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Out-of-Bounds Write in libssh2 ssh2_transport_read()

IdentifiersCVE-2026-55200CWE-787

CVE-2026-55200 is an out-of-bounds write vulnerability in libssh2 affecting versions through 1.11.1. The flaw is in ssh2_transport_read() in transport.c, where the packet_length field is not properly constrained with an upper-bound check. A remote attacker can send crafted SSH packets containing an excessively large packet_length value, causing heap memory corruption during packet processing. The vulnerability is fixed by commit 7acf3df.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

Successful exploitation can corrupt heap memory and may lead to remote code execution in the context of the vulnerable process. The available scoring information indicates high impact to confidentiality, integrity, and availability. Exploitation is remote over the network and does not require privileges or user interaction.

Mitigation

If you can’t patch tonight, do this now.

Until a patched version is deployed, reduce exposure of applications that use libssh2 by limiting network reachability to trusted SSH peers, placing affected services behind network access controls, and monitoring for malformed or anomalously large SSH packets. Where feasible, add defensive packet-length validation in consuming applications or intermediary controls, but vendor/library patching is the primary mitigation.

Remediation

Patch, then assume compromise.

Upgrade libssh2 to a patched version that includes the fix introduced by commit 7acf3df. If building from source or maintaining a fork, apply the changes from commit 7acf3df and ensure packet_length is validated against appropriate upper bounds before memory operations are performed in ssh2_transport_read().
PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

EXPOSURE SURFACE

Affected products & vendors

Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.

VendorProductType
Libssh2Libssh2application

Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.

ACTIVITY FEED

Recent activity

7 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.

7 SOURCESView more in app
What this page doesn’t show

The version that knows your environment.

This page is what’s public. Mallory adds the parts that aren’t: which of your assets are affected, which adversaries are exploiting it right now, which detections to deploy, and what to do tonight.
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Exposure mapping

Query your assets running an affected version, and investigate the blast radius.

Threat actor evidence

Every observed campaign linking this CVE to a named adversary.

Associated malware

Malware families riding this exploit, with evidence and IOCs.

Detection signatures

YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.

Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity6

Community discussion across Reddit, Mastodon, and other social sources.

EXTERNAL REFERENCES
MITRE CVE
cve.org · CVE-2026-55200
NVD
nvd.nist.gov/vuln/detail/CVE-2026-55200
MITRE CWE · CWE-787
cwe.mitre.org