diff --git a/pkg/model/github_context.go b/pkg/model/github_context.go
index 199a9c6..c68aaa1 100644
--- a/pkg/model/github_context.go
+++ b/pkg/model/github_context.go
@@ -1,5 +1,12 @@
 package model
 
+import (
+	"fmt"
+
+	"github.com/nektos/act/pkg/common"
+	log "github.com/sirupsen/logrus"
+)
+
 type GithubContext struct {
 	Event            map[string]interface{} `json:"event"`
 	EventPath        string                 `json:"event_path"`
@@ -26,3 +33,109 @@ type GithubContext struct {
 	RunnerPerflog    string                 `json:"runner_perflog"`
 	RunnerTrackingID string                 `json:"runner_tracking_id"`
 }
+
+func asString(v interface{}) string {
+	if v == nil {
+		return ""
+	} else if s, ok := v.(string); ok {
+		return s
+	}
+	return ""
+}
+
+func nestedMapLookup(m map[string]interface{}, ks ...string) (rval interface{}) {
+	var ok bool
+
+	if len(ks) == 0 { // degenerate input
+		return nil
+	}
+	if rval, ok = m[ks[0]]; !ok {
+		return nil
+	} else if len(ks) == 1 { // we've reached the final key
+		return rval
+	} else if m, ok = rval.(map[string]interface{}); !ok {
+		return nil
+	} else { // 1+ more keys
+		return nestedMapLookup(m, ks[1:]...)
+	}
+}
+
+func withDefaultBranch(b string, event map[string]interface{}) map[string]interface{} {
+	repoI, ok := event["repository"]
+	if !ok {
+		repoI = make(map[string]interface{})
+	}
+
+	repo, ok := repoI.(map[string]interface{})
+	if !ok {
+		log.Warnf("unable to set default branch to %v", b)
+		return event
+	}
+
+	// if the branch is already there return with no changes
+	if _, ok = repo["default_branch"]; ok {
+		return event
+	}
+
+	repo["default_branch"] = b
+	event["repository"] = repo
+
+	return event
+}
+
+var findGitRef = common.FindGitRef
+var findGitRevision = common.FindGitRevision
+
+func (ghc *GithubContext) SetRefAndSha(defaultBranch string, repoPath string) {
+	// https://docs.github.com/en/actions/learn-github-actions/events-that-trigger-workflows
+	// https://docs.github.com/en/developers/webhooks-and-events/webhooks/webhook-events-and-payloads
+	switch ghc.EventName {
+	case "pull_request_target":
+		ghc.Ref = ghc.BaseRef
+		ghc.Sha = asString(nestedMapLookup(ghc.Event, "pull_request", "base", "sha"))
+	case "pull_request", "pull_request_review", "pull_request_review_comment":
+		ghc.Ref = fmt.Sprintf("refs/pull/%s/merge", ghc.Event["number"])
+	case "deployment", "deployment_status":
+		ghc.Ref = asString(nestedMapLookup(ghc.Event, "deployment", "ref"))
+		ghc.Sha = asString(nestedMapLookup(ghc.Event, "deployment", "sha"))
+	case "release":
+		ghc.Ref = asString(nestedMapLookup(ghc.Event, "release", "tag_name"))
+	case "push", "create", "workflow_dispatch":
+		ghc.Ref = asString(ghc.Event["ref"])
+		if deleted, ok := ghc.Event["deleted"].(bool); ok && !deleted {
+			ghc.Sha = asString(ghc.Event["after"])
+		}
+	default:
+		ghc.Ref = asString(nestedMapLookup(ghc.Event, "repository", "default_branch"))
+	}
+
+	if ghc.Ref == "" {
+		ref, err := findGitRef(repoPath)
+		if err != nil {
+			log.Warningf("unable to get git ref: %v", err)
+		} else {
+			log.Debugf("using github ref: %s", ref)
+			ghc.Ref = ref
+		}
+
+		// set the branch in the event data
+		if defaultBranch != "" {
+			ghc.Event = withDefaultBranch(defaultBranch, ghc.Event)
+		} else {
+			ghc.Event = withDefaultBranch("master", ghc.Event)
+		}
+
+		if ghc.Ref == "" {
+			ghc.Ref = asString(nestedMapLookup(ghc.Event, "repository", "default_branch"))
+		}
+	}
+
+	if ghc.Sha == "" {
+		_, sha, err := findGitRevision(repoPath)
+		if err != nil {
+			log.Warningf("unable to get git revision: %v", err)
+		} else {
+			ghc.Sha = sha
+		}
+	}
+}
diff --git a/pkg/model/github_context_test.go b/pkg/model/github_context_test.go
new file mode 100644
index 0000000..c3c8b56
--- /dev/null
+++ b/pkg/model/github_context_test.go
@@ -0,0 +1,132 @@
+package model
+
+import (
+	"fmt"
+	"testing"
+
+	log "github.com/sirupsen/logrus"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSetRefAndSha(t *testing.T) {
+	log.SetLevel(log.DebugLevel)
+
+	oldFindGitRef := findGitRef
+	oldFindGitRevision := findGitRevision
+	defer func() { findGitRef = oldFindGitRef }()
+	defer func() { findGitRevision = oldFindGitRevision }()
+
+	findGitRef = func(file string) (string, error) {
+		return "refs/heads/master", nil
+	}
+
+	findGitRevision = func(file string) (string, string, error) {
+		return "", "1234fakesha", nil
+	}
+
+	tables := []struct {
+		eventName string
+		event     map[string]interface{}
+		ref       string
+		sha       string
+	}{
+		{
+			eventName: "pull_request_target",
+			event: map[string]interface{}{
+				"pull_request": map[string]interface{}{
+					"base": map[string]interface{}{
+						"sha": "pr-base-sha",
+					},
+				},
+			},
+			ref: "master",
+			sha: "pr-base-sha",
+		},
+		{
+			eventName: "pull_request",
+			event: map[string]interface{}{
+				"number": "1234",
+			},
+			ref: "refs/pull/1234/merge",
+			sha: "1234fakesha",
+		},
+		{
+			eventName: "deployment",
+			event: map[string]interface{}{
+				"deployment": map[string]interface{}{
+					"ref": "refs/heads/somebranch",
+					"sha": "deployment-sha",
+				},
+			},
+			ref: "refs/heads/somebranch",
+			sha: "deployment-sha",
+		},
+		{
+			eventName: "release",
+			event: map[string]interface{}{
+				"release": map[string]interface{}{
+					"tag_name": "v1.0.0",
+				},
+			},
+			ref: "v1.0.0",
+			sha: "1234fakesha",
+		},
+		{
+			eventName: "push",
+			event: map[string]interface{}{
+				"ref":     "refs/heads/somebranch",
+				"after":   "push-sha",
+				"deleted": false,
+			},
+			ref: "refs/heads/somebranch",
+			sha: "push-sha",
+		},
+		{
+			eventName: "unknown",
+			event: map[string]interface{}{
+				"repository": map[string]interface{}{
+					"default_branch": "main",
+				},
+			},
+			ref: "main",
+			sha: "1234fakesha",
+		},
+		{
+			eventName: "no-event",
+			event:     map[string]interface{}{},
+			ref:       "refs/heads/master",
+			sha:       "1234fakesha",
+		},
+	}
+
+	for _, table := range tables {
+		t.Run(table.eventName, func(t *testing.T) {
+			ghc := &GithubContext{
+				EventName: table.eventName,
+				BaseRef:   "master",
+				Event:     table.event,
+			}
+
+			ghc.SetRefAndSha("main", "/some/dir")
+
+			assert.Equal(t, table.ref, ghc.Ref)
+			assert.Equal(t, table.sha, ghc.Sha)
+		})
+	}
+
+	t.Run("no-default-branch", func(t *testing.T) {
+		findGitRef = func(file string) (string, error) {
+			return "", fmt.Errorf("no default branch")
+		}
+
+		ghc := &GithubContext{
+			EventName: "no-default-branch",
+			Event:     map[string]interface{}{},
+		}
+
+		ghc.SetRefAndSha("", "/some/dir")
+
+		assert.Equal(t, "master", ghc.Ref)
+		assert.Equal(t, "1234fakesha", ghc.Sha)
+	})
+}
diff --git a/pkg/runner/run_context.go b/pkg/runner/run_context.go
index 5949dae..58cb2ac 100644
--- a/pkg/runner/run_context.go
+++ b/pkg/runner/run_context.go
@@ -542,13 +542,6 @@ func (rc *RunContext) getGithubContext() *model.GithubContext {
 		}
 	}
 
