PatchSiren cyber security CVE debrief
CVE-2025-8277 Red Hat CVE debrief
A memory leak vulnerability exists in libssh's key exchange (KEX) handling. When a client repeatedly sends incorrect KEX guesses, the library fails to free memory during rekey operations, leading to gradual memory exhaustion. This can cause client-side crashes, particularly when libgcrypt is in use, impacting application stability and availability. The vulnerability was published on September 9, 2025, and last modified on May 19, 2026. Red Hat has assigned this issue a CVSS 3.1 score of 3.1 (Low severity). The weakness is categorized as CWE-401 (Missing Release of Memory after Effective Lifetime).
- Vendor
- Red Hat
- Product
- Red Hat Enterprise Linux 9
- CVSS
- LOW 3.1
- CISA KEV
- Not listed in stored evidence
- Original CVE published
- 2025-09-09
- Original CVE updated
- 2026-05-19
- Advisory published
- 2025-09-09
- Advisory updated
- 2026-05-19
Who should care
Organizations running applications that use libssh as an SSH client library, particularly those with long-lived connections or automated SSH client processes. System administrators managing environments where libssh with libgcrypt is deployed should monitor for stability issues. Developers integrating libssh into client applications should assess their KEX handling and plan for patch integration. Security teams should include this in low-priority patching cycles given the CVSS Low severity rating and client-side-only impact.
Technical summary
The vulnerability stems from improper memory management in libssh's key exchange implementation. During rekey operations, when a client submits incorrect KEX guesses repeatedly, allocated memory is not released. This leads to a gradual accumulation of unreleased memory, eventually exhausting available system resources. The crash manifestation is particularly pronounced when libgcrypt serves as the cryptographic backend. The attack requires network access and repeated incorrect KEX guesses, with high attack complexity due to the need for legitimate client interaction patterns. The impact is confined to availability degradation through client-side crashes, with no direct confidentiality or integrity compromise.
Defensive priority
low
Recommended defensive actions
- Monitor libssh deployments for unusual memory consumption patterns, particularly in long-lived SSH client connections
- Apply vendor-provided patches when available from distribution maintainers or upstream libssh
- Consider implementing connection rate limiting or anomaly detection for SSH client behavior if feasible
- Review application logs for repeated KEX negotiation failures that may indicate exploitation attempts
- Prioritize patching for environments using libssh with libgcrypt where client stability is critical
Evidence notes
The vulnerability description is sourced from official CVE metadata published by NVD and Red Hat. The memory leak behavior is specifically tied to repeated incorrect KEX guesses during rekey operations. The impact is limited to client-side crashes and availability degradation, with no confidentiality or integrity impact. The CVSS vector confirms network attack vector with high attack complexity, low privileges required, and low availability impact.
Official resources
public