fix(security): prevent on: pull_request actions from mutating caches of other workflow events (#925)

Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/925

act/artifactcache/handler.go

@@ -181,11 +181,19 @@ func (h *Handler) find(w http.ResponseWriter, r *http.Request, params httprouter
 	}
 	defer db.Close()
 
-	cache, err := findCache(db, repo, keys, version)
+	cache, err := findCache(db, repo, keys, version, rundata.WriteIsolationKey)
 	if err != nil {
 		h.responseJSON(w, r, 500, err)
 		return
 	}
+	// If read was scoped to WriteIsolationKey and didn't find anything, we can fallback to the non-isolated cache read
+	if cache == nil && rundata.WriteIsolationKey != "" {
+		cache, err = findCache(db, repo, keys, version, "")
+		if err != nil {
+			h.responseJSON(w, r, 500, err)
+			return
+		}
+	}
 	if cache == nil {
 		h.responseJSON(w, r, 204)
 		return
@@ -236,6 +244,7 @@ func (h *Handler) reserve(w http.ResponseWriter, r *http.Request, params httprou
 	cache.CreatedAt = now
 	cache.UsedAt = now
 	cache.Repo = repo
+	cache.WriteIsolationKey = rundata.WriteIsolationKey
 	if err := insertCache(db, cache); err != nil {
 		h.responseJSON(w, r, 500, err)
 		return
@@ -275,6 +284,10 @@ func (h *Handler) upload(w http.ResponseWriter, r *http.Request, params httprout
 		h.responseJSON(w, r, 500, fmt.Errorf("cache repo is not valid"))
 		return
 	}
+	if cache.WriteIsolationKey != rundata.WriteIsolationKey {
+		h.responseJSON(w, r, 403, fmt.Errorf("cache authorized for write isolation %q, but attempting to operate on %q", rundata.WriteIsolationKey, cache.WriteIsolationKey))
+		return
+	}
 
 	if cache.Complete {
 		h.responseJSON(w, r, 400, fmt.Errorf("cache %v %q: already complete", cache.ID, cache.Key))
@@ -326,6 +339,10 @@ func (h *Handler) commit(w http.ResponseWriter, r *http.Request, params httprout
 		h.responseJSON(w, r, 500, fmt.Errorf("cache repo is not valid"))
 		return
 	}
+	if cache.WriteIsolationKey != rundata.WriteIsolationKey {
+		h.responseJSON(w, r, 403, fmt.Errorf("cache authorized for write isolation %q, but attempting to operate on %q", rundata.WriteIsolationKey, cache.WriteIsolationKey))
+		return
+	}
 
 	if cache.Complete {
 		h.responseJSON(w, r, 400, fmt.Errorf("cache %v %q: already complete", cache.ID, cache.Key))
@@ -386,6 +403,11 @@ func (h *Handler) get(w http.ResponseWriter, r *http.Request, params httprouter.
 		h.responseJSON(w, r, 500, fmt.Errorf("cache repo is not valid"))
 		return
 	}
+	// reads permitted against caches w/ the same isolation key, or no isolation key
+	if cache.WriteIsolationKey != rundata.WriteIsolationKey && cache.WriteIsolationKey != "" {
+		h.responseJSON(w, r, 403, fmt.Errorf("cache authorized for write isolation %q, but attempting to operate on %q", rundata.WriteIsolationKey, cache.WriteIsolationKey))
+		return
+	}
 
 	if err := h.useCache(id); err != nil {
 		h.responseJSON(w, r, 500, fmt.Errorf("cache useCache: %w", err))
@@ -417,7 +439,7 @@ func (h *Handler) middleware(handler httprouter.Handle) httprouter.Handle {
 }
 