-	_, sha, err := common.FindGitRevision(repoPath)
-	if err != nil {
-		log.Warningf("unable to get git revision: %v", err)
-	} else {
-		ghc.Sha = sha
-	}
-
 	if rc.EventJSON != "" {
 		err = json.Unmarshal([]byte(rc.EventJSON), &ghc.Event)
 		if err != nil {
@@ -556,32 +549,13 @@ func (rc *RunContext) getGithubContext() *model.GithubContext {
 		}
 	}
 
-	maybeRef := nestedMapLookup(ghc.Event, ghc.EventName, "ref")
-	if maybeRef != nil {
-		log.Debugf("using github ref from event: %s", maybeRef)
-		ghc.Ref = maybeRef.(string)
-	} else {
-		ref, err := common.FindGitRef(repoPath)
-		if err != nil {
-			log.Warningf("unable to get git ref: %v", err)
-		} else {
-			log.Debugf("using github ref: %s", ref)
-			ghc.Ref = ref
-		}
-
-		// set the branch in the event data
-		if rc.Config.DefaultBranch != "" {
-			ghc.Event = withDefaultBranch(rc.Config.DefaultBranch, ghc.Event)
-		} else {
-			ghc.Event = withDefaultBranch("master", ghc.Event)
-		}
-	}
-
 	if ghc.EventName == "pull_request" {
 		ghc.BaseRef = asString(nestedMapLookup(ghc.Event, "pull_request", "base", "ref"))
 		ghc.HeadRef = asString(nestedMapLookup(ghc.Event, "pull_request", "head", "ref"))
 	}
 
+	ghc.SetRefAndSha(rc.Config.DefaultBranch, repoPath)
+
 	return ghc
 }
 
@@ -637,29 +611,6 @@ func nestedMapLookup(m map[string]interface{}, ks ...string) (rval interface{})
 	}
 }
 
-func withDefaultBranch(b string, event map[string]interface{}) map[string]interface{} {
-	repoI, ok := event["repository"]
-	if !ok {
-		repoI = make(map[string]interface{})
-	}
-
-	repo, ok := repoI.(map[string]interface{})
-	if !ok {
-		log.Warnf("unable to set default branch to %v", b)
-		return event
-	}
-
-	// if the branch is already there return with no changes
-	if _, ok = repo["default_branch"]; ok {
-		return event
-	}
-
-	repo["default_branch"] = b
-	event["repository"] = repo
-
-	return event
-}
-
 func (rc *RunContext) withGithubEnv(env map[string]string) map[string]string {
 	github := rc.getGithubContext()
 	env["CI"] = "true"