1:00 PM PST -- Young Buck tells TMZ, "It's nobody's fault but my own."
Buck literally got busted with his pants down according to a probation officer -- who says he saw YB using a tube to shoot a "yellow substance" into his drug testing cup last week.
According to docs ... he first tried to use the ol’ secondhand smoke excuse, but ultimately caved and admitted he'd gotten high with Snoop. Now, a judge will decide whether Buck has to do jail time for the violation.
Shame ... Buck had been on a winning streak, pissing clean for his previous 21 mandatory tests since November 2013.
Bottom line? Snoop’s weed will mess you up. In multiple ways.