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feat: add SubjectExpression CEL field to Identity for dynamic keyless signing#64

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feat: add SubjectExpression CEL field to Identity for dynamic keyless signing#64
jzeng4 wants to merge 1 commit intokyverno:mainfrom
jzeng4:juzeng/cel

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@jzeng4 jzeng4 commented Mar 13, 2026 •
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Summary

  • Add a new optional subjectExpression field to the Identity struct in ImageValidatingPolicy
  • The field accepts a CEL expression evaluated at admission time; the result is used as a regexp match against the certificate SAN URI
  • This is a backward-compatible addition: existing subject, subjectRegExp, issuer, and issuerRegExp fields are unchanged
  • Follows the message/messageExpression convention used elsewhere in Kyverno

Motivation

When verifying keyless (Fulcio/Sigstore) image signatures in multi-tenant clusters, the expected certificate SAN URI often needs to be derived from runtime context — for example, the namespace of the requesting object or the image reference itself. The static subjectRegExp field cannot express this.

Related issue

Fixes kyverno/kyverno#15398

Next PR: kyverno/kyverno#15479

Example

identities:
  - issuer: https://token.actions.githubusercontent.com
    # derive the subject regexp from the namespace at admission time
    subjectExpression: '"^https://github\\.com/my-org/" + object.metadata.namespace + "/.*"'

Test plan

  • Unit tests in pkg/imageverification/variables/attestors_test.go cover static values, subject-from-namespace, subject-from-image, and error cases
  • No changes to existing field types — API compatibility preserved

@jzeng4 jzeng4 force-pushed the juzeng/cel branch 2 times, most recently from 3faccc1 to 37ae598 Compare March 13, 2026 23:57
@jzeng4
feat: add SubjectExpression CEL field to Identity for dynamic keyless…
8129dcf 
… signing

Add a new optional SubjectExpression field to the Identity struct, which accepts a CEL expression evaluated at admission time. The result is used as a regexp match against the certificate SAN URI, enabling dynamic subject matching (e.g. based on namespace or image name) without changing existing Subject/SubjectRegExp fields.

Signed-off-by: Junyuan Zeng <jzeng04@gmail.com>
@jzeng4 jzeng4 mentioned this pull request Mar 14, 2026
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jzeng4 commented Mar 21, 2026

@eddycharly Could you take a look and merge when you get a chance? This has been approved by @fjogeleit. Thanks!

NETIZEN-11 added a commit to NETIZEN-11/kyverno that referenced this pull request Mar 29, 2026
@NETIZEN-11
feat(api): change Identity.Subject and SubjectRegExp to *StringOrExpr…
19dfbd1 
…ession

Update the Identity struct in the kyverno/api local fork so that
Subject and SubjectRegExp accept either a static string value or a
CEL expression, matching the pattern already used by cert/certChain.

- Subject string  -> Subject *StringOrExpression
- SubjectRegExp string -> SubjectRegExp *StringOrExpression
- Update DeepCopyInto to handle the new pointer fields
- Add go.mod replace directive pointing to local fork

Implements: kyverno/api#64
Related: kyverno#15398
Signed-off-by: NETIZEN_11 <niteshkumar121411@gmail.com>
NETIZEN-11 added a commit to NETIZEN-11/kyverno that referenced this pull request Mar 30, 2026
@NETIZEN-11
fix: address Copilot review comments on CEL identity expressions PR
dce7f81 
- Remove local ../kyverno-api replace directive; use jzeng4/api VCS
  pseudo-version (kyverno/api#64) so CI can resolve the dependency
- Adapt code to jzeng4/api API design: SubjectExpression string field
  instead of Subject/SubjectRegExp as *StringOrExpression
- Fix CompileAttestors: remove identity compilation using wrong env;
  identity expressions are compiled via CompileAttestorIdentities with
  the dedicated NewIdentityExprEnv() only
- Fix identPath in CompileAttestorIdentities: add missing 'cosign'
  segment so field errors point to correct path
- Remove stale 'no-op' NOTE comment from CompileAttestorIdentities
- Fix ImageKey constant: alias ImageDataKey instead of duplicating
  the string literal to avoid ambiguity
- Update all tests to use plain string fields and SubjectExpression
NETIZEN-11 added a commit to NETIZEN-11/kyverno that referenced this pull request Mar 30, 2026
@NETIZEN-11
feat(api): change Identity.Subject and SubjectRegExp to *StringOrExpr…
d697529 
…ession

Update the Identity struct in the kyverno/api local fork so that
Subject and SubjectRegExp accept either a static string value or a
CEL expression, matching the pattern already used by cert/certChain.

