This ruby 1.8.7 patch was derived from the ruby 1.9.3 fix for:

https://www.ruby-lang.org/en/news/2013/11/22/heap-overflow-in-floating-point-parsing-cve-2013-4164/

as seen here:
http://bugs.ruby-lang.org/projects/ruby-trunk/repository/diff/util.c?rev=43780&rev_to=41757

CVE-2013-4164

Heap-based buffer overflow in Ruby 1.8, 1.9 before 1.9.3-p484, 2.0
before 2.0.0-p353, 2.1 before 2.1.0 preview2, and trunk before revision
43780 allows context-dependent attackers to cause a denial of service
(segmentation fault) and possibly execute arbitrary code via a string
that is converted to a floating point value, as demonstrated using (1)
the to_f method or (2) JSON.parse.

--- ruby-1.8.7-p374-orig/util.c	2010-11-21 23:21:34.000000000 -0800
+++ ruby-1.8.7-p374/util.c	2013-12-02 16:58:32.995038000 -0800
@@ -892,6 +892,11 @@
 #else
 #define MALLOC malloc
 #endif
+#ifdef FREE
+extern void FREE(void*);
+#else
+#define FREE free
+#endif
 
 #ifndef Omit_Private_Memory
 #ifndef PRIVATE_MEM
@@ -1176,7 +1181,7 @@
 #endif
 
     ACQUIRE_DTOA_LOCK(0);
-    if ((rv = freelist[k]) != 0) {
+    if (k <= Kmax && (rv = freelist[k]) != 0) {
         freelist[k] = rv->next;
     }
     else {
@@ -1186,7 +1191,7 @@
 #else
         len = (sizeof(Bigint) + (x-1)*sizeof(ULong) + sizeof(double) - 1)
                 /sizeof(double);
-        if (pmem_next - private_mem + len <= PRIVATE_mem) {
+        if (k <= Kmax && pmem_next - private_mem + len <= PRIVATE_mem) {
             rv = (Bigint*)pmem_next;
             pmem_next += len;
         }
@@ -1205,6 +1210,10 @@
 Bfree(Bigint *v)
 {
     if (v) {
+        if (v->k > Kmax) {
+            FREE(v);
+            return;
+        }
         ACQUIRE_DTOA_LOCK(0);
         v->next = freelist[v->k];
         freelist[v->k] = v;
@@ -2200,6 +2209,7 @@
         for (; c >= '0' && c <= '9'; c = *++s) {
 have_dig:
             nz++;
+            if (nf > DBL_DIG * 4) continue;
             if (c -= '0') {
                 nf += nz;
                 for (i = 1; i < nz; i++)