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the hang again... nice. what may be the reason of this? edit: in ssh2_knownhost_fuzzer. |
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Pull request overview
Adds two new OSS-Fuzz harnesses to broaden libssh2’s fuzz coverage beyond the existing handshake fuzzer, and wires them into the Autotools/CMake OSS-Fuzz build and packaging scripts.
Changes:
- Add
ssh2_sftp_packet_fuzzerto drive banner reception and SSH binary packet parsing/dispatch using a synthetic server banner + wrapped fuzz payload. - Add
ssh2_knownhost_fuzzerto exerciselibssh2_knownhost_readline()and the writeline path. - Update OSS-Fuzz build scripts (Autotools + CMake + ossfuzz.sh) to build and export the new fuzz targets.
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
| tests/ossfuzz/ssh2_sftp_packet_fuzzer.cc | New fuzzer harness for SSH transport packet parsing/dispatch (with fixed server banner). |
| tests/ossfuzz/ssh2_knownhost_fuzzer.cc | New fuzzer harness for known_hosts line parsing and serialization. |
| tests/ossfuzz/ossfuzz.sh | Copy the additional fuzz binaries into $OUT. |
| tests/ossfuzz/Makefile.am | Build the two new fuzzers when OSS-Fuzz targets are enabled. |
| tests/ossfuzz/CMakeLists.txt | Generalize fuzzer target creation/linking via a loop for all fuzzers. |
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vszakats
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Aug 5, 2026
"MAX_PAYLOAD_SIZE is larger than LIBSSH2_PACKET_MAXPAYLOAD (40000). For inputs >~39995 bytes, packet_length exceeds libssh2's inbound limit and _libssh2_transport_read() returns OUT_OF_BOUNDARY early, which reduces the harness's ability to reach deeper parsing/dispatch code." libssh2#2509 (comment)
vszakats
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to vszakats/libssh2
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Aug 5, 2026
"MAX_PAYLOAD_SIZE is larger than LIBSSH2_PACKET_MAXPAYLOAD (40000). For inputs >~39995 bytes, packet_length exceeds libssh2's inbound limit and _libssh2_transport_read() returns OUT_OF_BOUNDARY early, which reduces the harness's ability to reach deeper parsing/dispatch code." libssh2#2509 (comment)
Member
Author
|
After making the knownhost fuzzer a no-op, it still hangs the same way: #include "libssh2.h"
extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size)
{
return 0;
} |
## Summary Adds two new libFuzzer harnesses targeting previously uncovered parsing paths in libssh2. ### New harness: `tests/ossfuzz/ssh2_sftp_packet_fuzzer.cc` Complements the existing `ssh2_client_fuzzer` by prepending a valid `SSH-2.0` server banner so that `banner_receive()` in `session.c` always completes and the code enters the SSH binary-packet transport layer with fuzz-controlled payload bytes. **Coverage added:** - Server banner/version string parsing (`session.c:banner_receive`) - SSH binary packet transport reader (`transport.c:_libssh2_transport_read`) - SSH message type dispatch (`packet.c:_libssh2_packet_add`) including: - `SSH_MSG_DISCONNECT` string parsing - `SSH_MSG_DEBUG` and `SSH_MSG_IGNORE` handlers - `SSH_MSG_EXT_INFO` extension negotiation - `SSH_MSG_GLOBAL_REQUEST` parsing - `SSH_MSG_CHANNEL_*` family (DATA, EOF, CLOSE, REQUEST, OPEN, WINDOW_ADJUST) - `SSH_MSG_USERAUTH_BANNER` string extraction - `SSH_MSG_KEXINIT` algorithm name-list parser in `kex.c` ### New harness: `tests/ossfuzz/ssh2_knownhost_fuzzer.cc` Exercises `libssh2_knownhost_readline()` which parses lines from an OpenSSH `known_hosts` file. Uses only the public API; no network access required. **Coverage added:** - Plain and comma-separated hostname parsing (`knownhost.c:oldstyle_hostline`) - Hashed hostname parsing (`knownhost.c:hashed_hostline`, `|1|salt|hash` format) - Key-type token recognition and base64 public-key blob decoding (`misc.c:_libssh2_base64_decode`) - HMAC-SHA1 computation for hashed host verification - `libssh2_knownhost_writeline()` round-trip for successfully parsed entries ### Why this matters The existing `ssh2_client_fuzzer` exercises the client-side SSH handshake but: - Does not bypass banner parsing (banner data and binary-packet data are interleaved in the same fuzz corpus, making it hard for the fuzzer to reach post-banner code) - Does not cover the known-hosts parser (`knownhost_readline`, `hashed_hostline`, base64 decode) — reachable whenever an application calls `libssh2_knownhost_readfile()` Both paths process attacker-controlled data and are reachable in production deployments. ### Build Both harnesses integrate with the existing `tests/ossfuzz/ossfuzz.sh` build system and require no new dependencies. Verified to compile cleanly with OpenSSL + zlib on Debian Bookworm. Ref 1953
"MAX_PAYLOAD_SIZE is larger than LIBSSH2_PACKET_MAXPAYLOAD (40000). For inputs >~39995 bytes, packet_length exceeds libssh2's inbound limit and _libssh2_transport_read() returns OUT_OF_BOUNDARY early, which reduces the harness's ability to reach deeper parsing/dispatch code." libssh2#2509 (comment)
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🟡 Changes recommended
Packet framing is usually invalid, and advertised SFTP and HMAC coverage is not implemented.
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Review details
- Files reviewed: 6/6 changed files
- Comments generated: 4
- Review effort level: Balanced
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| const uint8_t padding_length = 4; | ||
| uint32_t packet_length = (uint32_t)(1 + payload_len + padding_length); | ||
| size_t pkt_buf_len = 4 + 1 + payload_len + padding_length; |
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| ssh2_sftp_packet_fuzzer \ | ||
| ssh2_knownhost_fuzzer |
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| * All of these fields involve string parsing, base64 decoding, and | ||
| * (for hashed hosts) HMAC computation - paths that process | ||
| * attacker-controlled data in any deployment that calls | ||
| * libssh2_knownhost_readfile() or libssh2_knownhost_readline(). |
| * SPDX-License-Identifier: BSD-3-Clause | ||
| */ | ||
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| /* Fuzz harness for libssh2 SSH binary packet dispatch and SFTP packet parsing. |
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