CVE-2009-0654

Tor 0.2.0.28, and probably 0.2.0.34 and earlier, allows remote attackers,
with control of an entry router and an exit router, to confirm that a
sender and receiver are communicating via vectors involving (1) replaying,
(2) modifying, (3) inserting, or (4) deleting a single cell, and then
observing cell recognition errors at the exit router. NOTE: the vendor
disputes the significance of this issue, noting that the product’s design
“accepted end-to-end correlation as an attack that is too expensive to
solve.”