 // if not found, return (nil, nil) instead of an error.
-func findCache(db *bolthold.Store, repo string, keys []string, version string) (*Cache, error) {
+func findCache(db *bolthold.Store, repo string, keys []string, version, writeIsolationKey string) (*Cache, error) {
 	cache := &Cache{}
 	for _, prefix := range keys {
 		// if a key in the list matches exactly, don't return partial matches
@@ -425,6 +447,7 @@ func findCache(db *bolthold.Store, repo string, keys []string, version string) (
 			bolthold.Where("Repo").Eq(repo).Index("Repo").
 				And("Key").Eq(prefix).
 				And("Version").Eq(version).
+				And("WriteIsolationKey").Eq(writeIsolationKey).
 				And("Complete").Eq(true).
 				SortBy("CreatedAt").Reverse()); err == nil || !errors.Is(err, bolthold.ErrNotFound) {
 			if err != nil {
@@ -441,6 +464,7 @@ func findCache(db *bolthold.Store, repo string, keys []string, version string) (
 			bolthold.Where("Repo").Eq(repo).Index("Repo").
 				And("Key").RegExp(re).
 				And("Version").Eq(version).
+				And("WriteIsolationKey").Eq(writeIsolationKey).
 				And("Complete").Eq(true).
 				SortBy("CreatedAt").Reverse()); err != nil {
 			if errors.Is(err, bolthold.ErrNotFound) {
@@ -644,5 +668,6 @@ func runDataFromHeaders(r *http.Request) cacheproxy.RunData {
 		RunNumber:          r.Header.Get("Forgejo-Cache-RunNumber"),
 		Timestamp:          r.Header.Get("Forgejo-Cache-Timestamp"),
 		RepositoryMAC:      r.Header.Get("Forgejo-Cache-MAC"),
+		WriteIsolationKey:  r.Header.Get("Forgejo-Cache-WriteIsolationKey"),
 	}
 }

act/artifactcache/handler_test.go

@@ -22,26 +22,35 @@ const (
 	cacheRepo      = "testuser/repo"
 	cacheRunnum    = "1"
 	cacheTimestamp = "0"
-	cacheMac       = "c13854dd1ac599d1d61680cd93c26b77ba0ee10f374a3408bcaea82f38ca1865"
+	cacheMac       = "bc2e9167f9e310baebcead390937264e4c0b21d2fdd49f5b9470d54406099360"
 )
 
 var handlerExternalURL string
 
 type AuthHeaderTransport struct {
 	T                  http.RoundTripper
+	WriteIsolationKey  string
+	OverrideDefaultMac string
 }
 
 func (t *AuthHeaderTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	req.Header.Set("Forgejo-Cache-Repo", cacheRepo)
 	req.Header.Set("Forgejo-Cache-RunNumber", cacheRunnum)
 	req.Header.Set("Forgejo-Cache-Timestamp", cacheTimestamp)
+	if t.OverrideDefaultMac != "" {
+		req.Header.Set("Forgejo-Cache-MAC", t.OverrideDefaultMac)
+	} else {
 		req.Header.Set("Forgejo-Cache-MAC", cacheMac)
+	}
 	req.Header.Set("Forgejo-Cache-Host", handlerExternalURL)
+	if t.WriteIsolationKey != "" {
+		req.Header.Set("Forgejo-Cache-WriteIsolationKey", t.WriteIsolationKey)
+	}
 	return t.T.RoundTrip(req)
 }
 
 var (
-	httpClientTransport = AuthHeaderTransport{http.DefaultTransport}
+	httpClientTransport = AuthHeaderTransport{T: http.DefaultTransport}
 	httpClient          = http.Client{Transport: &httpClientTransport}
 )
 
@@ -88,7 +97,7 @@ func TestHandler(t *testing.T) {
 		content := make([]byte, 100)
 		_, err := rand.Read(content)
 		require.NoError(t, err)
-		uploadCacheNormally(t, base, key, version, content)
+		uploadCacheNormally(t, base, key, version, "", content)
 	})
 
 	t.Run("clean", func(t *testing.T) {
@@ -380,7 +389,7 @@ func TestHandler(t *testing.T) {
 		_, err := rand.Read(content)
 		require.NoError(t, err)
 
-		uploadCacheNormally(t, base, key, version, content)
+		uploadCacheNormally(t, base, key, version, "", content)
 
 		// Perform the request with the custom `httpClient` which will send correct MAC data
 		resp, err := httpClient.Get(fmt.Sprintf("%s/cache?keys=%s&version=%s", base, key, version))
@@ -416,7 +425,7 @@ func TestHandler(t *testing.T) {
 		for i := range contents {
 			_, err := rand.Read(contents[i])
 			require.NoError(t, err)
-			uploadCacheNormally(t, base, keys[i], version, contents[i])
+			uploadCacheNormally(t, base, keys[i], version, "", contents[i])
 			time.Sleep(time.Second) // ensure CreatedAt of caches are different
 		}
 
