Cosign Certificate Chain Expiry Validation Issue Allows Issuing Certificate Expiry to Be Overlooked

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Cosign provides code signing and transparency for containers and binaries. In versions 3.0.4 and below, an issuing certificate with a validity that expires before the leaf certificate will be considered valid during verification even if the provided timestamp would mean the issuing certificate should be considered expired. When verifying artifact signatures using a certificate, Cosign first verifies the certificate chain using the leaf certificate's "not before" timestamp and later checks expiry of the leaf certificate using either a signed timestamp provided by the Rekor transparency log or from a timestamp authority, or using the current time. The root and all issuing certificates are assumed to be valid during the leaf certificate's validity. There is no impact to users of the public Sigstore infrastructure. This may affect private deployments with customized PKIs. This issue has been fixed in version 3.0.5.

Source published

19 Feb 2026 22:27:08 UTC

Source modified

20 Feb 2026 15:41:03 UTC

Saved here

23 Oct 2026 15:48:27 UTC

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CVSS 3.1 · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Cosign provides code signing and transparency for containers and binaries. In versions 3.0.4 and below, an issuing certificate with a validity that expires before the leaf certificate will be considered valid during verification even if the provided timestamp would mean the issuing certificate should be considered expired. When verifying artifact signatures using a certificate, Cosign first verifies the certificate chain using the leaf certificate's "not before" timestamp and later checks expiry of the leaf certificate using either a signed timestamp provided by the Rekor transparency log or from a timestamp authority, or using the current time. The root and all issuing certificates are assumed to be valid during the leaf certificate's validity. There is no impact to users of the public Sigstore infrastructure. This may affect private deployments with customized PKIs. This issue has been fixed in version 3.0.5.

Source published

Feb 19, 2026, 10:27:08 PM (timezone not stated)

Source modified

Feb 20, 2026, 3:41:03 PM (timezone not stated)

Saved here

15 Nov 2026 07:38:10 UTC

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ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Cosign provides code signing and transparency for containers and binaries. In versions 3.0.4 and below, an issuing certificate with a validity that expires before the leaf certificate will be considered valid during verification even if the provided timestamp would mean the issuing certificate should be considered expired. When verifying artifact signatures using a certificate, Cosign first verifies the certificate chain using the leaf certificate's "not before" timestamp and later checks expiry of the leaf certificate using either a signed timestamp provided by the Rekor transparency log or from a timestamp authority, or using the current time. The root and all issuing certificates are assumed to be valid during the leaf certificate's validity. There is no impact to users of the public Sigstore infrastructure. This may affect private deployments with customized PKIs. This issue has been fixed in version 3.0.5.

Source published

19 Feb 2026 23:16:24 UTC

Source modified

17 Jun 2026 10:22:39 UTC

Saved here

25 Oct 2026 05:17:36 UTC

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CVSS 3.1 · [email protected] · Secondary · 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

## Summary When verifying artifact signatures using a certificate, Cosign first verifies the certificate chain using the leaf certificate's "not before" timestamp and later checks expiry of the leaf certificate using either a signed timestamp provided by the Rekor transparency log or from a timestamp authority, or using the current time. The root and all issuing certificates are assumed to be valid during the leaf certificate's validity. An issuing certificate with a validity that expires before the leaf certificate will be considered valid during verification even if the provided timestamp would mean the issuing certificate should be considered expired. ## Impact No impact to users of the public Sigstore infrastructure. This may affect private deployments with customized PKIs. In practice, this is unlikely to occur as CAs should not be issuing certificates that outlive the validity of the CA and its parents. ## Workarounds Upgrade to the latest release, or verify the certificate chain out of band. ## Example to Reproduce * Root CA certificate is valid from 12pm-2pm * Intermediate CA certificate is valid from 12:30pm-1:30pm * Leaf certificate is valid from 1pm-3pm - **Note that this is unlikely to happen in practice**, as a CA shouldn't issue a certificate that would be valid after the issuing CA certificate expires * Signature generated at 2:30pm with a signed timestamp * During verification, the leaf certificate's not before time (1pm) is used to verify the chain - 1pm is in the validity windows for the root and intermediate CA certificates * The timestamp's time is checked to be in the validity window of only the leaf certificate - 2:30pm is in the validity window for the leaf * Even though the root and intermediate would be expired at 2:30pm, verification succeeds

Source published

19 Feb 2026 22:09:12 UTC

Source modified

10 Sep 2026 03:50:36 UTC

Saved here

24 Oct 2026 13:19:41 UTC

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GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N

Publisher rating · LOW

Cosign considered signatures valid with expired intermediate certificates when transparency log verification is skipped in github.com/sigstore/cosign

Source published

23 Feb 2026 18:23:15 UTC

Source modified

16 Jun 2026 16:44:28 UTC

Saved here

24 Oct 2026 13:19:41 UTC

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OSV · upstream source license applies

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