libssh2 Heap Buffer Overflow via ETM Cipher Negotiation
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 minus one bytes via memcpy, causing an overflow that on 32-bit glibc writes attacker-controlled bytes into an adjacent chunk's SIZE field, enabling tcache bin confusion, overlapping live objects, and function pointer overwrite during the session handshake before authentication.
- Published
- Jul 24, 2026
- Updated
- Jul 30, 2026
- EPSS
- 0.6% · 44th percentile
- CWE
- CWE-122
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Report tools
Action score
30
Monitor
Low priority for now.
- CVSS
- 30 / 40 · 7.7 / 10
- CISA KEV
- 0 / 30 · Not listed
- EPSS
- 0 / 30 · 0.6%
CISA SSVC decision
- Exploitation
- none
- Automatable
- no
- Technical impact
- total
Vulnrichment: CISA's decision-tree inputs.
CNA vs NVD score
- NVD
- —
- CNA · VulnCheck
- 7.7
- NVD has not scored this yet; the score shown is the CNA’s.
CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.
Affected systems
| Vendor | Product | CPE |
|---|---|---|
| libssh2 | libssh2 | cpe:2.3:a:libssh2:libssh2 |
Affected versions
NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.
- libssh2 libssh2up to 1.11.1 inclusive
Versions reported by the vendor
Affected version ranges reported by the assigning authority (VulnCheck). Independent of NVD's CPE analysis and usually ahead of it.
libssh2 libssh2
- 42e33d81577ed4b95d4b4f6f845e5ee8efe5eeb4not affected
- up to and including 1.11.1affected · semver
Package-level exposure
OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.
| Ecosystem | Package | Affected range | Fix |
|---|---|---|---|
| Debian:13 | libssh2 | before 1.11.1-1+deb13u2 | 1.11.1-1+deb13u2 |
| Debian:14 | libssh2 | before 1.11.1-5 | 1.11.1-5 |
| openSUSE:Leap 16.0 | libssh2_org | before 1.11.1-160000.5.1 | 1.11.1-160000.5.1 |
| openSUSE:Tumbleweed | libssh2_org | before 1.11.1-4.1 | 1.11.1-4.1 |
| Red Hat:hummingbird:1 | libssh2 | before 0:1.11.1-10.3.hum1 | 0:1.11.1-10.3.hum1 |
| Red Hat:hummingbird:1 | libssh2-docs | before 0:1.11.1-10.3.hum1 | 0:1.11.1-10.3.hum1 |
| SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOS | libssh2_org | before 1.11.0-150000.4.35.1 | 1.11.0-150000.4.35.1 |
| SUSE:Linux Enterprise Micro 5.3 | libssh2_org | before 1.11.0-150200.9.8.1 | 1.11.0-150200.9.8.1 |
| SUSE:Linux Enterprise Module for Basesystem 15 SP7 | libssh2_org | before 1.11.0-150600.20.6.1 | 1.11.0-150600.20.6.1 |
| SUSE:Linux Enterprise Server 12 SP5-LTSS | libssh2_org | before 1.11.0-29.18.1 | 1.11.0-29.18.1 |
| SUSE:Linux Micro 6.0 | libssh2_org | before 1.11.0-4.1 | 1.11.0-4.1 |
| SUSE:Linux Micro 6.1 | libssh2_org | before 1.11.0-slfo.1.1_2.1 | 1.11.0-slfo.1.1_2.1 |
| SUSE:Linux Micro 6.2 | libssh2_org | before 1.11.1-160000.5.1 | 1.11.1-160000.5.1 |
Same product
libssh2: all recordsOther highest-scoring records for the same primary product.
- CVE-2019-3861An out of bounds read flaw was discovered in libssh2 before 1.8.1 in the way SSH packets with a padding length value greater than the packet38Monitor
- CVE-2019-3860An out of bounds read flaw was discovered in libssh2 before 1.8.1 in the way SFTP packets with empty payloads are parsed.38Monitor
- CVE-2019-3858An out of bounds read flaw was discovered in libssh2 before 1.8.1 when a specially crafted SFTP packet is received from the server.38Monitor
- CVE-2019-3855An integer overflow flaw which could lead to an out of bounds write was discovered in libssh2 before 1.8.1 in the way packets are read from 38Monitor
- CVE-2019-3862An out of bounds read flaw was discovered in libssh2 before 1.8.1 in the way SSH_MSG_CHANNEL_REQUEST packets with an exit status message and38Monitor
- CVE-2019-3859An out of bounds read flaw was discovered in libssh2 before 1.8.1 in the _libssh2_packet_require and _libssh2_packet_requirev functions.38Monitor
Remediation
Which version to upgrade to
Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.
| Product / package | Fixed version | Source |
|---|---|---|
| debian:libssh2 | 1.11.1-1+deb13u2 · Debian:13 | Package registry (OSV) |
| openSUSE:libssh2_org | 1.11.1-4.1 · openSUSE:Tumbleweed | Package registry (OSV) |
| red hat:libssh2 | 0:1.11.1-10.3.hum1 · Red Hat:hummingbird:1 | Package registry (OSV) |
| red hat:libssh2-docs | 0:1.11.1-10.3.hum1 · Red Hat:hummingbird:1 | Package registry (OSV) |
| SUSE:libssh2_org | 1.11.1-160000.5.1 · SUSE:Linux Micro 6.2 | Package registry (OSV) |
Exploit status
No known public exploit
No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.
Research context
For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.
Timeline
From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.
No dated events beyond publication.
FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).
Patch and commit links
Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.
Credits
All researchersFinders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.
- VladimirEliTokarevfinder
Variant candidates
Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.
No nightly-computed relations.
Chain candidates
An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.
—
Bug bounty scope
No known public program.
Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).
Technical details
Attack conditions
- Anyone who can reach it over the internet can trigger it.
- No account or password is required.
- Special conditions such as timing or configuration are required.
If successful
- Confidentiality
- Integrity
- Availability
- Attack vector
- Network
- Attack complexity
- High
- Privileges required
- None
- User interaction
- P
- Scope
- X
Weakness class (CWE)
CWE-122 · Heap-based Buffer OverflowCVSS vector
CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
nvd-secondary
Attack context
MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.
Attack patterns (CAPEC)
ATT&CK techniques
—
Noroxi analysis
No Noroxi analysis for this record yet
We don't hand-write analysis for all 385,000+ vulnerabilities; that wouldn't be honest. For notable, high-impact vulnerabilities our team writes the mechanism, detection and remediation steps.
We use this product, ask for helpChange log
- Fix✗ → ✓
For records you follow, these changes also arrive as notifications. →
References
- github.com/libssh2/libssh2/commit/42e33d81577ed4b95d4b4f6f845e5ee8efe5eeb4
- github.com/libssh2/libssh2/pull/2198
- www.vulncheck.com/advisories/libssh2-heap-buffer-overflow-via-etm-cipher-negotiation
Vendor advisories and official records. Exploit/PoC links are deliberately left out.