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Cybersecurity · head to head

Cosign vs Greasemonkey

Cosign logo

Cosign

Cybersecurity

Signs and verifies container images and artifacts, with or without managing keys

From
Free
Rated
-
Full reviewWebsite
Greasemonkey logo

Greasemonkey

Browser Extensions

The original userscript manager, for Firefox

From
Free
Rated
-
Full reviewWebsite

The short version

  • Each has a real cost: Cosign keyless signing inherits every weakness of the identity provider behind it. Sigstore’s own threat model states that if an identity provider is compromised, Sigstore will issue certificates to those identities, so a compromised account produces perfectly valid signatures.; Greasemonkey firefox only. There is no Chrome, Edge or Safari build and there never has been.
  • They diverge on capability: Cosign covers Keyless signing, Greasemonkey covers Userscript execution.
  • Prices and features above were last checked on 17 September 2026.

Where they differ

Only the attributes on which Cosign and Greasemonkey actually diverge.

Attributes where Cosign and Greasemonkey differ
AttributeCosignGreasemonkey
Pricing modelOpen source, no licence feeopen-source
PlatformsmacOS, Linux, Windows, DockerFirefox
CategoryCybersecurityBrowser Extensions

Identical on both: starting price (Free), free tier (Yes), user rating (Not yet rated).

What each one covers

Drawn from each product's published feature list. An absence here means we hold no record of it - not that the product lacks it.

Only in Cosign

  • Keyless signing
  • Key and KMS signing
  • Registry-native storage
  • In-toto attestations
  • Offline verification
  • Trusted root and signing config

Only in Greasemonkey

  • Userscript execution
  • Built-in script editor
  • GM. API
  • @resource support
  • Script localisation
  • Firefox for Android

What people use each for

The jobs each tool is most often brought in to do.

Cosign

  • Signing container images in a build pipeline without managing long-lived private keysnot Greasemonkey
  • Attaching a signed bill of materials to a release so consumers can verify its provenancenot Greasemonkey
  • Meeting a customer or regulatory requirement for signed artifactsnot Greasemonkey
  • Verifying third-party images before they enter an internal registrynot Greasemonkey

Greasemonkey

  • Restyling or de-cluttering a site you use dailynot Cosign
  • Automating a repetitive action on a web applicationnot Cosign
  • Adding a missing feature to a site you do not controlnot Cosign
  • Running userscripts on Firefox for Androidnot Cosign
  • Writing and testing your own scripts in the browsernot Cosign

Where each one falls short

Documented limitations, not opinions. Every one is a constraint you would hit in normal use.

Cosign

  • Keyless signing inherits every weakness of the identity provider behind it. Sigstore’s own threat model states that if an identity provider is compromised, Sigstore will issue certificates to those identities, so a compromised account produces perfectly valid signatures.
  • A signature proves who signed, never whether they should have. The documentation is explicit that Sigstore cannot determine authorisation, so every consumer must write and maintain their own identity and issuer policy or verification means nothing.
  • Nothing is enforced without an admission controller. Signing changes what you can prove, not what runs, and the official policy controller has a small maintainer base for a component sitting in a cluster admission path.
  • Upgrades break pipelines. Version 3 changed defaults, version 4 is announced as removing legacy functionality and roughly half the command line flags, and two official client libraries still lacked support for the new log format as of mid 2026.
  • Signatures do not expire. An artifact signed before a maintainer account was compromised and one signed after are indistinguishable unless somebody is actively monitoring the transparency log, and almost nobody is.

Greasemonkey

  • Firefox only. There is no Chrome, Edge or Safari build and there never has been.
  • Its API diverged from the Tampermonkey conventions most shared scripts are written against, so a script from a sharing site may need adjusting to run.
  • The dashboard is plainer than Tampermonkey's, with no diff view when a script updates itself.
  • No cloud sync of the kind Tampermonkey and Violentmonkey offer; moving machines means exporting.
  • A userscript manager can read and rewrite every page you visit, so the security of the arrangement depends entirely on the scripts you choose to install.

