diff --git a/archives_test.go b/archives_test.go
index 524e674..8618263 100644
--- a/archives_test.go
+++ b/archives_test.go
@@ -6,6 +6,7 @@ import (
 	"bytes"
 	"compress/gzip"
 	"encoding/base64"
+	"encoding/binary"
 	"errors"
 	"fmt"
 	"io"
@@ -858,6 +859,67 @@ func TestOpenTarRejectsCumulativeOverflow(t *testing.T) {
 	}
 }
 
+func TestOpenTarRejectsTooManyEntries(t *testing.T) {
+	oldMax := maxArchiveEntries
+	maxArchiveEntries = 2
+	defer func() { maxArchiveEntries = oldMax }()
+
+	var buf bytes.Buffer
+	tw := tar.NewWriter(&buf)
+	for i := 0; i < 3; i++ {
+		_ = tw.WriteHeader(&tar.Header{Name: fmt.Sprintf("empty-%d", i), Mode: 0644})
+	}
+	_ = tw.Close()
+
+	_, err := openTar(buf.Bytes(), "")
+	if !errors.Is(err, ErrEntryLimit) {
+		t.Fatalf("expected ErrEntryLimit, got: %v", err)
+	}
+}
+
+func TestOpenZipRejectsTooManyEntries(t *testing.T) {
+	oldMax := maxArchiveEntries
+	maxArchiveEntries = 2
+	defer func() { maxArchiveEntries = oldMax }()
+
+	var buf bytes.Buffer
+	zw := zip.NewWriter(&buf)
+	for i := 0; i < 3; i++ {
+		_, _ = zw.Create(fmt.Sprintf("empty-%d", i))
+	}
+	_ = zw.Close()
+	raw := buf.Bytes()
+	directoryEnd := bytes.LastIndex(raw, []byte{'P', 'K', 0x05, 0x06})
+	if directoryEnd < 0 {
+		t.Fatal("zip end-of-directory record not found")
+	}
+	binary.LittleEndian.PutUint16(raw[directoryEnd+8:], 1)
+	binary.LittleEndian.PutUint16(raw[directoryEnd+10:], 1)
+
+	_, err := OpenBytes("too-many.zip", raw)
+	if !errors.Is(err, ErrEntryLimit) {
+		t.Fatalf("expected ErrEntryLimit, got: %v", err)
+	}
+}
+
+func TestOpenGemRejectsTooManyEntries(t *testing.T) {
+	oldMax := maxArchiveEntries
+	maxArchiveEntries = 2
+	defer func() { maxArchiveEntries = oldMax }()
+
+	var buf bytes.Buffer
+	tw := tar.NewWriter(&buf)
+	for i := 0; i < 3; i++ {
+		_ = tw.WriteHeader(&tar.Header{Name: fmt.Sprintf("empty-%d", i), Mode: 0644})
+	}
+	_ = tw.Close()
+
+	_, err := openGem(buf.Bytes())
+	if !errors.Is(err, ErrEntryLimit) {
+		t.Fatalf("expected ErrEntryLimit, got: %v", err)
+	}
+}
+
 func TestOpenGemRejectsOversizedData(t *testing.T) {
 	oldMax := maxDecompressedSize
 	maxDecompressedSize = 512
diff --git a/conda.go b/conda.go
index dba636f..c4786f4 100644
--- a/conda.go
+++ b/conda.go
@@ -18,10 +18,16 @@ import (
 // merged tarReader with raw pointing at the outer .conda bytes so Hash
 // matches the digest anaconda.org publishes in repodata.json.
 func openConda(raw []byte) (*tarReader, error) {
+	if err := checkZipEntryCount(raw); err != nil {
+		return nil, err
+	}
 	zr, err := zip.NewReader(bytes.NewReader(raw), int64(len(raw)))
 	if err != nil {
 		return nil, fmt.Errorf("opening conda zip: %w", err)
 	}
+	if err := checkArchiveEntryCount(len(zr.File)); err != nil {
+		return nil, err
+	}
 
 	var files []tarFileEntry
 	var total int64
@@ -32,7 +38,7 @@ func openConda(raw []byte) (*tarReader, error) {
 		if !strings.HasPrefix(f.Name, "pkg-") && !strings.HasPrefix(f.Name, "info-") {
 			continue
 		}
-		entries, size, err := readCondaMember(f)
+		entries, size, err := readCondaMember(f, len(files))
 		if err != nil {
 			return nil, err
 		}
@@ -56,7 +62,7 @@ func openConda(raw []byte) (*tarReader, error) {
 	return &tarReader{raw: raw, files: files, index: index}, nil
 }
 
