CVE-2026-66035: libssh2 through 1.11.1, fixed in commit 42e33d8, contains a pre-authentication heap buffer overflow vulnerabil

Q: How do I fix CVE-2026-66035?

libssh2: upgrade to newer than 1.11.1. Exact commands for each OS are on this page.

Q: Is CVE-2026-66035 being exploited?

CVE-2026-66035 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-66035 affect?

CVE-2026-66035 affects libssh2. Severity is high (CVSS 7.5). libssh2 through 1.11.1, fixed in commit 42e33d8, contains a pre-authentication heap buffer overflow vulnerability that allows a malicious SSH server to corrupt heap metadata in any connecting client by sending a packet with a packet_length

Routine

HIGH 7.5UPGRADE

CVE-2026-66035 is a high-severity flaw (CVSS 7.5) affecting libssh2. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

libssh2 through 1.11.1, fixed in commit 42e33d8, contains a pre-authentication heap buffer overflow vulnerability that allows a malicious SSH server to corrupt heap metadata in any connecting client by sending a packet with a packet_length smaller than the cipher's block size during Encrypt-then-MAC cipher negotiation. In the fullpacket() function in src/transport.c, the ETM path allocates a buffer of packet_length bytes but copies blocksize minu

Why this priority

High severity

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.5% (top 59%)
CISA exploited list	No
Weakness	CWE-122
Published	2026-07-24
Last modified	2026-07-30

Publisher (CNA): VulnCheck · Vendor: libssh2

Last updated 2026-07-30 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-24 17:17 UTCPublished CVE record published
2026-07-30 15:41 UTCUpdated Record last modified

libssh2 libssh2 UPGRADE

Affected: <= 1.11.1

Fixed in: newer than 1.11.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy libssh2 || rpm -q libssh2 || libssh2 --version

2. UPGRADE to newer than 1.11.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade libssh2

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade libssh2

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-66035?

libssh2: upgrade to newer than 1.11.1. Exact commands for each OS are on this page.

Is CVE-2026-66035 being exploited?

CVE-2026-66035 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-66035 affect?

CVE-2026-66035 affects libssh2. Severity is high (CVSS 7.5). libssh2 through 1.11.1, fixed in commit 42e33d8, contains a pre-authentication heap buffer overflow vulnerability that allows a malicious SSH server to corrupt heap metadata in any connecting client by sending a packet with a packet_length

How urgent is CVE-2026-66035?

Priority 36/100. High severity

Related CVEs

CVE-2026-58050 HIGH 7.0 UPGRADE libssh2 through 1.11.1 reads an attacker-controlled 32-bit attribute count from a publickey-subsystem response
CVE-2026-58051 MEDIUM 6.5 UPGRADE libssh2 through 1.11.1 grows its publickey list with SSH2_REALLOC but does not zero-initialize new entries bef
CVE-2026-55200 HIGH 8.1 DOWNGRADE libssh2 1.11.1, fixed in commit 97acf3d, contains an out-of-bounds write vulnerability in ssh2_transport_read(
CVE-2026-66032 HIGH 8.8 UPGRADE libssh2 through 1.11.1, fixed in commit 5e47761, contains a double-free vulnerability in the sftp_open() funct
CVE-2026-66033 HIGH 7.5 UPGRADE libssh2 through 1.11.1, fixed in commit a2ed82d, contains a pre-authentication integer underflow vulnerability

References

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