Pricing, plan by plan

Cosign

Free
  • CosignFree
    • Apache-2.0
    • Public Sigstore infrastructure free to use
    • No usage limits published

Greasemonkey

Free
  • Open sourceFree
    • Unlimited scripts
    • Built-in editor
    • No account required

Which should you pick?

Choose Cosign if

  • You need keyless signing.
  • You want to start without paying.
  • You work on macOS, Linux, Windows, Docker.
  • You also want key and kms signing.
Cosign review

Choose Greasemonkey if

  • You need userscript execution.
  • You want to start without paying.
  • You work on Firefox.
  • You also want built-in script editor.
Greasemonkey review

Questions people ask

Is Cosign or Greasemonkey better?
Neither clearly leads. Cosign starts at Free and Greasemonkey at Free, and user ratings are close enough to be indistinguishable. Choose on capability and platform support.
Which is cheaper, Cosign or Greasemonkey?
Cosign starts at Free and Greasemonkey at Free.
Does Cosign or Greasemonkey run on more platforms?
Cosign runs on macOS, Linux, Windows, Docker. Greasemonkey runs on Firefox.
Can I use Cosign for free?
Both have a free tier, so you can try either at no cost before committing.
What is Cosign best used for?
Cosign is most often used for signing container images in a build pipeline without managing long-lived private keys, attaching a signed bill of materials to a release so consumers can verify its provenance, meeting a customer or regulatory requirement for signed artifacts, verifying third-party images before they enter an internal registry. Of those, signing container images in a build pipeline without managing long-lived private keys and attaching a signed bill of materials to a release so consumers can verify its provenance are not what Greasemonkey is typically brought in for.
What can Cosign do that Greasemonkey cannot?
Cosign covers Keyless signing, Key and KMS signing, Registry-native storage, In-toto attestations. Greasemonkey covers Userscript execution, Built-in script editor, GM. API, @resource support.

Answered from the vendors’ own pages

Cosign: Does Cosign tell me if an image is vulnerable?

No. It has no vulnerability knowledge whatsoever. It can carry an SBOM as a signed attestation but never reads it. Pair it with a scanner.

Greasemonkey: Is Greasemonkey still maintained?

Yes. Version 4.14 was released on 2 June 2026, and the project has an active repository, bug tracker and discussion groups.

Source
Cosign: Is signing alone enough?

No. Verification is a command somebody runs. Without an admission controller enforcing it, an unsigned image still runs.

Greasemonkey: Does Greasemonkey work on Chrome?

No. It is a Firefox extension and has never shipped for Chrome. Violentmonkey and Tampermonkey are the equivalents there.

Source
Cosign: What does a bare cosign verify actually prove?

Very little. Without a pinned certificate identity and OIDC issuer, it accepts a valid signature from any identity at all.

Greasemonkey: Will Tampermonkey scripts run in Greasemonkey?

Often, but not always. Greasemonkey's API diverged from the conventions most shared scripts are written against, so a script may need its header or its GM calls adjusted.

Source
Cosign: What is the risk of keyless signing?

Your OIDC provider becomes the root of trust. Compromise of that account yields genuine, verifiable signatures, so account security is the control that matters.

Greasemonkey: Does Greasemonkey run on mobile?

On Firefox for Android, from version 4.12. There is no iOS build, because Firefox on iOS cannot load extensions of this kind.

Source
Cosign: Should we expect breaking changes?

Yes. Version 4 is announced to remove roughly half the flags, and a post-quantum migration is named as a further breaking change after that.

Greasemonkey: What is the GM API?

The set of functions a userscript manager exposes to scripts, named after Greasemonkey, which defined it. Greasemonkey supports the promise-based GM. form including GM.xmlHttpRequest and GM.registerMenuCommand.

Source
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