@@ -454,13 +463,98 @@ func TestHandler(t *testing.T) {
 		assert.Equal(t, contents[except], content)
 	})
 
+	t.Run("find can't match without WriteIsolationKey match", func(t *testing.T) {
+		defer func() { httpClientTransport.WriteIsolationKey = "" }()
+
+		version := "c19da02a2bd7e77277f1ac29ab45c09b7d46a4ee758284e26bb3045ad11d9d20"
+		key := strings.ToLower(t.Name())
+
+		uploadCacheNormally(t, base, key, version, "TestWriteKey", make([]byte, 64))
+
+		func() {
+			defer overrideWriteIsolationKey("AnotherTestWriteKey")()
+			resp, err := httpClient.Get(fmt.Sprintf("%s/cache?keys=%s&version=%s", base, key, version))
+			require.NoError(t, err)
+			require.Equal(t, 204, resp.StatusCode)
+		}()
+
+		{
+			resp, err := httpClient.Get(fmt.Sprintf("%s/cache?keys=%s&version=%s", base, key, version))
+			require.NoError(t, err)
+			require.Equal(t, 204, resp.StatusCode)
+		}
+
+		func() {
+			defer overrideWriteIsolationKey("TestWriteKey")()
+			resp, err := httpClient.Get(fmt.Sprintf("%s/cache?keys=%s&version=%s", base, key, version))
+			require.NoError(t, err)
+			require.Equal(t, 200, resp.StatusCode)
+		}()
+	})
+
+	t.Run("find prefers WriteIsolationKey match", func(t *testing.T) {
+		version := "c19da02a2bd7e77277f1ac29ab45c09b7d46a4ee758284e26bb3045ad11d9d21"
+		key := strings.ToLower(t.Name())
+
+		// Between two values with the same `key`...
+		uploadCacheNormally(t, base, key, version, "TestWriteKey", make([]byte, 64))
+		uploadCacheNormally(t, base, key, version, "", make([]byte, 128))
+
+		// We should read the value with the matching WriteIsolationKey from the cache...
+		func() {
+			defer overrideWriteIsolationKey("TestWriteKey")()
+
+			resp, err := httpClient.Get(fmt.Sprintf("%s/cache?keys=%s&version=%s", base, key, version))
+			require.NoError(t, err)
+			require.Equal(t, 200, resp.StatusCode)
+
+			got := struct {
+				ArchiveLocation string `json:"archiveLocation"`
+			}{}
+			require.NoError(t, json.NewDecoder(resp.Body).Decode(&got))
+			contentResp, err := httpClient.Get(got.ArchiveLocation)
+			require.NoError(t, err)
+			require.Equal(t, 200, contentResp.StatusCode)
+			content, err := io.ReadAll(contentResp.Body)
+			require.NoError(t, err)
+			// Which we finally check matches the correct WriteIsolationKey's content here.
+			assert.Equal(t, make([]byte, 64), content)
+		}()
+	})
+
+	t.Run("find falls back if matching WriteIsolationKey not available", func(t *testing.T) {
+		version := "c19da02a2bd7e77277f1ac29ab45c09b7d46a4ee758284e26bb3045ad11d9d21"
+		key := strings.ToLower(t.Name())
+
+		uploadCacheNormally(t, base, key, version, "", make([]byte, 128))
+
+		func() {
+			defer overrideWriteIsolationKey("TestWriteKey")()
+
+			resp, err := httpClient.Get(fmt.Sprintf("%s/cache?keys=%s&version=%s", base, key, version))
+			require.NoError(t, err)
+			require.Equal(t, 200, resp.StatusCode)
+
+			got := struct {
+				ArchiveLocation string `json:"archiveLocation"`
+			}{}
+			require.NoError(t, json.NewDecoder(resp.Body).Decode(&got))
+			contentResp, err := httpClient.Get(got.ArchiveLocation)
+			require.NoError(t, err)
+			require.Equal(t, 200, contentResp.StatusCode)
+			content, err := io.ReadAll(contentResp.Body)
+			require.NoError(t, err)
+			assert.Equal(t, make([]byte, 128), content)
+		}()
+	})
+
 	t.Run("case insensitive", func(t *testing.T) {
 		version := "c19da02a2bd7e77277f1ac29ab45c09b7d46a4ee758284e26bb3045ad11d9d20"
 		key := strings.ToLower(t.Name())
 		content := make([]byte, 100)
 		_, err := rand.Read(content)
 		require.NoError(t, err)
-		uploadCacheNormally(t, base, key+"_ABC", version, content)
+		uploadCacheNormally(t, base, key+"_ABC", version, "", content)
 