-func readCondaMember(f *zip.File) ([]tarFileEntry, int64, error) {
+func readCondaMember(f *zip.File, initialEntryCount int) ([]tarFileEntry, int64, error) {
 	rc, err := f.Open()
 	if err != nil {
 		return nil, 0, fmt.Errorf("opening %s: %w", f.Name, err)
@@ -71,7 +77,7 @@ func readCondaMember(f *zip.File) ([]tarFileEntry, int64, error) {
 		return nil, 0, fmt.Errorf("%w: %s exceeds %d bytes", ErrDecompressLimit, f.Name, maxDecompressedSize)
 	}
 
-	tr, err := openTar(data, "zstd")
+	tr, err := openTarWithInitialEntryCount(data, "zstd", initialEntryCount)
 	if err != nil {
 		return nil, 0, fmt.Errorf("opening %s: %w", f.Name, err)
 	}
diff --git a/conda_test.go b/conda_test.go
index 1b09dcf..f284514 100644
--- a/conda_test.go
+++ b/conda_test.go
@@ -186,6 +186,58 @@ func TestOpenCondaRejectsCumulativeOverflow(t *testing.T) {
 	}
 }
 
+func TestOpenCondaRejectsTooManyEntriesAcrossMembers(t *testing.T) {
+	oldMax := maxArchiveEntries
+	maxArchiveEntries = 3
+	defer func() { maxArchiveEntries = oldMax }()
+
+	_, err := openConda(createTestConda(t))
+	if !errors.Is(err, ErrEntryLimit) {
+		t.Fatalf("expected ErrEntryLimit, got: %v", err)
+	}
+}
+
+func TestOpenCondaStopsAtCombinedEntryLimit(t *testing.T) {
+	oldEntryMax := maxArchiveEntries
+	oldSizeMax := maxDecompressedSize
+	maxArchiveEntries = 2
+	maxDecompressedSize = 512
+	defer func() {
+		maxArchiveEntries = oldEntryMax
+		maxDecompressedSize = oldSizeMax
+	}()
+
+	members := []struct {
+		name string
+		data []byte
+	}{
+		{"pkg-a-1.tar.zst", writeTarZst(t, map[string]string{"first": "", "second": ""})},
+		{"info-a-1.tar.zst", writeTarZst(t, map[string]string{"large": strings.Repeat("x", 1024)})},
+	}
+	var buf bytes.Buffer
+	zw := zip.NewWriter(&buf)
+	for _, member := range members {
+		w, err := zw.CreateHeader(&zip.FileHeader{Name: member.name, Method: zip.Store})
+		if err != nil {
+			t.Fatal(err)
+		}
+		if _, err := w.Write(member.data); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := zw.Close(); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err := OpenBytes("a.conda", buf.Bytes())
+	if !errors.Is(err, ErrEntryLimit) {
+		t.Fatalf("expected ErrEntryLimit before reading the next member, got: %v", err)
+	}
+	if !strings.Contains(err.Error(), "count 3 exceeds 2") {
+		t.Fatalf("expected combined entry count in error, got: %v", err)
+	}
+}
+
 func TestOpenDoesNotInferConda(t *testing.T) {
 	reader, err := OpenBytes("artifact", createTestConda(t))
 	if err != nil {
diff --git a/gem.go b/gem.go
index b7706c5..c5edcc8 100644
--- a/gem.go
+++ b/gem.go
@@ -17,6 +17,7 @@ type gemReader struct {
 
 func openGem(raw []byte) (*gemReader, error) {
 	tr := tar.NewReader(bytes.NewReader(raw))
+	entryCount := 0
 
