libssh2: Integer Overflow to Buffer Overflow
CVE-2026-55200 is a high-severity integer overflow to buffer overflow vulnerability in libssh2, with a CVSS 3.1 base score of 8.3 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, but only if a user takes part, for example by opening a link or file. The record lists version 97acf3dfda80c91c3a8c9f2372546301d4a1a7a8 as the first without the flaw, so updating libssh2 to that release or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 3.88% chance of exploitation within 30 days, which is in the top 9.62% of all scored CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update libssh2 to 97acf3dfda80c91c3a8c9f2372546301d4a1a7a8 or a later version.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
- Because the attack needs a user to take part, warn users about unexpected links and files related to this product until it is patched.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Libssh2 | libssh2 | 1.11.1; 1c3f1b7da588f2652260285529ec3c1f1125eb4e up to, but not including, 97acf3dfda80c91c3a8c9f2372546301d4a1a7a8 | 97acf3dfda80c91c3a8c9f2372546301d4a1a7a8 |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- RequiredA user must take an action such as opening a file or link.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- LowPerformance or availability is reduced.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:L. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 9.2 | Critical | CNA (vulncheck.com) |
| CVSS 3.1 | 8.1 | High | CNA (vulncheck.com) |
| CVSS 3.1 | 8.3 | High | NVD |
Official description
libssh2 1.11.1, fixed in commit 97acf3d, contains an out-of-bounds write vulnerability in ssh2_transport_read() that fails to enforce upper bounds on packet_length field. Remote attackers can send crafted SSH packets with excessively large packet_length values to corrupt heap memory and achieve remote code execution.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Integer Overflow to Buffer Overflow
The product performs a calculation to determine how much memory to allocate, but an integer overflow can occur that causes less memory to be allocated than expected, leading to a buffer overflow.
More Integer Overflow to Buffer Overflow vulnerabilities (CWE-680)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: poc, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
Third-party and government advisories
Technical write-ups (may include proof of concept)
- web.archive.org/web/20260623211210/https://github.com/bikini/exploitarium/tree/main/libssh2-cve-2026-55200-pocExploit, Third Party Advisory
Other vulnerabilities in libssh2
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 8.8 High severity
- 7.5 High severity
- 7.5 High severity
Recent Integer Overflow to Buffer Overflow vulnerabilities
- 9.8 Critical severity
- 7.5 High severity
- 8.8 High severity
- 8.8 High severity
- 8.8 High severity
- 8.8 High severity
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