” Qualys said. “An enterprise facing this vulnerability could see critical servers become unreachable, interrupting routine operations and stalling essential maintenance tasks.”

Qualys said its researchers discovered the flaws, and reported them directly to OpenSSH on Jan. 31, after which the security advisory and patches were distributed to the operating system distribution security mailing list on Feb. 10.

On Tuesday, Qualys published full details and proof-of-concept exploit code for both flaws, timed to coincide with OpenSSH’s patch release, and emphasized that OpenSSH 9.9p2 patches the vulnerabilities. “To ensure continued security, we strongly advise upgrading affected systems to 9.9p2 as soon as possible,” it said.