This indicates an attack attempt to exploit a Format String vulnerability on vulnerable router.
The vulnerability is caused by an improper validation of user supplied data when the vulnerable router handles a maliciously crafted request. An attacker can exploit this to execute arbitrary code in the context of the vulnerable router via a crafted request.
Cisco Linksys WRT54GL
System Compromise: Remote attackers can gain control of vulnerable systems
Currently we are unaware of any vendor supplied patch for this issue.