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Malicious artifacts can cause machine-wide DoS

Moderate
bobcallaway published GHSA-95pr-fxf5-86gv Apr 10, 2024

Package

gomod github.com/sigstore/cosign (Go)

Affected versions

<= 2.2.3

Patched versions

2.2.4

Description

Maliciously-crafted software artifacts can cause denial of service of the machine running Cosign, thereby impacting all services on the machine. The root cause is that Cosign creates slices based on the number of signatures, manifests or attestations in untrusted artifacts. As such, the untrusted artifact can control the amount of memory that Cosign allocates.

As an example, these lines demonstrate the problem:

func (s *sigs) Get() ([]oci.Signature, error) {
m, err := s.Manifest()
if err != nil {
return nil, err
}
signatures := make([]oci.Signature, 0, len(m.Layers))
for _, desc := range m.Layers {
layer, err := s.Image.LayerByDigest(desc.Digest)
if err != nil {
return nil, err
}
signatures = append(signatures, signature.New(layer, desc))
}
return signatures, nil
}

This Get() method gets the manifest of the image, allocates a slice equal to the length of the layers in the manifest, loops through the layers and adds a new signature to the slice.

The exact issue is Cosign allocates excessive memory on the lines that creates a slice of the same length as the manifests.

Remediation

Update to the latest version of Cosign, where the number of attestations, signatures and manifests has been limited to a reasonable value.

Cosign PoC

In the case of this API (also referenced above):

func (s *sigs) Get() ([]oci.Signature, error) {
m, err := s.Manifest()
if err != nil {
return nil, err
}
signatures := make([]oci.Signature, 0, len(m.Layers))
for _, desc := range m.Layers {
layer, err := s.Image.LayerByDigest(desc.Digest)
if err != nil {
return nil, err
}
signatures = append(signatures, signature.New(layer, desc))
}
return signatures, nil
}

… The first line can contain a length that is safe for the system and will not throw a runtime panic or be blocked by other safety mechanisms. For the sake of argument, let’s say that the length of m, err := s.Manifest() is the max allowed (by the machine without throwing OOM panics) manifests minus 1. When Cosign then allocates a new slice on this line: signatures := make([]oci.Signature, 0, len(m.Layers)), Cosign will allocate more memory than is available and the machine will be denied of service, causing Cosign and all other services on the machine to be unavailable.

To illustrate the issue here, we run a modified version of TestSignedImageIndex() in pkg/oci/remote:

func TestSignedImageIndex(t *testing.T) {
ri := remote.Image
rix := remote.Index
t.Cleanup(func() {
remoteImage = ri
remoteIndex = rix
})
wantLayers := int64(7)
wantImages := int64(1)
l1, err := random.Image(300 /* byteSize */, wantLayers)
if err != nil {
t.Fatalf("random.Index() = %v", err)
}
l2, err := random.Index(300 /* byteSize */, wantLayers, wantImages)
if err != nil {
t.Fatalf("random.Index() = %v", err)
}
l3 := mutate.AppendManifests(
empty.Index,
mutate.IndexAddendum{
Add: l2,
},
mutate.IndexAddendum{
Add: l1,
},
)

Here, wantLayers is the number of manifests from these lines:

func (s *sigs) Get() ([]oci.Signature, error) {
m, err := s.Manifest()
if err != nil {
return nil, err
}

To test this, we want to make wantLayers high enough to not cause a memory on its own but still trigger the machine-wide OOM when a slice gets create with the same length. On my local machine, it would take hours to create a slice of layers that fulfils that criteria, so instead I modify the Cosign production code to reflect a long list of manifests:

// Get implements oci.Signatures
func (s *sigs) Get() ([]oci.Signature, error) {
        m, err := s.Manifest()
        if err != nil {
                return nil, err
        }
        // Here we imitate a long list of manifests
        ms := make([]byte, 2600000000) // imitate a long list of manifests
        signatures := make([]oci.Signature, 0, len(ms))
        panic("Done")
        //signatures := make([]oci.Signature, 0, len(m.Layers))
        for _, desc := range m.Layers {

With this modified code, if we can cause an OOM without triggering the panic("Done"), we have succeeded.

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
High
Privileges required
High
User interaction
Required
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:N/I:N/A:H

CVE ID

CVE-2024-29903

Weaknesses

No CWEs

Credits