How severe is CVE-2026-55199?

CVE-2026-55199 has a CVSS 3.x base score of 5.9, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2026-55199 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (33rd percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-55199?

CVE-2026-55199 affects Libssh2. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-55199?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2026-55199 have an EU (EUVD) identifier?

Yes. CVE-2026-55199 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-37782.

When was CVE-2026-55199 published?

CVE-2026-55199 was published on 2026-06-17 and last updated on 2026-06-26.

CVE-2026-55199

CVE-2026-55199 is a medium-severity vulnerability in Libssh2 with a CVSS 3.x base score of 5.9. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-835.

Key facts

Severity: Medium (CVSS 3.x base score 5.9)
CVSS v4: 8.2
EPSS exploit prediction: 0% (33rd percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-37782
Weakness: CWE-835
Affected product: Libssh2
Published: 2026-06-17
Last modified: 2026-06-26

Description

libssh2 through 1.11.1, fixed in commit 1762685, contains a pre-authentication denial of service vulnerability in the SSH_MSG_EXT_INFO handler in src/packet.c that allows a malicious SSH server to cause a client CPU exhaustion loop by sending a crafted extension count value. A malicious server can set nr_extensions to 0xFFFFFFFF during key exchange, causing the client to spin in a tight CPU loop for over 60 seconds because return values from _libssh2_get_string() are unchecked and the session timeout does not apply to CPU-bound loops.

Frequently asked questions

What is CVE-2026-55199?: libssh2 through 1.11.1, fixed in commit 1762685, contains a pre-authentication denial of service vulnerability in the SSH_MSG_EXT_INFO handler in src/packet.c that allows a malicious SSH server to cause a client CPU exhaustion loop by sending a crafted extension count value. A malicious server can set nr_extensions to 0xFFFFFFFF during key exchange, causing the client to spin in a tight CPU loop for over 60 seconds because return values from _libssh2_get_string() are unchecked and the session timeout does not apply to CPU-bound loops.
How severe is CVE-2026-55199?: CVE-2026-55199 has a CVSS 3.x base score of 5.9, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2026-55199 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (33rd percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-55199?: CVE-2026-55199 affects Libssh2. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-55199?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-55199 have an EU (EUVD) identifier?: Yes. CVE-2026-55199 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-37782.
When was CVE-2026-55199 published?: CVE-2026-55199 was published on 2026-06-17 and last updated on 2026-06-26.

References

Affected products (1)

cpe:2.3:a:libssh2:libssh2:*:*:*:*:*:*:*:*

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