Date: June 2024
Author: Zephyr Security Explorer
Zephyr RTOS is gaining popularity in the Internet of Things (IoT) space, thanks to its flexibility and open-source model. But as with any widely used piece of software, vulnerabilities do surface. In this long-read, we’ll break down *CVE-2023-4259*, a crucial security flaw rooted in the eS-WiFi driver included with Zephyr. If you develop or manage IoT devices, read this carefully!
What is CVE-2023-4259?
CVE-2023-4259 highlights two potential buffer overflow vulnerabilities in the eswifi Wi-Fi driver provided by Zephyr OS. A malicious actor could leverage these to crash the system or potentially run their own code on the device.
Component: Drivers > Wi-Fi > eS-WiFi
Original disclosure and patch:
- Zephyr Github Pull Request #64378
- Zephyr Security Advisory
1. eswifi->ssid_len Assignment
Located in drivers/wifi/eswifi/core.c (Zephyr tree), the driver parses input for SSID length. Here’s a simplified version:
/* Vulnerable snippet */
eswifi->ssid_len = strlen(ssid);
memcpy(eswifi->ssid, ssid, eswifi->ssid_len);
Risk: No bounds checking. If ssid is longer than the destination buffer (eswifi->ssid[]), this causes a buffer overflow.
Located in the same driver, the password entry shows a similar weakness
strncpy(eswifi->pass, pass, strlen(pass));
Risk: Again, if pass is longer than the expected buffer, eswifi->pass[], this copies too much data, risking a buffer overrun.
Demonstrating The Exploit (Proof-of-Concept)
Suppose you're developing an IoT device using Zephyr and your application passes Wi-Fi credentials provided over a serial, UART link, or HTTP (for initial configuration). What happens if an attacker sends deliberately oversized SSID or password data?
#define MAX_SSID_LEN 32
#define MAX_PASS_LEN 64
struct eswifi_dev {
char ssid[MAX_SSID_LEN];
size_t ssid_len;
char pass[MAX_PASS_LEN];
};
void wifi_connect(const char *ssid, const char *pass) {
struct eswifi_dev *eswifi = get_device();
// VULNERABLE USAGE
eswifi->ssid_len = strlen(ssid);
memcpy(eswifi->ssid, ssid, eswifi->ssid_len);
strncpy(eswifi->pass, pass, strlen(pass)); // VULNERABLE
}
If someone sends ssid=AAAAAAAAAAAAAAAA... (hundreds of 'A's), memcpy and strncpy happily write past the end of the arrays. At best, this will crash your application. Worse still, with some clever payloads, an attacker might execute their own code.
Here’s how an attacker could exploit the above issue (pseudocode)
# Attacker crafts a payload longer than the expected example.
ssid = "A" * 200 # overflow eswifi->ssid
passw = "B" * 300 # overflow eswifi->pass
# These are input via whatever setup process the device uses,
# e.g., device's web server or UART setup tool
# The driver does not check length, so entire data is copied.
If the stack is laid out as follows
| eswifi->ssid (32 bytes) | eswifi->ssid_len | eswifi->pass (64 bytes) | ...
The excessive data will blow through these fields, corrupting neighboring memory.
Denial of Service: The device could crash, becoming unresponsive.
- Code Execution: If an attacker can control what's written to nearby memory and later influence code flow, such as a return address, they could hijack control.
- Network Exposure: Management interfaces (serial/UART or network) can be security boundaries — don't trust their input!
Never use unchecked memcpy or strncpy on external input. Always include length limits
// Safe version
#define MAX_SSID_LEN 32
#define MAX_PASS_LEN 64
eswifi->ssid_len = strnlen(ssid, MAX_SSID_LEN - 1);
memcpy(eswifi->ssid, ssid, eswifi->ssid_len);
eswifi->ssid[eswifi->ssid_len] = '\'; // Ensure null-terminate
strncpy(eswifi->pass, pass, MAX_PASS_LEN);
eswifi->pass[MAX_PASS_LEN-1] = '\'; // Ensure null-terminate
Additionally, validate incoming data
if (eswifi->ssid_len == || eswifi->ssid_len > MAX_SSID_LEN - 1) {
// Invalid SSID length, handle error
}
Reported: August 2023
- Patched: Zephyr 3.5.+ (GitHub Commit)
- Official Advisory: GHSA-c9j7-f64f-q785
More Reading
- Common Buffer Overflow Exploits
- Zephyr Security: Vulnerability Management
- Firmware Updates: Why They Matter
Conclusion
*CVE-2023-4259* is a classic example of how dangerous buffer overflows remain in embedded IoT software. These issues are easy to introduce but equally easy to prevent by following secure coding practices and always validating inputs. If you’re building with Zephyr or managing Zephyr-based devices, make patching and input validation a priority.
Stay secure out there!
*This post is based on original research and Zephyr security advisories. For questions or new vulnerabilities, contact Zephyr security or open a responsible disclosure ticket on their GitHub.*
Timeline
Published on: 09/26/2023 00:15:11 UTC
Last modified on: 11/14/2023 03:15:09 UTC