 	// Find data.tar.gz in the gem
 	for {
@@ -27,6 +28,10 @@ func openGem(raw []byte) (*gemReader, error) {
 		if err != nil {
 			return nil, fmt.Errorf("reading gem tar: %w", err)
 		}
+		entryCount++
+		if err := checkArchiveEntryCount(entryCount); err != nil {
+			return nil, err
+		}
 
 		// Look for data.tar.gz
 		if header.Name == "data.tar.gz" {
diff --git a/tar.go b/tar.go
index e33a710..742aba2 100644
--- a/tar.go
+++ b/tar.go
@@ -15,9 +15,13 @@ import (
 	"github.com/ulikunitz/xz"
 )
 
-var maxDecompressedSize int64 = 512 << 20 // 512 MiB
+var (
+	maxDecompressedSize int64 = 512 << 20 // 512 MiB
+	maxArchiveEntries         = 100_000
+)
 
 var ErrDecompressLimit = errors.New("decompressed content exceeds size limit")
+var ErrEntryLimit = errors.New("archive entry count exceeds limit")
 
 type tarReader struct {
 	raw   []byte
@@ -31,6 +35,10 @@ type tarFileEntry struct {
 }
 
 func openTar(raw []byte, compression string) (*tarReader, error) {
+	return openTarWithInitialEntryCount(raw, compression, 0)
+}
+
+func openTarWithInitialEntryCount(raw []byte, compression string, initialEntryCount int) (*tarReader, error) {
 	content := bytes.NewReader(raw)
 	r := io.Reader(content)
 
@@ -71,6 +79,9 @@ func openTar(raw []byte, compression string) (*tarReader, error) {
 		if err != nil {
 			return nil, fmt.Errorf("reading tar: %w", err)
 		}
+		if err := checkArchiveEntryCount(initialEntryCount + len(files) + 1); err != nil {
+			return nil, err
+		}
 
 		// FileInfo().Mode() combines header.Mode permission bits with type
 		// bits derived from Typeflag. It reports hard links as regular
@@ -119,6 +130,13 @@ func openTar(raw []byte, compression string) (*tarReader, error) {
 	return &tarReader{raw: raw, files: files, index: index}, nil
 }
 
+func checkArchiveEntryCount(count int) error {
+	if count > maxArchiveEntries {
+		return fmt.Errorf("%w: count %d exceeds %d", ErrEntryLimit, count, maxArchiveEntries)
+	}
+	return nil
+}
+
 func (t *tarReader) List() ([]FileInfo, error) {
 	files := make([]FileInfo, len(t.files))
 	for i, f := range t.files {
diff --git a/zip.go b/zip.go
index 025dd88..838fff9 100644
--- a/zip.go
+++ b/zip.go
@@ -3,6 +3,7 @@ package archives
 import (
 	"archive/zip"
 	"bytes"
+	"encoding/binary"
 	"fmt"
 	"io"
 	"strings"
@@ -15,10 +16,22 @@ import (
 // then returns a synthesised 0666/0444 that callers should not treat as a
 // stored mode.
 const (
-	zipUnixModeShift = 16
-	zipCreatorShift  = 8
-	zipCreatorUnix   = 3
-	zipCreatorMacOSX = 19
+	zipUnixModeShift             = 16
+	zipCreatorShift              = 8
+	zipCreatorUnix               = 3
+	zipCreatorMacOSX             = 19
+	zipSignatureLen              = 4
+	zipDirectoryHeaderLen        = 46
+	zipDirectoryEndLen           = 22
+	zipDirectory64EndLen         = 56
+	zipDirectory64LocatorLen     = 20
+	zipDirectoryHeaderSignature  = 0x02014b50
+	zipDirectoryEndSignature     = 0x06054b50
+	zipDirectory64EndSignature   = 0x06064b50
+	zipDirectory64LocSignature   = 0x07064b50
+	zipDirectoryEndSearchWindow  = 65 * 1024
+	zipDirectoryRecords64Marker  = 0xffff
+	zipDirectorySizeOffsetMarker = 0xffffffff
 )
 
 type zipReader struct {
@@ -28,10 +41,16 @@ type zipReader struct {
 }
 
 func openZip(raw []byte) (*zipReader, error) {
+	if err := checkZipEntryCount(raw); err != nil {
+		return nil, err
+	}
 	reader, err := zip.NewReader(bytes.NewReader(raw), int64(len(raw)))
 	if err != nil {
 		return nil, fmt.Errorf("opening zip: %w", err)
 	}
+	if err := checkArchiveEntryCount(len(reader.File)); err != nil {
+		return nil, err
+	}
 
 	index := make(map[string]*zip.File, len(reader.File))
 	for _, f := range reader.File {
@@ -47,6 +66,88 @@ func openZip(raw []byte) (*zipReader, error) {
 	}, nil
 }
 
