# ksmbd – Fuzzing Improvements and Vulnerability Discovery (2/3)

02 Sep 2025 – Posted by Norbert Szetei

## Introduction

This is a follow-up to the article originally published here.

Our initial research uncovered several unauthenticated bugs, but we had only touched the attack surface lightly. Even after patching the code to bypass authentication, most interesting operations required interacting with handlers and state we initially omitted. In this part, we explain how we increased coverage and applied different fuzzing strategies to identify more bugs.

Some functionalities require additional configuration options. We tried to enable many available features to maximize the exposed attack surface. This helped us trigger code paths that are disabled in the minimalistic configuration example. However, to simplify our setup, we did not consider features like Kerberos support or RDMA. These could be targets for further improvement.

## Configuration-Dependent Attack Surface

The following functionalities helped expand the attack surface. Only oplocks are enabled by default.

**G** = Global scope only

**S** = Per-share, but can also be set globally as a default

– **durable handles (G)**
– **oplocks (S)**
– **server multi channel support (G)**
– **smb2 leases (G)**
– **vfs objects (S)**

From a code perspective, in addition to `smb2pdu.c`, these source files were involved:

– `ndr.c`– NDR encoding/decoding used in SMB structures
– `oplock.c`– Oplock request and break handling
– `smbacl.c`– Parsing and enforcement of SMB ACLs
– `vfs.c`– Interface to virtual file system operations
– `vfs_cache.c`– Cache layer for file and directory lookups

The remaining files in the `fs/smb/server` directory were either part of standard communication or exercising them required a more complex setup, as in the case of various authentication schemes.

## Fuzzer Improvements

SMB3 expects a valid session setup before most operations, and its authentication flow is multi-step, requiring correct ordering. Implementing valid Kerberos authentication was impractical for fuzzing.

As described in the first part, we patched the NTLMv2 authentication to be able to interact with resources. We also explicitly allowed guest accounts and specified `map to guest = bad user` to allow a fallback to “guest” when credentials were invalid. After reporting CVE-2024-50285: ksmbd: check outstanding simultaneous SMB operations, credit limitations became more strict, so we patched that out as well to avoid rate limiting.

When we restarted syzkaller with a larger corpus, a few minutes later, all remaining candidates were rejected. After some investigation, we realized it was due to the default `max connections = 128`, which we had to increase to the maximum value 65536. No other limits were changed.

### State Management

SMB interactions are stateful, relying on sessions, TreeIDs, and FileIDs. Fuzzing required simulating valid transitions like `smb2_create` ⇢ `smb2_ioctl` ⇢ `smb2_close`. When we initiated operations such as `smb2_tree_connect`, `smb2_sess_setup`, or `smb2_create`, we manually parsed responses in the pseudo-syscall to extract resource identifiers and reused them in subsequent calls. Our harness was programmed to send multiple messages per pseudo-syscall.

Example code for resources parsing is displayed below:

“`
// process response. does not contain +4B PDU length void process_buffer(int msg_no, const char *buffer, size_t received) { // .. snip .. // Extract SMB2 command uint16_t cmd_rsp = u16((const uint8_t *)(buffer + CMD_OFFSET)); debug(“Response command: 0x%04xn”, cmd_rsp); switch (cmd_rsp) { case SMB2_TREE_CONNECT: if (received >= TREE_ID_OFFSET + sizeof(uint32_t)) { tree_id = u32((const uint8_t *)(buffer + TREE_ID_OFFSET)); debug(“Obtained tree_id: 0x%xn”, tree_id); } break; case SMB2_SESS_SETUP: // First session setup response carries session_id if (msg_no == 0x01 && received >= SESSION_ID_OFFSET + sizeof(uint64_t)) { session_id = u64((const uint8_t *)(buffer + SESSION_ID_OFFSET)); debug(“Obtained session_id: 0x%llxn”, session_id); } break; case SMB2_CREATE: if (received >= CREATE_VFID_OFFSET + sizeof(uint64_t)) { persistent_file_id = u64((const uint8_t *)(buffer + CREATE_PFID_OFFSET)); volatile_file_id = u64((const uint8_t *)(buffer + CREATE_VFID_OFFSET)); debug(“Obtained p_fid: 0x%llx, v_fid: 0x%llxn”, persistent_file_id, volatile_file_id); } break; default: debug(“Unknown command (0x%04x)n”, cmd_rsp); break; } }
“`

