dhclient: Check for unexpected characters in some DHCP server options

Some options are written directly to the lease file, which may be parsed
by subsequent dhclient invocations.  We must make sure that a malicious
server can't control the "medium" field of a lease definition, otherwise
they can achieve RCE by injecting one into the lease file, whereupon it
will be passed to dhclient-script, which passes it through eval.

Approved by:	so
Security:	FreeBSD-SA-26:12.dhclient
Security:	CVE-2026-42511
Reported by:	Joshua Rogers of AISLE Research Team (https://aisle.com/)

sbin/dhclient/dhclient.c

@@ -1226,6 +1226,12 @@ packet_to_lease(struct packet *packet)
 		}
 		memcpy(lease->server_name, packet->raw->sname, DHCP_SNAME_LEN);
 		lease->server_name[DHCP_SNAME_LEN]='\0';
+		if (strchr(lease->server_name, '"') != NULL ||
+		    strchr(lease->server_name, '\\') != NULL) {
+			warning("dhcpoffer: server name contains invalid characters.");
+			free_client_lease(lease);
+			return (NULL);
+		}
 	}
 
 	/* Ditto for the filename. */
@@ -1241,6 +1247,12 @@ packet_to_lease(struct packet *packet)
 		}
 		memcpy(lease->filename, packet->raw->file, DHCP_FILE_LEN);
 		lease->filename[DHCP_FILE_LEN]='\0';
+		if (strchr(lease->filename, '"') != NULL ||
+		    strchr(lease->filename, '\\') != NULL) {
+			warning("dhcpoffer: filename contains invalid characters.");
+			free_client_lease(lease);
+			return (NULL);
+		}
 	}
 	return lease;
 }