 		{
 			reqKey := key + "_aBc"
@@ -494,7 +588,7 @@ func TestHandler(t *testing.T) {
 		for i := range contents {
 			_, err := rand.Read(contents[i])
 			require.NoError(t, err)
-			uploadCacheNormally(t, base, keys[i], version, contents[i])
+			uploadCacheNormally(t, base, keys[i], version, "", contents[i])
 			time.Sleep(time.Second) // ensure CreatedAt of caches are different
 		}
 
@@ -545,7 +639,7 @@ func TestHandler(t *testing.T) {
 		for i := range contents {
 			_, err := rand.Read(contents[i])
 			require.NoError(t, err)
-			uploadCacheNormally(t, base, keys[i], version, contents[i])
+			uploadCacheNormally(t, base, keys[i], version, "", contents[i])
 			time.Sleep(time.Second) // ensure CreatedAt of caches are different
 		}
 
@@ -581,9 +675,166 @@ func TestHandler(t *testing.T) {
 		require.NoError(t, err)
 		assert.Equal(t, contents[expect], content)
 	})
+
+	t.Run("upload across WriteIsolationKey", func(t *testing.T) {
+		defer overrideWriteIsolationKey("CorrectKey")()
+
+		key := strings.ToLower(t.Name())
+		version := "c19da02a2bd7e77277f1ac29ab45c09b7d46a4ee758284e26bb3045ad11d9d20"
+		content := make([]byte, 256)
+
+		var id uint64
+		// reserve
+		{
+			body, err := json.Marshal(&Request{
+				Key:     key,
+				Version: version,
+				Size:    int64(len(content)),
+			})
+			require.NoError(t, err)
+			resp, err := httpClient.Post(fmt.Sprintf("%s/caches", base), "application/json", bytes.NewReader(body))
+			require.NoError(t, err)
+			assert.Equal(t, 200, resp.StatusCode)
+
+			got := struct {
+				CacheID uint64 `json:"cacheId"`
+			}{}
+			require.NoError(t, json.NewDecoder(resp.Body).Decode(&got))
+			id = got.CacheID
+		}
+		// upload, but with the incorrect write isolation key relative to the cache obj created
+		func() {
+			defer overrideWriteIsolationKey("WrongKey")()
+			req, err := http.NewRequest(http.MethodPatch,
+				fmt.Sprintf("%s/caches/%d", base, id), bytes.NewReader(content))
+			require.NoError(t, err)
+			req.Header.Set("Content-Type", "application/octet-stream")
+			req.Header.Set("Content-Range", "bytes 0-99/*")
+			resp, err := httpClient.Do(req)
+			require.NoError(t, err)
+			assert.Equal(t, 403, resp.StatusCode)
+		}()
+	})
+
+	t.Run("commit across WriteIsolationKey", func(t *testing.T) {
+		defer overrideWriteIsolationKey("CorrectKey")()
+
+		key := strings.ToLower(t.Name())
+		version := "c19da02a2bd7e77277f1ac29ab45c09b7d46a4ee758284e26bb3045ad11d9d20"
+		content := make([]byte, 256)
+
+		var id uint64
+		// reserve
+		{
+			body, err := json.Marshal(&Request{
+				Key:     key,
+				Version: version,
+				Size:    int64(len(content)),
+			})
+			require.NoError(t, err)
+			resp, err := httpClient.Post(fmt.Sprintf("%s/caches", base), "application/json", bytes.NewReader(body))
+			require.NoError(t, err)
+			assert.Equal(t, 200, resp.StatusCode)
+
+			got := struct {
+				CacheID uint64 `json:"cacheId"`
+			}{}
+			require.NoError(t, json.NewDecoder(resp.Body).Decode(&got))
+			id = got.CacheID
+		}