Another issue we had to solve was that ksmbd relies on global state-memory pools or session tables, which makes fuzzing less deterministic. We tried enabling the experimental reset_acc_state feature to reset accumulated state, but it slowed down fuzzing significantly. We decided to not care much about reproducibility, since each bug typically appeared in dozens or even hundreds of test cases. For the rest, we used focused fuzzing, as described below.

### Protocol Specification

We based our harness on the official SMB protocol specification by implementing a grammar for all supported SMB commands. Microsoft publishes detailed technical documents for SMB and other protocols as part of its Open Specifications program.

As an example, the wire format of the SMB2 IOCTL Request is shown below:

We then manually rewrote this specification into our grammar, which allowed our harness to automatically construct valid SMB2 IOCTL requests:

“`
smb2_ioctl_req { Header_Prefix SMB2Header_Prefix Command const[0xb, int16] Header_Suffix SMB2Header_Suffix StructureSize const[57, int16] Reserved const[0, int16] CtlCode union_control_codes PersistentFileId const[0x4, int64] VolatileFileId const[0x0, int64] InputOffset offsetof[Input, int32] InputCount bytesize[Input, int32] MaxInputResponse const[65536, int32] OutputOffset offsetof[Output, int32] OutputCount len[Output, int32] MaxOutputResponse const[65536, int32] Flags int32[0:1] Reserved2 const[0, int32] Input array[int8] Output array[int8] } [packed]
“`

We did a final check against the source code to identify and verify possible mismatches during our translation.

## Fuzzing Strategies

Since we were curious about the bugs that might be missed when using only the default syzkaller configuration with a corpus generated from scratch, we explored different fuzzing approaches, each of which is described in the following subsections.

### FocusAreas

Occasionally, we triggered a bug that we were not able to reproduce, and it was not immediately clear from the crash log why it occurred. In other cases, we wanted to focus on a parsing function that had weak coverage. The experimental function focus_areas allows exactly that.

For instance, by targeting smb_check_perm_dacl with

“`
“focus_areas”: [ {“filter”: {“functions”: [“smb_check_perm_dacl”]}, “weight”: 20.0}, {“filter”: {“files”: [“^fs/smb/server/”]}, “weight”: 2.0}, {“weight”: 1.0} ]
“`

we identified multiple integer overflows and were able to quickly suggest and confirm the patch.

To reach the vulnerable code, syzkaller constructed an ACL that passed validation and led to an integer overflow. After rewriting it in Python, it looked like this:

“`
def build_sd(): sd = bytearray(0x14) sd[0x00] = 0x00 sd[0x01] = 0x00 struct.pack_into(” packets.json os.makedirs(“corpus”, exist_ok=True) def load_packets(json_file): with open(json_file, ‘r’) as file: data = json.load(file) packets = [entry[“_source”][“layers”][“tcp.payload”] for entry in data] return packets if __name__ == “__main__”: json_file = “packets.json” packets = load_packets(json_file) for i, packet in enumerate(packets): pdu_size = len(packet[0]) filename = f”corpus/packet_{i:03d}.txt” with open(filename, “w”) as f: f.write(f”syz_ksmbd_send_req(&(0x7f0000000340)=ANY=[@ANYBLOB=”{packet[0]}”], {hex(pdu_size)}, 0x0, 0x0)”)
“`

After that, we used syz-db to pack all candidates into the corpus database and resumed fuzzing.

With that, we were able to immediately trigger ksmbd: fix use-after-free in ksmbd_sessions_deregister() and improve overall coverage by a few percent.

