diff --git a/internal/cli/scan_history.go b/internal/cli/scan_history.go
index 5ba6493..c5b4362 100644
--- a/internal/cli/scan_history.go
+++ b/internal/cli/scan_history.go
@@ -7,10 +7,13 @@ import (
 	"fmt"
 	"io"
 	"strings"
+	"time"
 
 	service "github.com/devr-tools/codeguard/pkg/codeguard"
 )
 
+const scanHistoryTimeout = 5 * time.Minute
+
 func runScanHistory(args []string, stdout io.Writer, stderr io.Writer) int {
 	fs := flag.NewFlagSet("scan-history", flag.ContinueOnError)
 	fs.SetOutput(stderr)
@@ -30,7 +33,9 @@ func runScanHistory(args []string, stdout io.Writer, stderr io.Writer) int {
 		cfg = service.ExampleConfig()
 	}
 
-	report, err := service.ScanGitHistory(context.Background(), cfg, service.HistoryScanOptions{
+	ctx, cancel := context.WithTimeout(context.Background(), scanHistoryTimeout)
+	defer cancel()
+	report, err := service.ScanGitHistory(ctx, cfg, service.HistoryScanOptions{
 		RepoPath:   *repoPath,
 		MaxCommits: *maxCommits,
 		AllRefs:    *allRefs,
diff --git a/internal/codeguard/history/history.go b/internal/codeguard/history/history.go
index 54bcca3..b3a84d9 100644
--- a/internal/codeguard/history/history.go
+++ b/internal/codeguard/history/history.go
@@ -8,6 +8,7 @@ package history
 import (
 	"bufio"
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"os/exec"
@@ -20,6 +21,11 @@ import (
 
 const commitMarker = "@@CG-COMMIT@@ "
 
+const (
+	maxHistoryLineBytes   = 64 * 1024
+	maxHistoryOutputBytes = 256 * 1024 * 1024
+)
+
 var hunkHeader = regexp.MustCompile(`^@@ -\d+(?:,\d+)? \+(\d+)`)
 
 // Scan walks git history and returns deduplicated secret findings. Findings are
@@ -39,7 +45,9 @@ func Scan(ctx context.Context, opts Options) (Report, error) {
 		args = append(args, fmt.Sprintf("-n%d", opts.MaxCommits))
 	}
 
-	cmd := exec.CommandContext(ctx, "git", args...) //nolint:gosec // fixed git log subcommand; args are tool-controlled constants plus the scan repo path
+	cmdCtx, cancel := context.WithCancel(ctx)
+	defer cancel()
+	cmd := exec.CommandContext(cmdCtx, "git", args...) //nolint:gosec // fixed git log subcommand; args are tool-controlled constants plus the scan repo path
 	stdout, err := cmd.StdoutPipe()
 	if err != nil {
 		return Report{}, err
@@ -48,7 +56,13 @@ func Scan(ctx context.Context, opts Options) (Report, error) {
 		return Report{}, fmt.Errorf("git log: %w", err)
 	}
 
-	report := parseLog(stdout, opts.Scanner)
+	limited := &io.LimitedReader{R: stdout, N: maxHistoryOutputBytes + 1}
+	report := parseLog(limited, opts.Scanner)
+	if limited.N == 0 {
+		cancel()
+		_ = cmd.Wait()
+		return Report{}, fmt.Errorf("git log output exceeds %d MiB limit", maxHistoryOutputBytes/(1024*1024))
+	}
 
 	if err := cmd.Wait(); err != nil {
 		return Report{}, fmt.Errorf("git log: %w", err)
@@ -69,19 +83,16 @@ type logParser struct {
 }
 
 func parseLog(reader io.Reader, scanner security.Scanner) Report {
-	// bufio.Reader.ReadString grows for arbitrarily long lines, unlike
-	// bufio.Scanner, which silently stops on an over-long token — a dangerous
-	// failure mode for a security scan (it would skip the rest of history).
-	buf := bufio.NewReaderSize(reader, 64*1024)
+	buf := bufio.NewReaderSize(reader, maxHistoryLineBytes)
 	parser := &logParser{
 		scanner: scanner,
 		seen:    make(map[string]struct{}),
 		commits: make(map[string]struct{}),
 	}
 	for {
-		raw, err := buf.ReadString('\n')
+		raw, err := readBoundedLine(buf)
 		if len(raw) > 0 {
-			parser.handleLine(strings.TrimRight(raw, "\r\n"))
+			parser.handleLine(strings.TrimRight(string(raw), "\r\n"))
 		}
 		if err != nil {
 			break
@@ -91,6 +102,26 @@ func parseLog(reader io.Reader, scanner security.Scanner) Report {
 	return parser.report
 }
 
+// readBoundedLine retains at most maxHistoryLineBytes from a diff line while
+// draining the rest, so an attacker-controlled blob cannot cause an unbounded
+// allocation or make the parser silently skip the remainder of history.
+func readBoundedLine(buf *bufio.Reader) ([]byte, error) {
+	var line []byte
+	for {
+		fragment, err := buf.ReadSlice('\n')
+		remaining := maxHistoryLineBytes - len(line)
+		if remaining > 0 {
+			if len(fragment) < remaining {
+				remaining = len(fragment)
+			}
+			line = append(line, fragment[:remaining]...)
+		}
+		if !errors.Is(err, bufio.ErrBufferFull) {
+			return line, err
+		}
+	}
+}
+
 // handleLine advances parser state for one line of `git log -p -U0` output.
 func (p *logParser) handleLine(line string) {
 	switch {
diff --git a/internal/codeguard/history/history_test.go b/internal/codeguard/history/history_test.go
new file mode 100644
index 0000000..ba77324
--- /dev/null
+++ b/internal/codeguard/history/history_test.go
@@ -0,0 +1,28 @@
+package history
+
+import (
+	"bufio"
+	"strings"
+	"testing"
+)
+
+func TestReadBoundedLineDrainsOversizedLine(t *testing.T) {
+	input := "+" + strings.Repeat("x", maxHistoryLineBytes*2) + "\nnext\n"
+	buf := strings.NewReader(input)
+	reader := bufio.NewReaderSize(buf, maxHistoryLineBytes)
+
+	line, err := readBoundedLine(reader)
+	if err != nil {
+		t.Fatalf("read oversized line: %v", err)
+	}
+	if len(line) != maxHistoryLineBytes {
+		t.Fatalf("line length = %d, want %d", len(line), maxHistoryLineBytes)
+	}
+	next, err := readBoundedLine(reader)
+	if err != nil {
+		t.Fatalf("read following line: %v", err)
+	}
+	if got := string(next); got != "next\n" {
+		t.Fatalf("following line = %q, want %q", got, "next\\n")
+	}
+}