4.3

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-39395

CVE-2026-39395: Cosign's verify-blob-attestation reports false positive when payload parsing fails

Cosign provides code signing and transparency for containers and binaries. Prior to 3.0.6 and 2.6.3, cosign verify-blob-attestation may erroneously report a "Verified OK" result for attestations with malformed payloads or mismatched predicate types. For old-format bundles and detached signatures, this was due to a logic flaw in the error handling of the predicate type validation. For new-format bundles, the predicate type validation was bypassed completely. This vulnerability is fixed in 3.0.6 and 2.6.3.

(GitHub Advisory)

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CVE-2026-39395

CVE-2026-39395:

4.3

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
github.com/sigstore/cosign	go	>= 3.0.0, < 3.0.6	3.0.6
github.com/sigstore/cosign	go	< 2.6.3	2.6.3

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The analysis of the security advisory and the associated patch commit f1ad3ee952313be5d74a49d67ba0aa8d0d5e351f clearly indicates that the vulnerability is located in the Exec method of the VerifyBlobAttestationCommand struct. The vulnerability has two facets, both of which are addressed in the patch within this single function.

Improper Error Handling (Old Bundles/Detached Signatures): The original code had a flawed conditional check: if b, gotPredicateType, err := policy.AttestationToPayloadJSON(...); b == nil && err == nil. This logic ignored any non-nil err returned by AttestationToPayloadJSON (which occurs with malformed payloads), leading to a false "Verified OK" result. The patch corrects this by checking for err != nil immediately after the function call.
Missing Validation (New Bundles): For new-format bundles, the code path completely lacked a call to policy.AttestationToPayloadJSON to validate the predicate type. The patch adds the necessary logic to extract the attestation from the bundle and perform this validation, preventing the acceptance of attestations with mismatched or malformed types.

Therefore, the (*VerifyBlobAttestationCommand).Exec function is the precise location of the vulnerability. During exploitation, this function would be on the call stack as it processes the malicious attestation, making it the key runtime indicator.

Vulnerable functions

(*VerifyBlobAttestationCommand).Exec

cmd/cosign/cli/verify/verify_blob_attestation.go

The vulnerability lies in the `Exec` method of the `VerifyBlobAttestationCommand`. For old-format bundles and detached signatures, the function improperly handled errors returned from `policy.AttestationToPayloadJSON`. It would only report an error if both the returned payload was nil and the error was nil, effectively ignoring parsing errors that would cause the verification to incorrectly succeed. For new-format bundles, the predicate type validation was completely skipped. An attacker could craft a malformed attestation that would be accepted as valid.

Vulnerability Intelligence
Miggo AI

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WAF Rule

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Reasoning

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