diff --git a/mercurial/parsers.c b/mercurial/parsers.c
--- a/mercurial/parsers.c
+++ b/mercurial/parsers.c
@@ -752,8 +752,8 @@ static const char *index_deref(indexObje
 	return PyString_AS_STRING(self->data) + pos * v1_hdrsize;
 }
 
-static inline void index_get_parents(indexObject *self, Py_ssize_t rev,
-				int *ps)
+static inline int index_get_parents(indexObject *self, Py_ssize_t rev,
+				    int *ps, int maxrev)
 {
 	if (rev >= self->length - 1) {
 		PyObject *tuple = PyList_GET_ITEM(self->added,
@@ -765,6 +765,13 @@ static inline void index_get_parents(ind
 		ps[0] = getbe32(data + 24);
 		ps[1] = getbe32(data + 28);
 	}
+	/* If index file is corrupted, ps[] may point to invalid revisions. So
+	 * there is a risk of buffer overflow to trust them unconditionally. */
+	if (ps[0] > maxrev || ps[1] > maxrev) {
+		PyErr_SetString(PyExc_ValueError, "parent out of range");
+		return -1;
+	}
+	return 0;
 }
 
 
@@ -1149,7 +1156,8 @@ static PyObject *compute_phases_map_sets
 	if (minrevallphases != -1) {
 		int parents[2];
 		for (i = minrevallphases; i < len; i++) {
-			index_get_parents(self, i, parents);
+			if (index_get_parents(self, i, parents, len - 1) < 0)
+				goto release_phasesetlist;
 			set_phase_from_parents(phases, parents[0], parents[1], i);
 		}
 	}
@@ -1248,7 +1256,8 @@ static PyObject *index_headrevs(indexObj
 			continue;
 		}
 
-		index_get_parents(self, i, parents);
+		if (index_get_parents(self, i, parents, len - 1) < 0)
+			goto bail;
 		for (j = 0; j < 2; j++) {
 			if (parents[j] >= 0)
 				nothead[parents[j]] = 1;
@@ -1716,7 +1725,8 @@ static PyObject *find_gca_candidates(ind
 				}
 			}
 		}
-		index_get_parents(self, v, parents);
+		if (index_get_parents(self, v, parents, maxrev) < 0)
+			goto bail;
 
 		for (i = 0; i < 2; i++) {
 			int p = parents[i];
@@ -1813,7 +1823,8 @@ static PyObject *find_deepest(indexObjec
 			continue;
 
 		sv = seen[v];
-		index_get_parents(self, v, parents);
+		if (index_get_parents(self, v, parents, maxrev) < 0)
+			goto bail;
 
 		for (i = 0; i < 2; i++) {
 			int p = parents[i];
diff --git a/tests/test-parseindex.t b/tests/test-parseindex.t
--- a/tests/test-parseindex.t
+++ b/tests/test-parseindex.t
@@ -59,3 +59,62 @@ We approximate that by reducing the read
   26333235a41c
 
   $ cd ..
+
+Test corrupted p1/p2 fields that could cause SEGV at parsers.c:
+
+  $ mkdir invalidparent
+  $ cd invalidparent
+
+  $ hg clone --pull -q --config phases.publish=False ../a limit
+  $ hg clone --pull -q --config phases.publish=False ../a segv
+  $ rm -R limit/.hg/cache segv/.hg/cache
+
+  $ python <<EOF
+  > data = open("limit/.hg/store/00changelog.i", "rb").read()
+  > for n, p in [('limit', '\0\0\0\x02'), ('segv', '\0\x01\0\0')]:
+  >     # corrupt p1 at rev0 and p2 at rev1
+  >     d = data[:24] + p + data[28:127 + 28] + p + data[127 + 32:]
+  >     open(n + "/.hg/store/00changelog.i", "wb").write(d)
+  > EOF
+
+  $ hg debugindex -f1 limit/.hg/store/00changelog.i
+     rev flag   offset   length     size   base   link     p1     p2       nodeid
+       0 0000        0       63       62      0      0      2     -1 7c31755bf9b5
+       1 0000       63       66       65      1      1      0      2 26333235a41c
+  $ hg debugindex -f1 segv/.hg/store/00changelog.i
+     rev flag   offset   length     size   base   link     p1     p2       nodeid
+       0 0000        0       63       62      0      0  65536     -1 7c31755bf9b5
+       1 0000       63       66       65      1      1      0  65536 26333235a41c
+
+  $ cat <<EOF > test.py
+  > import sys
+  > from mercurial import changelog, scmutil
+  > cl = changelog.changelog(scmutil.vfs(sys.argv[1]))
+  > n0, n1 = cl.node(0), cl.node(1)
+  > ops = [
+  >     ('compute_phases_map_sets', lambda: cl.computephases([[0], []])),
+  >     ('index_headrevs', lambda: cl.headrevs()),
+  >     ('find_gca_candidates', lambda: cl.commonancestorsheads(n0, n1)),
+  >     ('find_deepest', lambda: cl.ancestor(n0, n1)),
+  >     ]
+  > for l, f in ops:
+  >     print l + ':',
+  >     try:
+  >         f()
+  >         print 'uncaught buffer overflow?'
+  >     except ValueError, inst:
+  >         print inst
+  > EOF
+
+  $ python test.py limit/.hg/store
+  compute_phases_map_sets: parent out of range
+  index_headrevs: parent out of range
+  find_gca_candidates: parent out of range
+  find_deepest: parent out of range
+  $ python test.py segv/.hg/store
+  compute_phases_map_sets: parent out of range
+  index_headrevs: parent out of range
+  find_gca_candidates: parent out of range
+  find_deepest: parent out of range
+
+  $ cd ..