+		// upload
+		{
+			req, err := http.NewRequest(http.MethodPatch,
+				fmt.Sprintf("%s/caches/%d", base, id), bytes.NewReader(content))
+			require.NoError(t, err)
+			req.Header.Set("Content-Type", "application/octet-stream")
+			req.Header.Set("Content-Range", "bytes 0-99/*")
+			resp, err := httpClient.Do(req)
+			require.NoError(t, err)
+			assert.Equal(t, 200, resp.StatusCode)
+		}
+		// commit, but with the incorrect write isolation key relative to the cache obj created
+		func() {
+			defer overrideWriteIsolationKey("WrongKey")()
+			resp, err := httpClient.Post(fmt.Sprintf("%s/caches/%d", base, id), "", nil)
+			require.NoError(t, err)
+			assert.Equal(t, 403, resp.StatusCode)
+		}()
+	})
+
+	t.Run("get across WriteIsolationKey", func(t *testing.T) {
+		defer func() { httpClientTransport.WriteIsolationKey = "" }()
+
+		version := "c19da02a2bd7e77277f1ac29ab45c09b7d46a4ee758284e26bb3045ad11d9d21"
+		key := strings.ToLower(t.Name())
+		uploadCacheNormally(t, base, key, version, "", make([]byte, 128))
+		keyIsolated := strings.ToLower(t.Name()) + "_isolated"
+		uploadCacheNormally(t, base, keyIsolated, version, "CorrectKey", make([]byte, 128))
+
+		// Perform the 'get' without the right WriteIsolationKey for the cache entry... should be OK for `key` since it
+		// was written with WriteIsolationKey "" meaning it is available for non-isolated access
+		func() {
+			defer overrideWriteIsolationKey("WhoopsWrongKey")()
+
+			resp, err := httpClient.Get(fmt.Sprintf("%s/cache?keys=%s&version=%s", base, key, version))
+			require.NoError(t, err)
+			require.Equal(t, 200, resp.StatusCode)
+			got := struct {
+				ArchiveLocation string `json:"archiveLocation"`
+			}{}
+			require.NoError(t, json.NewDecoder(resp.Body).Decode(&got))
+
+			contentResp, err := httpClient.Get(got.ArchiveLocation)
+			require.NoError(t, err)
+			require.Equal(t, 200, contentResp.StatusCode)
+			httpClientTransport.WriteIsolationKey = "CorrectKey" // reset for next find
+		}()
+
+		// Perform the 'get' without the right WriteIsolationKey for the cache entry... should be 403 for `keyIsolated`
+		// because it was written with a different WriteIsolationKey.
+		{
+			got := func() struct {
+				ArchiveLocation string `json:"archiveLocation"`
+			} {
+				defer overrideWriteIsolationKey("CorrectKey")() // for test purposes make the `find` successful...
+				resp, err := httpClient.Get(fmt.Sprintf("%s/cache?keys=%s&version=%s", base, keyIsolated, version))
+				require.NoError(t, err)
+				require.Equal(t, 200, resp.StatusCode)
+				got := struct {
+					ArchiveLocation string `json:"archiveLocation"`
+				}{}
+				require.NoError(t, json.NewDecoder(resp.Body).Decode(&got))
+				return got
+			}()
+
+			func() {
+				defer overrideWriteIsolationKey("WhoopsWrongKey")() // but then access w/ the wrong key for `get`
+				contentResp, err := httpClient.Get(got.ArchiveLocation)
+				require.NoError(t, err)
+				require.Equal(t, 403, contentResp.StatusCode)
+			}()
+		}
+	})
 }
 