### Sanitizer Coverage Beyond KASAN

In addition to KASAN, we tried other sanitizers such as KUBSAN and KCSAN. There was no significant improvement: KCSAN produced many false positives or reported bugs in unrelated components with seemingly no security impact. Interestingly, KUBSAN was able to identify one additional issue that KASAN did not detect:

“`
id = le32_to_cpu(psid->sub_auth[psid->num_subauth – 1]);
“`

In this case, the user was able to set `psid->num_subauth` to `0`, which resulted in an incorrect read `psid->sub_auth[-1]`. Although this access still fell within the same struct allocation ( `smb_sid`), UBSAN’s array index bounds check considered the declared bounds of the array

“`
struct smb_sid { __u8 revision; /* revision level */ __u8 num_subauth; __u8 authority[NUM_AUTHS]; __le32 sub_auth[SID_MAX_SUB_AUTHORITIES]; /* sub_auth[num_subauth] */ } __attribute__((packed));
“`

and was therefore able to catch the bug.

## Coverage

One unresolved issue was fuzzing with multiple processes. Due to various locking mechanisms, and because we reused the same authentication state, we noticed that fuzzing was more stable and coverage increased faster when using only one process. We sent multiple requests within a single invocation, but initially worried that this would cause us to miss race conditions.

If we check the execution log, we see that syzkaller creates multiple threads inside one process, the same way it does when calling standard syscalls:

“`
1.887619984s ago: executing program 0 (id=1628): syz_ksmbd_send_req(&(0x7f0000000d40)={0xee, @smb2_read_req={{}, 0x8, {0x1, 0x0, 0x0, 0x0, 0x0, 0x1, 0x1, “fbac8eef056a860726ca964fb4f60999”}, 0x31, 0x6, 0x2, 0x7e, 0x70, 0x4, 0x0, 0xffffffff, 0x2, 0x7, 0xee, 0x0, “1cad48fb0cba2f253915fe074290eb3e10ed9ac895dde2a575e4caabc1f3a537e265fea8a440acfd66cf5e249b1ccaae941160f24282c81c9df0260d0403bb44b0461da80509bd756c155b191718caa5eabd4bd89aa9bed58bf87d42ef49bca4c9f08f22d495b601c9c025631b815bf6cbeb0aa4785aec4abf776d75e5be”}}, 0xf2, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0) syz_ksmbd_send_req(&(0x7f0000000900)=ANY=[@ANYRES16=0x0], 0xf0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0) (async, rerun: 32) syz_ksmbd_send_req(&(0x7f0000001440)=ANY=[@ANYBLOB=”000008c0fe534d4240000000000000000b0001000000000000000000030000000000000000000000010000000100000000000000684155244ffb955e3201e88679ed735a39000000040214000400000000000000000000000000000078000000480800000000010000000000000000000000010001″], 0x8c4, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0) (async, rerun: 32) syz_ksmbd_send_req(&(0x7f0000000200)={0x58, @smb2_oplock_break_req={{}, 0x12, {0x1, 0x0, 0x0, 0x9, 0x0, 0x1, 0x1, “3c66dd1fe856ec397e7f8d7c8c293fd6″}, 0x24}}, 0x5c, &(0x7f0000000000)=ANY=[@ANYBLOB=”00000080fe534d424000010000000000050001000800000000000000040000000000000000000000010000000100000000000000b31fae29f7ea148ad156304f457214a539000000020000000000000000000000000000000000000000000002″], 0x84, &(0x7f0000000100)=ANY=[@ANYBLOB=”00000062fe534d4240000000000000000e00010000000000000000000700000000000000000000000100000001000000000000000002000000ffff0000000000000000002100030a08000000040000000000000000000000000000006000020009000000aedf”], 0x66, 0x0, 0x0) (async) …
“`

Observe the `async` keyword automatically added during the fuzzing process, which allows running commands in parallel without blocking, implemented in this commit fd8caa5. Hence, no UAF was missed due to the seemingly absent parallelism.

In the end, based on syzkaller’s benchmark, we executed 20-30 processes per second in 20 VMs, which still potentially meant running several hundred commands. For reference, we used a server with an average configuration – nothing particularly optimized for fuzzing performance.