- Subject string  -> Subject *StringOrExpression
- SubjectRegExp string -> SubjectRegExp *StringOrExpression
- Update DeepCopyInto to handle the new pointer fields
- Add go.mod replace directive pointing to local fork

Implements: kyverno/api#64
Related: kyverno#15398
Signed-off-by: NETIZEN_11 <niteshkumar121411@gmail.com>
NETIZEN-11 added a commit to NETIZEN-11/kyverno that referenced this pull request Mar 30, 2026
@NETIZEN-11
fix: address Copilot review comments on CEL identity expressions PR
d88f88d 
- Remove local ../kyverno-api replace directive; use jzeng4/api VCS
  pseudo-version (kyverno/api#64) so CI can resolve the dependency
- Adapt code to jzeng4/api API design: SubjectExpression string field
  instead of Subject/SubjectRegExp as *StringOrExpression
- Fix CompileAttestors: remove identity compilation using wrong env;
  identity expressions are compiled via CompileAttestorIdentities with
  the dedicated NewIdentityExprEnv() only
- Fix identPath in CompileAttestorIdentities: add missing 'cosign'
  segment so field errors point to correct path
- Remove stale 'no-op' NOTE comment from CompileAttestorIdentities
- Fix ImageKey constant: alias ImageDataKey instead of duplicating
  the string literal to avoid ambiguity
- Update all tests to use plain string fields and SubjectExpression

Signed-off-by: NETIZEN_11 <niteshkumar121411@gmail.com>
NETIZEN-11 added a commit to NETIZEN-11/kyverno that referenced this pull request Mar 30, 2026
@NETIZEN-11
feat(api): change Identity.Subject and SubjectRegExp to *StringOrExpr…
f270937 
…ession

Update the Identity struct in the kyverno/api local fork so that
Subject and SubjectRegExp accept either a static string value or a
CEL expression, matching the pattern already used by cert/certChain.

- Subject string  -> Subject *StringOrExpression
- SubjectRegExp string -> SubjectRegExp *StringOrExpression
- Update DeepCopyInto to handle the new pointer fields
- Add go.mod replace directive pointing to local fork

Implements: kyverno/api#64
Related: kyverno#15398
Signed-off-by: NETIZEN_11 <niteshkumar121411@gmail.com>
NETIZEN-11 added a commit to NETIZEN-11/kyverno that referenced this pull request Mar 30, 2026
@NETIZEN-11
fix: address Copilot review comments on CEL identity expressions PR
87f5f55 
- Remove local ../kyverno-api replace directive; use jzeng4/api VCS
  pseudo-version (kyverno/api#64) so CI can resolve the dependency
- Adapt code to jzeng4/api API design: SubjectExpression string field
  instead of Subject/SubjectRegExp as *StringOrExpression
- Fix CompileAttestors: remove identity compilation using wrong env;
  identity expressions are compiled via CompileAttestorIdentities with
  the dedicated NewIdentityExprEnv() only
- Fix identPath in CompileAttestorIdentities: add missing 'cosign'
  segment so field errors point to correct path
- Remove stale 'no-op' NOTE comment from CompileAttestorIdentities
- Fix ImageKey constant: alias ImageDataKey instead of duplicating
  the string literal to avoid ambiguity
- Update all tests to use plain string fields and SubjectExpression

Signed-off-by: NETIZEN_11 <niteshkumar121411@gmail.com>
@jzeng4
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jzeng4 commented Apr 11, 2026

@fjogeleit — could you please share the plan or timeline for merging and releasing this version with the PR included?

Thank you!

@realshuting realshuting enabled auto-merge (squash) April 14, 2026 03:07
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@JimBugwadia JimBugwadia JimBugwadia approved these changes

+3 more reviewers

@lucchmielowski lucchmielowski lucchmielowski approved these changes

@fjogeleit fjogeleit fjogeleit approved these changes

@aerosouund aerosouund aerosouund approved these changes

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Feature Request: CEL Expression Support for Identity Fields in ImageValidatingPolicy

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@jzeng4 @JimBugwadia @lucchmielowski @fjogeleit @aerosouund