-func uploadCacheNormally(t *testing.T, base, key, version string, content []byte) {
+func overrideWriteIsolationKey(writeIsolationKey string) func() {
+	originalWriteIsolationKey := httpClientTransport.WriteIsolationKey
+	originalMac := httpClientTransport.OverrideDefaultMac
+
+	httpClientTransport.WriteIsolationKey = writeIsolationKey
+	httpClientTransport.OverrideDefaultMac = computeMac("secret", cacheRepo, cacheRunnum, cacheTimestamp, httpClientTransport.WriteIsolationKey)
+
+	return func() {
+		httpClientTransport.WriteIsolationKey = originalWriteIsolationKey
+		httpClientTransport.OverrideDefaultMac = originalMac
+	}
+}
+
+func uploadCacheNormally(t *testing.T, base, key, version, writeIsolationKey string, content []byte) {
+	if writeIsolationKey != "" {
+		defer overrideWriteIsolationKey(writeIsolationKey)()
+	}
+
 	var id uint64
 	{
 		body, err := json.Marshal(&Request{

act/artifactcache/mac.go

@@ -22,7 +22,7 @@ func (h *Handler) validateMac(rundata cacheproxy.RunData) (string, error) {
 		return "", ErrValidation
 	}
 
-	expectedMAC := computeMac(h.secret, rundata.RepositoryFullName, rundata.RunNumber, rundata.Timestamp)
+	expectedMAC := computeMac(h.secret, rundata.RepositoryFullName, rundata.RunNumber, rundata.Timestamp, rundata.WriteIsolationKey)
 	if hmac.Equal([]byte(expectedMAC), []byte(rundata.RepositoryMAC)) {
 		return rundata.RepositoryFullName, nil
 	}
@@ -40,12 +40,14 @@ func validateAge(ts string) bool {
 	return true
 }
 
-func computeMac(secret, repo, run, ts string) string {
+func computeMac(secret, repo, run, ts, writeIsolationKey string) string {
 	mac := hmac.New(sha256.New, []byte(secret))
 	mac.Write([]byte(repo))
 	mac.Write([]byte(">"))
 	mac.Write([]byte(run))
 	mac.Write([]byte(">"))
 	mac.Write([]byte(ts))
+	mac.Write([]byte(">"))
+	mac.Write([]byte(writeIsolationKey))
 	return hex.EncodeToString(mac.Sum(nil))
 }

act/artifactcache/mac_test.go

@@ -19,7 +19,7 @@ func TestMac(t *testing.T) {
 		run := "1"
 		ts := strconv.FormatInt(time.Now().Unix(), 10)
 
-		mac := computeMac(handler.secret, name, run, ts)
+		mac := computeMac(handler.secret, name, run, ts, "")
 		rundata := cacheproxy.RunData{
 			RepositoryFullName: name,
 			RunNumber:          run,
@@ -37,7 +37,7 @@ func TestMac(t *testing.T) {
 		run := "1"
 		ts := "9223372036854775807" // This should last us for a while...
 
-		mac := computeMac(handler.secret, name, run, ts)
+		mac := computeMac(handler.secret, name, run, ts, "")
 		rundata := cacheproxy.RunData{
 			RepositoryFullName: name,
 			RunNumber:          run,
@@ -72,9 +72,12 @@ func TestMac(t *testing.T) {
 		run := "42"
 		ts := "1337"
 
-		mac := computeMac(secret, name, run, ts)
-		expectedMac := "f666f06f917acb7186e152195b2a8c8d36d068ce683454be0878806e08e04f2b" // * Precomputed, anytime the computeMac function changes this needs to be recalculated
+		mac := computeMac(secret, name, run, ts, "")
+		expectedMac := "4754474b21329e8beadd2b4054aa4be803965d66e710fa1fee091334ed804f29" // * Precomputed, anytime the computeMac function changes this needs to be recalculated
+		require.Equal(t, expectedMac, mac)
 
-		require.Equal(t, mac, expectedMac)
+		mac = computeMac(secret, name, run, ts, "refs/pull/12/head")
+		expectedMac = "9ca8f4cb5e1b083ee8cd215215bc00f379b28511d3ef7930bf054767de34766d" // * Precomputed, anytime the computeMac function changes this needs to be recalculated
+		require.Equal(t, expectedMac, mac)
 	})
 }

act/artifactcache/model.go

@@ -32,4 +32,5 @@ type Cache struct {
 	Complete          bool   `json:"complete"`
 	UsedAt            int64  `json:"usedAt"`
 	CreatedAt         int64  `json:"createdAt"`
+	WriteIsolationKey string `json:"writeIsolationKey"`
 }

act/cacheproxy/handler.go

@@ -51,15 +51,17 @@ type RunData struct {
 	RunNumber          string
 	Timestamp          string
 	RepositoryMAC      string
+	WriteIsolationKey  string
 }
 
-func (h *Handler) CreateRunData(fullName, runNumber, timestamp string) RunData {
-	mac := computeMac(h.cacheSecret, fullName, runNumber, timestamp)
+func (h *Handler) CreateRunData(fullName, runNumber, timestamp, writeIsolationKey string) RunData {
+	mac := computeMac(h.cacheSecret, fullName, runNumber, timestamp, writeIsolationKey)
 	return RunData{
 		RepositoryFullName: fullName,
 		RunNumber:          runNumber,
 		Timestamp:          timestamp,
 		RepositoryMAC:      mac,
+		WriteIsolationKey:  writeIsolationKey,
 	}
 }
 
