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-rw-r--r--src/timedata.cpp18
1 files changed, 18 insertions, 0 deletions
diff --git a/src/timedata.cpp b/src/timedata.cpp
index 8a095d26dc..6c3bd9a48d 100644
--- a/src/timedata.cpp
+++ b/src/timedata.cpp
@@ -49,6 +49,24 @@ void AddTimeData(const CNetAddr& ip, int64_t nTime)
static CMedianFilter<int64_t> vTimeOffsets(200,0);
vTimeOffsets.input(nOffsetSample);
LogPrintf("Added time data, samples %d, offset %+d (%+d minutes)\n", vTimeOffsets.size(), nOffsetSample, nOffsetSample/60);
+
+ // There is a known issue here (see issue #4521):
+ //
+ // - The structure vTimeOffsets contains up to 200 elements, after which
+ // any new element added to it will not increase its size, replacing the
+ // oldest element.
+ //
+ // - The condition to update nTimeOffset includes checking whether the
+ // number of elements in vTimeOffsets is odd, which will never happen after
+ // there are 200 elements.
+ //
+ // But in this case the 'bug' is protective against some attacks, and may
+ // actually explain why we've never seen attacks which manipulate the
+ // clock offset.
+ //
+ // So we should hold off on fixing this and clean it up as part of
+ // a timing cleanup that strengthens it in a number of other ways.
+ //
if (vTimeOffsets.size() >= 5 && vTimeOffsets.size() % 2 == 1)
{
int64_t nMedian = vTimeOffsets.median();