The tempo/session cooperative close handler validated the close voucher amount using < instead of <= against the on-chain settled amount. An attacker could submit a close voucher exactly equal to the settled amount, which would be accepted without committing any new funds, effectively closing or griefing the channel for free.
Fixed in 0.4.11.
There are no workarounds available for this vulnerability.
0.4.11Exploitability
AV:NAC:LPR:NUI:NScope
S:UImpact
C:NI:HA:N7.5/CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N