The only other Reds baserunner to reach safely was Drew Stubbs, who drew walks in both the first and fourth innings. Following Hanigan's hit, there was a sacrifice bunt by Cueto, but it was 12 up, 12 down the rest of the night for Bumgarner.
All and all, it was a rough way to begin an 11-game West Coast swing for Cincinnati.
San Francisco, which swept its three games from the Dodgers with three straight shutouts, notched four straight shutouts for the first time in franchise history. The 36-inning scoreless streak is the longest for the franchise since 1948.
"It's a pretty good streak. They're in a good place right now," Reds manager Dusty Baker said.
Reds ace Johnny Cueto was on the wrong side of this momentum, as he gave up three runs, two earned, over six innings. Cueto walked a season-high four batters and struck out three while throwing 108 pitches.
Cueto notched two quick outs in the first inning and thought he was going to retire the side in order. A 3-2 fastball to Melky Cabrera was close but called high by home-plate umpire C.B. Bucknor for a walk to open the door for the Giants.
"I made a great pitch and I thought it was a strike," Cueto said through an interpreter. "But you know how things go. The umpire said it was high."
Next was Buster Posey, who hit a single through to right field. Angel Pagan did likewise past second baseman Miguel Cairo that scored Cabrera. Jay Bruce's throw to the plate skipped off of Hanigan for an error on the right fielder that let Posey score. Cueto, who couldn't hold on to Hanigan's throw while covering the plate, retrieved the ball and threw out Pagan trying for third base.
"Johnny pitched real well," Hanigan said. "He walked a couple of guys I don't think he was happy about. The first inning, it was kind of a broken-down swing single and kind of a snowball fight there to get two [runs]. For the most part, he managed the game, got back in the game. We just didn't get any offense. I thought he did fine."
Another defensive miscue cost the Reds with two outs in the sixth. Pablo Sandoval skied what appeared to be a routine fly to left field, where Todd Frazier misjudged it and took a couple of steps in. The ball sailed over his head and bounced to the wall for a RBI double and a 3-0 Giants lead. Two more runs crossed against Reds reliever Sam LeCure in the seventh.
Bumgarner, now 10-4 with a 2.85 ERA this season, came into the night with a 0-2 record and 8.56 ERA in three career starts against the Reds.
"This team has hurt me a couple games, so I just wanted to keep us in there," Bumgarner said. "I made pitches, stuck to the game plan, and it worked out."
In addition to striking out eight, Bumgarner notched 11 outs via ground balls.
"He's made adjustments from last year to this year, in terms with how he's throwing," Hanigan said. "He's throwing 70 percent cutters and a little cutter-slider thing. It's hard enough to get it in on your hands. I don't think we had the right approach on our team in general for the most part. We'll go get them tomorrow."
For the Reds, who now have a one-game lead over the Pirates in the National League Central, it was only the second time they've have been shut out this season, with the last one coming on May 18 against the Yankees. The last time they mustered one hit in a game was July 29, 2009, against the Padres and Mat Latos.
"You have to say the guy threw a great ballgame," Baker said. "You feel helpless sitting over there. There's not much you can do when you're not getting hits and not getting baserunners."