Libssh — CraftedSignal Threat Feed

Libssh Denial-of-Service Vulnerability via Inefficient Regular Expression Processing (CVE-2026-0967)

hello@craftedsignal.io — Fri, 01 May 2026 07:16:39 +0000

CVE-2026-0967 is a denial-of-service (DoS) vulnerability affecting libssh, a library implementing the SSH protocol. The root cause lies in the inefficient processing of regular expressions within the library’s code. An attacker could exploit this vulnerability by sending specially crafted input that triggers excessive resource consumption during regular expression matching, leading to a denial of service. Successful exploitation could potentially enable defense evasion by overwhelming security controls and negatively impacting the availability of systems relying on the vulnerable libssh library. The vulnerability affects both Linux and Windows platforms where libssh is used.

Attack Chain

The attacker identifies a service or application utilizing a vulnerable version of libssh.
The attacker crafts a malicious input string designed to trigger inefficient regular expression processing within libssh.
The attacker sends the crafted input to the vulnerable service via a network connection (e.g., SSH).
The libssh library attempts to process the malicious input using its regular expression engine.
The inefficient regular expression causes excessive CPU consumption or memory allocation.
The vulnerable service becomes unresponsive due to resource exhaustion, leading to a denial-of-service condition.
Subsequent legitimate requests to the service are blocked or delayed, further exacerbating the impact.

Impact

Successful exploitation of CVE-2026-0967 can result in a denial-of-service condition, rendering affected services or applications unavailable. The impact scope depends on the role of the affected system. For example, a critical server becoming unavailable could disrupt business operations. While the number of potential victims is unknown, any system utilizing a vulnerable version of libssh is susceptible. The defense evasion aspect could allow attackers to bypass security controls during the DoS.

Recommendation

Identify systems using libssh and determine the installed version.
Apply available patches or updates for libssh to remediate CVE-2026-0967 as released by Microsoft.
Deploy the Sigma rule “Detect Suspicious Libssh Regex Processing” to monitor for potential exploitation attempts.
Monitor CPU and memory usage on systems running libssh for unusual spikes, which may indicate a DoS attack.
Implement rate limiting on services using libssh to mitigate the impact of DoS attacks.

Multiple Vulnerabilities in libssh Allow File Manipulation and DoS

hello@craftedsignal.io — Thu, 16 Apr 2026 10:29:59 +0000

The libssh library, a widely used implementation of the SSH protocol, contains several vulnerabilities that could be exploited by a malicious actor. These vulnerabilities could allow an attacker to manipulate files on a system utilizing the vulnerable library, or cause a denial-of-service (DoS) condition, rendering the system or service unavailable. Given the widespread use of libssh in various applications and systems, these vulnerabilities pose a significant risk to organizations relying on this library for secure communication. The impact ranges from unauthorized data modification to complete service outages, impacting availability and data integrity. Publicly available exploit code may exist, increasing the likelihood of exploitation.

Attack Chain

The attacker identifies a system using a vulnerable version of libssh.
The attacker establishes an SSH connection to the target system.
The attacker exploits a vulnerability in libssh related to file handling (specific CVE details unavailable from provided source), potentially through crafted SSH commands.
Successful exploitation allows the attacker to modify arbitrary files on the system, potentially including configuration files or application data.
Alternatively, the attacker exploits a vulnerability related to resource management within libssh to trigger a denial-of-service.
This DoS is achieved by sending a specific sequence of SSH requests that consume excessive resources, such as memory or CPU time.
The targeted service becomes unresponsive, preventing legitimate users from accessing it.
The attacker maintains the DoS condition, disrupting the target’s operations.

Impact

Successful exploitation of these libssh vulnerabilities can have severe consequences. File manipulation could lead to data corruption, unauthorized access, or system compromise. A denial-of-service attack could disrupt critical services, leading to financial losses, reputational damage, and operational downtime. The number of potential victims is vast, considering the widespread use of libssh in servers, network devices, and embedded systems. The targeted systems and sectors are not specified in the source material.

Recommendation

Implement network monitoring to detect unusual SSH traffic patterns that may indicate exploitation attempts (review existing firewall and network connection logs).
Deploy the Sigma rule DetectSuspiciousSSHClientVersion to identify potentially malicious SSH clients connecting to your systems.
Monitor systems for unexpected file modifications, focusing on configuration files and application data (enable file integrity monitoring).

libssh Insecure Configuration Allows Local MITM Attacks (CVE-2025-14821)

hello@craftedsignal.io — Tue, 07 Apr 2026 17:16:25 +0000

A critical vulnerability, CVE-2025-14821, has been identified in the libssh library. This flaw arises from an insecure default configuration on Windows systems. Specifically, libssh automatically loads configuration files from the C:\etc directory. Critically, this directory can be created and modified by unprivileged local users. This allows a malicious local user to manipulate the SSH configuration, facilitating man-in-the-middle attacks, downgrading connection security, and manipulating trusted host information. Successful exploitation grants attackers the ability to intercept and potentially modify SSH communications, posing a significant risk to data confidentiality, integrity, and availability.

Attack Chain

Attacker creates the directory C:\etc if it does not already exist.
Attacker creates a malicious SSH configuration file (e.g., ssh_config) within the C:\etc directory. This configuration can specify settings to downgrade encryption or redirect connections.
A legitimate user initiates an SSH connection using an application that leverages the vulnerable libssh library.
libssh automatically loads the attacker-controlled configuration file from C:\etc\ssh_config.
The malicious configuration settings are applied, potentially downgrading the encryption algorithm used for the SSH connection.
The attacker intercepts the SSH traffic, performing a man-in-the-middle attack due to the weakened encryption or connection redirection.
The attacker can now eavesdrop on or modify the SSH communication, gaining unauthorized access to sensitive information or injecting malicious commands.
Attacker maintains persistent access or exfiltrates sensitive data obtained through the compromised SSH session.

Impact

Successful exploitation of CVE-2025-14821 allows a local attacker to perform man-in-the-middle attacks on SSH connections. This can lead to the compromise of sensitive data transmitted over SSH, such as credentials, configuration files, or confidential documents. The ability to manipulate trusted host information further exacerbates the risk, potentially allowing attackers to impersonate legitimate servers. The vulnerability affects any Windows system using a vulnerable version of libssh and could impact organizations across all sectors that rely on SSH for secure communication and remote administration.

Recommendation

Monitor for the creation or modification of files within the C:\etc directory, particularly configuration files like ssh_config, using file integrity monitoring (FIM) rules on Windows systems.
Implement the Sigma rule provided to detect the creation of the C:\etc directory by non-system processes.
Restrict write access to the C:\etc directory and its contents using appropriate file system permissions on Windows systems.