We measured coverage using syzkaller’s built-in function-level metrics. While we’re aware that this does not capture state transitions, which are critical in a protocol like SMB, it still provides a useful approximation of code exercised. Overall, the `fs/smb/server` directory reached around 60%. For `smb2pdu.c` specifically, which handles most SMB command parsing and dispatch, we reached 70%.

The screenshot below shows coverage across key files.

## Discovered Bugs

During our research period, **we reported a grand total of 23 bugs**. The majority of the bugs are use-after-frees or out-of-bounds read or write findings. Given this quantity, it is natural that the impact differs. For instance, _fix the warning from __kernel_write_iter_ is a simple warning that could only be used for DoS in a specific setup ( `kernel.panic_on_warn`), _validate zero num_subauth before sub_auth is accessed_ is a simple out-of-bounds 1-byte read, and _prevent rename with empty string_ will only cause a kernel oops.

There are additional issues where exploitability requires more thoughtful analysis (e.g., _fix type confusion via race condition when using ipc_msg_send_request_). Nevertheless, after evaluating potentially promising candidates, we were able to identify some powerful primitives, allowing an attacker to exploit the finding at least locally to gain remote code execution.

The list of the issues identified is reported hereby:

DescriptionCommitCVEprevent out-of-bounds stream writes by validating *pos0ca6df4CVE-2025-37947prevent rename with empty string53e3e5bCVE-2025-37956fix use-after-free in ksmbd_session_rpc_opena1f46c9CVE-2025-37926fix the warning from __kernel_write_iterb37f2f3CVE-2025-37775fix use-after-free in smb_break_all_levII_oplock()18b4facCVE-2025-37776fix use-after-free in __smb2_lease_break_noti()21a4e47CVE-2025-37777validate zero num_subauth before sub_auth is accessedbf21e29CVE-2025-22038fix overflow in dacloffset bounds checkbeff0bcCVE-2025-22039fix use-after-free in ksmbd_sessions_deregister()15a9605CVE-2025-22041fix r_count dec/increment mismatchddb7ea3CVE-2025-22074add bounds check for create lease contextbab703eCVE-2025-22042add bounds check for durable handle context542027eCVE-2025-22043prevent connection release during oplock break notification3aa660cCVE-2025-21955fix use-after-free in ksmbd_free_work_structbb39ed4CVE-2025-21967fix use-after-free in smb2_lock84d2d16CVE-2025-21945fix bug on trap in smb2_locke26e2d2CVE-2025-21944fix out-of-bounds in parse_sec_desc()d6e13e1CVE-2025-21946fix type confusion via race condition when using ipc_msg_send_req..e2ff19fCVE-2025-21947align aux_payload_buf to avoid OOB reads in cryptographic operati..06a0254-check outstanding simultaneous SMB operations0a77d94CVE-2024-50285fix slab-use-after-free in smb3_preauth_hash_rspb8fc56fCVE-2024-50283fix slab-use-after-free in ksmbd_smb2_session_createc119f4eCVE-2024-50286fix slab-out-of-bounds in smb2_allocate_rsp_buf0a77715CVE-2024-26980

Note that we are aware of the controversy around CVE assignment since the Linux kernel became a CVE Numbering Authority (CNA) in February 2024. My personal take is that, while there were many disputable cases, the current approach is pragmatic: CVEs are now assigned for fixes with potential security impact, particularly memory corruptions and other classes of bugs that could potentially be exploitable.

For more information, the whole process is described in detail in this great presentation, or the relevant article. Lastly, the voting process for CVE approval is implemented in the vulns.git repository.

### Conclusion

Our research yielded a few dozen bugs, although using pseudo-syscalls is generally discouraged and comes with several disadvantages. For instance, in all cases, we had to perform the triaging process manually by finding the relevant crash log entries, generating C programs, and minimizing them by hand.

Since syscalls can be tied using resources, this method could also be applied to ksmbd, which involves sending packets. It would be ideal for future research to explore this direction – SMB commands could yield resources that are then fed into different commands. Due to time restrictions, we followed the pseudo-syscall approach, relying on custom patches.

For the next and last part, we focus on exploiting CVE-2025-37947.