CVE-2025-5987: Libssh ChaCha20 Cipher Vulnerability with OpenSSL
A security vulnerability has been identified in libssh when used with the ChaCha20 cipher in conjunction with the OpenSSL library. This flaw, tracked as CVE-2025-5987, could potentially lead to data compromise and system instability.
🔍 TL;DR Summary
Libssh, when using the ChaCha20 cipher with OpenSSL, fails to properly handle heap exhaustion errors. An attacker exploiting this can cause libssh to use a partially initialized cipher context, leading to compromised data confidentiality, integrity, and potential crashes. Patch your libssh installation to mitigate this risk.
🚨 Vulnerability Details
- CVE ID: CVE-2025-5987
- Description: A flaw exists in libssh where the OpenSSL error code for heap exhaustion aliases with the SSH_OK code. This prevents libssh from correctly detecting memory allocation failures when using ChaCha20, potentially leading to the use of uninitialized cipher contexts.
- CVSS Score and Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L (Base Score: 5.0 MEDIUM)
- CVSS Explanation: This vulnerability has a MEDIUM severity because, while it is network-exploitable, it requires high attack complexity. An attacker needs to exhaust the heap which is not a trivial task. Privilege is low. If successful, impact is limited to low confidentiality, integrity, and availability.
- Exploit Requirements: Successful exploitation requires an attacker to exhaust the heap space available to libssh when performing ChaCha20 encryption using the OpenSSL library. This can be achieved by sending a large volume of data intended for encryption.
- Affected Vendor, Product, Version: libssh (Specific versions affected are still under investigation).
- CWE: CWE-393 - Security Bypass
- CWE Explanation: CWE-393 describes a situation where security features can be circumvented or bypassed. In this case, the OpenSSL error code aliasing causes a security check to be bypassed.
📅 Timeline of Events
- 2025-07-07: Vulnerability reported to Red Hat.
- 2025-07-07: CVE ID assigned (CVE-2025-5987).
- 2025-07-07: Initial public disclosure.
- [Future Date]: Patch release (expected).
🧠 Exploitability & Real-World Risk
While the exploitability of this vulnerability is considered high complexity due to the requirement of exhausting the heap, the potential impact is significant. In scenarios where libssh is used to handle sensitive data over network connections, a successful exploit could lead to the disclosure of encrypted information or data corruption. It is important to note that successful exploitation depends on specific system configurations and traffic patterns, which increases the difficulty. Real-world impact is high because libssh is an integral part of many systems.
🛠️ Recommendations
- Apply the Patch: Once a patch is available, apply it immediately. This is the most effective way to mitigate the vulnerability.
- Monitor System Resources: Implement monitoring to detect unusual memory usage patterns that could indicate an attempted heap exhaustion attack.
- Review SSH Configurations: Ensure SSH configurations adhere to security best practices to minimize the attack surface.
- Consider Alternative Ciphers: While patching is the priority, evaluating alternative cipher suites can provide defense in depth.
🧪 Technical Insight
The root cause of this vulnerability lies in the error handling within libssh. The library relies on the OpenSSL library for cryptographic operations. When OpenSSL encounters a heap exhaustion error, it returns a specific error code. However, this error code mistakenly aliases with the SSH_OK code. This mixup prevents libssh from properly detecting the memory allocation failure and leads to using potentially uninitialized cryptographic parameters. Because the cipher is not properly initialized, encrypted data will be predictable and corrupt, allowing an attacker to compromise the session.
🙌 Credit to Researcher(s)
This vulnerability was reported to Red Hat.
🔗 References
🧵 Tags
libssh, CVE-2025-5987, ChaCha20, OpenSSL, Vulnerability, Security, Heap Exhaustion, Red Hat
Summary: A vulnerability exists in libssh when using the ChaCha20 cipher with OpenSSL. Heap exhaustion can lead to libssh using a partially initialized cipher context, compromising data confidentiality and integrity. Apply patches when available and monitor system resources.
CVE ID: CVE-2025-5987
Risk Analysis: Successful exploitation can lead to data confidentiality loss, data integrity compromise, and service disruption. It is critical to apply patches and monitor resources.
Recommendation: Apply the patch to libssh once available. Monitor your system resources for abnormal memory usage. Review SSH configurations for optimal security.
Timeline
- 2025-07-07: Vulnerability reported to Red Hat and CVE ID assigned.