According to Rule 11-4-5 of the NFL Rule Book, "No article of any type may be placed on the gridiron, or used in any nature to assist a player in the completion of a field goal and (extra point) attempt."
Hence, an item such as pine tar is expressly illegal and cannot be used during an NFL game to give an unfair advantage to the kicking team. The league takes adherence to its rule book seriously, and there are panels regularly set up to look into potential violations and hand out steep punishments.

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NFL Pine Tar controversy in 2023
During a Week 10 game between the Buffalo Bills and Denver Broncos, Broncos placekicker Wil Lutz converted a 36-yard field goal to secure a 24-22 victory in MNF. This successful field goal put an end to an enthralling battle between two playoff-chasing teams. Moreover, the Broncos secured their second win in a row over an AFC Conference powerhouse.
However, on closer look, it seems that Denver punter Riley Dixon used a sticky substance to hold the ball for Lutz's kick. If this were proven true, then the Broncos would have been in big trouble due to Rule 11-4-5.
Luckily for the Broncos, further review of the video showed that Dixon had a turf burn on his hand. That likely occurred due to an earlier botched holding attempt during the game. The NFL has yet to make a statement since, so it's likely that the Broncos made the kick legitimately and didn't break any rules in the process.
A similar Pine Tar incident in last year's playoffs
The pine tar controversy in Week 10 of the ongoing regular season isn't the first of its kind. In fact, during last year's playoffs, the Philadelphia Eagles were accused of knowingly using some prop or tee during kicking attempts.
That raised a lot of eyebrows, and the NFL decided to issue a memorandum to all franchises about the rules. The Eagles weren't investigated openly, and we wonder if the Denver Broncos would be as well.
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