CVE-2026-53921: odhcpd: Stack buffer overflow in DHCPv6 IA reply serialization (2 independent sites)

Vulnerability summary

CVE-2026-53921 is a critical (CVSS 9.8) Stack-based Buffer Overflow vulnerability in openwrt odhcpd.

Timeline

1. 23-10-2026
   PublishedVulnerability first disclosed.

Description

odhcpd is the DHCPv6, DHCPv4, and router-advertisement server used by OpenWrt. Prior to version 25.12, odhcpd's DHCPv6 IA response handler writes past the end of a fixed 512-byte stack buffer (pdbuf) when a single DHCPv6 REQUEST carries enough IA options to nearly exhaust the reply space. Two independent overflow sites exist in the same call chain, both reachable from the network via a single unauthenticated UDP packet. odhcpd runs as root on OpenWrt; embedded targets often lack stack canaries and ASLR, making code execution realistic. This issue has been patched in version 25.12.

• CWE-121Stack-based Buffer OverflowA stack-based buffer overflow condition is a condition where the buffer being overwritten is allocated on the stack (i.e., is a local variable or, rarely, a parameter to a function).