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Researchers disclosed the Terrapin attack, a man-in-the-middle flaw in the SSH transport protocol tracked as CVE-2023-48795, that can corrupt the handshake and strip or disable negotiated security features. The attack exploits sequence number manipulation and prefix truncation in the SSH Binary Packet Protocol, affecting connections that use ChaCha20-Poly1305 and certain CBC with Encrypt-then-MAC configurations. The issue impacts OpenSSH before 9.6 and a broad set of SSH clients, servers, libraries, and products; researchers reported that 77% of Internet-exposed SSH servers supported at least one vulnerable mode and 57% preferred one.
The disclosure also highlighted implementation-specific flaws in AsyncSSH—CVE-2023-46445 and CVE-2023-46446—that could enable more severe outcomes, including redirecting a victim into an attacker-controlled shell account under the right conditions. Ruhr University Bochum researchers released technical details, proof-of-concept code, and a scanner after coordinated vendor fixes, while maintainers shipped updates including OpenSSH 9.6, PuTTY 0.8, libssh 0.10.6/0.9.8, and AsyncSSH 2.14.1/2.14.2. Defenders were urged to patch affected software and, where possible, disable vulnerable cipher modes in favor of safer options such as AES-GCM.
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The Terrapin attack was tracked as CVE-2023-48795, covering the SSH transport protocol weakness that can allow integrity-check bypass and downgrade of negotiated security features in affected implementations.
Alongside disclosure, the researchers published the Terrapin technical paper as well as proof-of-concept code and a scanner, providing detailed information on the attack and affected configurations.
Researchers publicly revealed Terrapin, a man-in-the-middle prefix truncation attack against SSH that can weaken or remove negotiated security protections during the handshake under specific cipher configurations.
Affected projects issued updates to address Terrapin and related implementation flaws, including OpenSSH 9.6, PuTTY 0.8, libssh 0.10.6 and 0.9.8, and AsyncSSH 2.14.1 and 2.14.2.
Researchers from Ruhr University Bochum privately reported the Terrapin SSH weakness to affected vendors in October 2023, beginning coordinated remediation before public release.
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OpenSSH released security updates to address two vulnerabilities tracked as **CVE-2025-26465** and **CVE-2025-26466**, with downstream advisories warning that affected deployments could face both server impersonation and service disruption. According to vendor and researcher disclosures, **CVE-2025-26465** can enable a man-in-the-middle attack when `VerifyHostKeyDNS` is enabled, allowing an attacker to bypass host verification, present a crafted key, impersonate a trusted SSH server, and potentially intercept sessions, access sensitive data, or execute commands. The second issue, **CVE-2025-26466**, is a pre-authentication denial-of-service flaw caused by unbounded memory allocation triggered through `SSH2_MSG_PING` messages, which can exhaust memory and disrupt SSH availability. Guidance from OpenSSH and national CSIRT reporting urges organizations to upgrade to **OpenSSH 9.9p2 or later**, disable `VerifyHostKeyDNS` unless it is strictly required, and harden exposed services with connection controls such as `LoginGraceTime`, `MaxStartups`, and `PerSourcePenalties` to reduce exploitation risk.
Jul 24, 2026
A critical flaw in **OpenSSH server**, tracked as `CVE-2024-6387` and dubbed **regreSSHion**, can allow an unauthenticated attacker to achieve remote code execution with **root** privileges under specific conditions. The issue affects OpenSSH versions **8.5p1 through 9.7p1**, and public reporting says it has been **actively exploited in the wild** with proof-of-concept code available. Researchers and national defenders also highlighted related issues, including `CVE-2024-6409`, a less severe code-execution variant identified during analysis, and `CVE-2024-7589`, a **FreeBSD-specific** variant patched separately. Defenders were urged to **upgrade to OpenSSH 9.8p1 or later** to remove exposure. Where immediate patching is not possible, guidance recommends temporarily restricting or disabling SSH access, or setting `LoginGraceTime` to `0` as a mitigation, while warning that this workaround can disrupt service availability and should be treated as temporary until systems are fully updated.
Jul 24, 2026
A severe flaw in **PuTTY** and several applications that reuse its code can expose private keys used with the NIST P-521 `ECDSA` algorithm. The weakness causes the software to generate insufficiently random nonces, allowing an attacker to recover a victim’s private key after collecting roughly 60 signatures from the vulnerable implementation. The issue affects PuTTY as well as tools including **FileZilla**, **WinSCP**, and **TortoiseGit**; users of **TortoiseSVN** were advised to replace `Plink` with the latest PuTTY version until an official fix becomes available. Organizations were urged to update immediately and treat any affected P-521 key pairs as potentially compromised because recovered keys could be reused for SSH authentication or other trusted public-key access. Separately, a critical compromise in **XZ Utils** introduced malicious code into versions `5.6.0` and `5.6.1`, creating a supply-chain backdoor that could enable unauthorized access on affected Linux systems, with **SSH** identified as a primary exposure path. Defenders were advised to downgrade to version `5.4.6` immediately, consider disabling SSH if rollback was not possible, and verify system integrity because patching alone may not be enough if the backdoor was already exploited. The alerts noted that some Linux distributions had shipped the tainted versions while others were not observed to be vulnerable or had already issued updates.
Jun 29, 2026