DHCP server unbounded option parsing - OOB read from packet buffer

[AsafMeizner]

RT-Thread Version

tested HEAD, commit 38c007a

Hardware Type/Architectures

independent (Bug is in lwip-dhcpd C implementation)

Develop Toolchain

Other

Describe the bug

Summary

A buffer over-read vulnerability exists in RT-Thread's DHCP server implementation (components/net/lwip-dhcpd/dhcp_server.c) where the DHCP option parsing loop has no bounds check on the input buffer position, allowing a crafted DHCP packet to read past the allocated buffer.

Details

File: components/net/lwip-dhcpd/dhcp_server.c, line 346

The DHCP options parsing loop uses while(finished == 0) to iterate through options in the received packet. The loop reads option type and length fields from the packet data but does NOT check whether the current read position exceeds the packet buffer boundary.

/* dhcp_server.c:346 */
while(finished == 0) {
    /* reads option type and length from packet data */
    /* NO check that position < packet_length */
}

Since DHCP packets are received over the network with no authentication, this is reachable pre-authentication on the local network segment.

PoC

Send a crafted DHCP packet on the local network segment with:

• Valid DHCP header (op=1, htype=1, hlen=6)
• Options section containing a chain of options where each option's length field points past the end of the packet
• No DHCP_OPTION_END (0xFF) marker

The server's option parsing loop will read past the packet buffer boundary until it hits unmapped memory (crash) or reads adjacent heap data (info leak).

Build and run the RT-Thread DHCP server component with AddressSanitizer enabled to observe the over-read:

# Cross-compile RT-Thread with ASAN, enable lwip-dhcpd,
# send crafted DHCP DISCOVER on the local interface

Other additional context

Impact

• Out-of-bounds read from the packet buffer
• Potential information disclosure from adjacent heap memory
• Potential crash (DoS) if reading into unmapped memory
• Affects any RT-Thread device running the built-in DHCP server
• Attack vector: adjacent network (DHCP is link-local), no authentication required

Suggested fix:

- while(finished == 0) {
+ while(finished == 0 && position < packet_length) {

Also validate that position + option_length does not exceed packet_length before reading option data.