CVE-2026-59844 in libssh
Summary
by MITRE • 07/21/2026
A flaw was found in libssh. A remote authenticated client can issue SSH_FXP_READ requests with an arbitrarily large length, causing a libssh SFTP server to allocate excessive memory and potentially exhaust it through repeated requests.
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Analysis
by VulDB Data Team • 07/21/2026
This vulnerability exists within the libssh library implementation of the Secure Shell File Transfer Protocol which serves as a critical component in secure file operations across networked systems. The flaw represents a classic denial of service condition that can be exploited by authenticated remote attackers who possess valid credentials to establish an SSH connection. The vulnerability manifests when the SFTP server processes SSH_FXP_READ requests with excessively large length parameters, allowing malicious actors to manipulate memory allocation behavior through crafted protocol interactions.
The technical nature of this vulnerability stems from inadequate input validation within the SFTP server implementation where the library fails to properly constrain the maximum size parameter in read operations. When an authenticated client submits a request with an arbitrarily large length value, the libssh server allocates memory proportional to that requested size without proper bounds checking. This memory allocation behavior can be repeatedly exploited through multiple sequential requests, leading to progressive memory consumption that eventually exhausts available system resources. The flaw operates at the protocol level within the SFTP subsystem, making it particularly dangerous as it requires only authentication credentials rather than privileged access.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise entire server availability and system stability. Attackers can systematically consume memory resources through repeated exploitation, causing the targeted SFTP server to become unresponsive or crash entirely. This creates a denial of service condition that can affect legitimate users attempting to perform authorized file transfers while simultaneously consuming significant computational resources. The vulnerability affects systems running libssh implementations where SFTP services are exposed to authenticated remote clients, making it particularly concerning for enterprise environments where multiple users maintain access to shared file systems.
Mitigation strategies should focus on immediate implementation of input validation controls within the SFTP server configuration and application-level restrictions on maximum request sizes. System administrators should implement rate limiting mechanisms to restrict the number of concurrent read operations and establish memory allocation caps that prevent excessive consumption. The most effective approach involves updating to patched versions of libssh where proper bounds checking has been implemented for all SFTP operations, particularly those involving file size parameters. Organizations should also consider implementing network-level monitoring to detect unusual patterns of large request sizes and establish automated alerting systems that can identify potential exploitation attempts.
This vulnerability aligns with CWE-770 which describes the allocation of resources without limits or with inadequate limits, representing a fundamental resource exhaustion issue within the software implementation. The attack pattern follows ATT&CK technique T1499.004 which encompasses network denial of service attacks through resource consumption, specifically targeting memory exhaustion conditions in network services. The flaw demonstrates how authenticated access can be leveraged to perform privilege escalation-like effects through resource manipulation rather than direct authentication bypass, making it particularly insidious for environments where user access controls are properly enforced but system resource management is not adequately protected.