+func checkZipEntryCount(raw []byte) error {
+	start, end, ok := zipCentralDirectoryBounds(raw)
+	if !ok {
+		return nil
+	}
+
+	count := 0
+	for offset := start; offset+zipDirectoryHeaderLen <= end; {
+		if binary.LittleEndian.Uint32(raw[offset:]) != zipDirectoryHeaderSignature {
+			break
+		}
+		nameLen := int(binary.LittleEndian.Uint16(raw[offset+28:]))
+		extraLen := int(binary.LittleEndian.Uint16(raw[offset+30:]))
+		commentLen := int(binary.LittleEndian.Uint16(raw[offset+32:]))
+		recordLen := zipDirectoryHeaderLen + nameLen + extraLen + commentLen
+		if recordLen > end-offset {
+			return nil
+		}
+
+		count++
+		if err := checkArchiveEntryCount(count); err != nil {
+			return err
+		}
+		offset += recordLen
+	}
+
+	return nil
+}
+
+func zipCentralDirectoryBounds(raw []byte) (int, int, bool) {
+	searchStart := len(raw) - zipDirectoryEndSearchWindow
+	if searchStart < 0 {
+		searchStart = 0
+	}
+
+	for offset := len(raw) - zipDirectoryEndLen; offset >= searchStart; offset-- {
+		if binary.LittleEndian.Uint32(raw[offset:]) != zipDirectoryEndSignature {
+			continue
+		}
+		commentLen := int(binary.LittleEndian.Uint16(raw[offset+20:]))
+		if offset+zipDirectoryEndLen+commentLen > len(raw) {
+			continue
+		}
+
+		directoryEnd := offset
+		directoryRecords := binary.LittleEndian.Uint16(raw[offset+10:])
+		directorySize := uint64(binary.LittleEndian.Uint32(raw[offset+12:]))
+		directoryOffset := uint64(binary.LittleEndian.Uint32(raw[offset+16:]))
+		if directoryRecords == zipDirectoryRecords64Marker ||
+			directorySize == zipDirectorySizeOffsetMarker ||
+			directoryOffset == zipDirectorySizeOffsetMarker {
+			locatorOffset := offset - zipDirectory64LocatorLen
+			if locatorOffset < 0 || binary.LittleEndian.Uint32(raw[locatorOffset:]) != zipDirectory64LocSignature {
+				return 0, 0, false
+			}
+			zip64Offset := binary.LittleEndian.Uint64(raw[locatorOffset+8:])
+			if len(raw) < zipDirectory64EndLen || zip64Offset > uint64(len(raw)-zipDirectory64EndLen) {
+				return 0, 0, false
+			}
+			directoryEnd = int(zip64Offset)
+			if binary.LittleEndian.Uint32(raw[directoryEnd:]) != zipDirectory64EndSignature {
+				return 0, 0, false
+			}
+			directorySize = binary.LittleEndian.Uint64(raw[directoryEnd+40:])
+			directoryOffset = binary.LittleEndian.Uint64(raw[directoryEnd+48:])
+		}
+
+		if directorySize > uint64(directoryEnd) {
+			return 0, 0, false
+		}
+		start := directoryEnd - int(directorySize)
+		if directoryOffset < uint64(directoryEnd)-directorySize &&
+			directoryOffset <= uint64(len(raw)-zipSignatureLen) &&
+			binary.LittleEndian.Uint32(raw[int(directoryOffset):]) == zipDirectoryHeaderSignature {
+			start = int(directoryOffset)
+		}
+		return start, directoryEnd, true
+	}
+
+	return 0, 0, false
+}
+
 func (z *zipReader) List() ([]FileInfo, error) {
 	files := make([]FileInfo, 0, len(z.reader.File))