@@ -152,6 +154,9 @@ func (h *Handler) newReverseProxy(targetHost string) (*httputil.ReverseProxy, er
 			r.Out.Header.Set("Forgejo-Cache-Timestamp", runData.Timestamp)
 			r.Out.Header.Set("Forgejo-Cache-MAC", runData.RepositoryMAC)
 			r.Out.Header.Set("Forgejo-Cache-Host", h.ExternalURL())
+			if runData.WriteIsolationKey != "" {
+				r.Out.Header.Set("Forgejo-Cache-WriteIsolationKey", runData.WriteIsolationKey)
+			}
 		},
 	}
 	return proxy, nil
@@ -207,12 +212,14 @@ func (h *Handler) Close() error {
 	return retErr
 }
 
-func computeMac(secret, repo, run, ts string) string {
+func computeMac(secret, repo, run, ts, writeIsolationKey string) string {
 	mac := hmac.New(sha256.New, []byte(secret))
 	mac.Write([]byte(repo))
 	mac.Write([]byte(">"))
 	mac.Write([]byte(run))
 	mac.Write([]byte(">"))
 	mac.Write([]byte(ts))
+	mac.Write([]byte(">"))
+	mac.Write([]byte(writeIsolationKey))
 	return hex.EncodeToString(mac.Sum(nil))
 }

internal/app/run/runner.go

@@ -265,8 +265,23 @@ func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.
 
 	// Register the run with the cacheproxy and modify the CACHE_URL
 	if r.cacheProxy != nil {
+		writeIsolationKey := ""
+
+		// When performing an action on an event from a PR, provide a "write isolation key" to the cache. The generated
+		// ACTIONS_CACHE_URL will be able to read the cache, and write to a cache, but its writes will be isolated to
+		// future runs of the PR's workflows and won't be shared with other pull requests or actions. This is a security
+		// measure to prevent a malicious pull request from poisoning the cache with secret-stealing code which would
+		// later be executed on another action.
+		if taskContext["event_name"].GetStringValue() == "pull_request" || taskContext["event_name"].GetStringValue() == "pull_request_target" {
+			// Ensure that `Ref` has the expected format so that we don't end up with a useless write isolation key
+			if !strings.HasPrefix(preset.Ref, "refs/pull/") {
+				return fmt.Errorf("write isolation key: expected preset.Ref to be refs/pull/..., but was %q", preset.Ref)
+			}
+			writeIsolationKey = preset.Ref
+		}
+
 		timestamp := strconv.FormatInt(time.Now().Unix(), 10)
-		cacheRunData := r.cacheProxy.CreateRunData(preset.Repository, preset.RunID, timestamp)
+		cacheRunData := r.cacheProxy.CreateRunData(preset.Repository, preset.RunID, timestamp, writeIsolationKey)
 		cacheRunID, err := r.cacheProxy.AddRun(cacheRunData)
 		if err == nil {
 			defer func() { _ = r.cacheProxy.RemoveRun(cacheRunID) }()

internal/app/run/runner_test.go

@@ -171,11 +171,7 @@ func TestRunnerCacheConfiguration(t *testing.T) {
 	// Must set up cache for our test
 	require.NotNil(t, runner.cacheProxy)
 
-	ctx, cancel := context.WithCancel(t.Context())
-	defer cancel()
-
-	// Run a given workflow w/ event...
-	runWorkflow := func(yamlContent, eventName, ref, description string) {
+	runWorkflow := func(ctx context.Context, cancel context.CancelFunc, yamlContent, eventName, ref, description string) {
 		task := &runnerv1.Task{
 			WorkflowPayload: []byte(yamlContent),
 			Context: &structpb.Struct{
@@ -195,6 +191,10 @@ func TestRunnerCacheConfiguration(t *testing.T) {
 		require.NoError(t, err, description)
 	}
 
+	t.Run("Cache accessible", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(t.Context())
+		defer cancel()
+
 		// Step 1: Populate shared cache with push workflow
 		populateYaml := `
 name: Cache Testing Action
@@ -212,7 +212,7 @@ jobs:
           mkdir -p cache_path_1
           echo "Hello from push workflow!" > cache_path_1/cache_content_1
 `
-	runWorkflow(populateYaml, "push", "refs/heads/main", "step 1: push cache populate expected to succeed")
+		runWorkflow(ctx, cancel, populateYaml, "push", "refs/heads/main", "step 1: push cache populate expected to succeed")
 
 		// Step 2: Validate that cache is accessible; mostly a sanity check that the test environment and mock context
 		// provides everything needed for the cache setup.
@@ -232,5 +232,108 @@ jobs:
           [[ -f cache_path_1/cache_content_1 ]] && echo "Step 2: cache file found." || echo "Step 2: cache file missing!"
           [[ -f cache_path_1/cache_content_1 ]] || exit 1
 `
-	runWorkflow(checkYaml, "push", "refs/heads/main", "step 2: push cache check expected to succeed")
+		runWorkflow(ctx, cancel, checkYaml, "push", "refs/heads/main", "step 2: push cache check expected to succeed")
+	})
+
+	t.Run("PR cache pollution prevented", func(t *testing.T) {
+		ctx, cancel := context.WithCancel(t.Context())
+		defer cancel()
+
+		// Step 1: Populate shared cache with push workflow
+		populateYaml := `
+name: Cache Testing Action
+on:
+  push:
+jobs:
+  job-cache-check-1:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/cache@v4
+        with:
+          path: cache_path_1
+          key: cache-key-1
+      - run: |
+          mkdir -p cache_path_1
+          echo "Hello from push workflow!" > cache_path_1/cache_content_1
+`
+		runWorkflow(ctx, cancel, populateYaml, "push", "refs/heads/main", "step 1: push cache populate expected to succeed")
+
+		// Step 2: Check if pull_request can read push cache, should be available as it's a trusted cache.
+		checkPRYaml := `
+name: Cache Testing Action
+on:
+  pull_request:
+jobs:
+  job-cache-check-2:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/cache@v4
+        with:
+          path: cache_path_1
+          key: cache-key-1
+      - run: |
+          [[ -f cache_path_1/cache_content_1 ]] && echo "Step 2: cache file found." || echo "Step 2: cache file missing!"
+          [[ -f cache_path_1/cache_content_1 ]] || exit 1
+`
+		runWorkflow(ctx, cancel, checkPRYaml, "pull_request", "refs/pull/1234/head", "step 2: PR should read push cache")
+
+		// Step 3: Pull request writes to cache; here we need to use a new cache key because we'll get a cache-hit like we
+		// did in step #2 if we keep the same key, and then the cache contents won't be updated.
+		populatePRYaml := `
+name: Cache Testing Action
+on:
+  pull_request:
+jobs:
+  job-cache-check-3:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/cache@v4
+        with:
+          path: cache_path_1
+          key: cache-key-2
+      - run: |
+          mkdir -p cache_path_1
+          echo "Hello from PR workflow!" > cache_path_1/cache_content_2
+`
+		runWorkflow(ctx, cancel, populatePRYaml, "pull_request", "refs/pull/1234/head", "step 3: PR cache populate expected to succeed")
+
+		// Step 4: Check if pull_request can read its own cache written by step #3.
+		checkPRKey2Yaml := `
+name: Cache Testing Action
+on:
+  pull_request:
+jobs:
+  job-cache-check-4:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/cache@v4
+        with:
+          path: cache_path_1
+          key: cache-key-2
+      - run: |
+          [[ -f cache_path_1/cache_content_2 ]] && echo "Step 4 cache file found." || echo "Step 4 cache file missing!"
+          [[ -f cache_path_1/cache_content_2 ]] || exit 1
+`
+		runWorkflow(ctx, cancel, checkPRKey2Yaml, "pull_request", "refs/pull/1234/head", "step 4: PR should read its own cache")
+
+		// Step 5: Check that the push workflow cannot access the isolated cache that was written by the pull_request in
+		// step #3, ensuring that it's not possible to pollute the cache by predicting cache keys.
+		checkKey2Yaml := `
+name: Cache Testing Action
+on:
+  push:
+jobs:
+  job-cache-check-6:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/cache@v4
+        with:
+          path: cache_path_1
+          key: cache-key-2
+      - run: |
+          [[ -f cache_path_1/cache_content_2 ]] && echo "Step 5 cache file found, oh no!" || echo "Step 5: cache file missing as expected."
+          [[ -f cache_path_1/cache_content_2 ]] && exit 1 || exit 0
+`
+		runWorkflow(ctx, cancel, checkKey2Yaml, "push", "refs/heads/main", "step 5: push cache should not be polluted by PR")
+	})
 }