Tekton Pipelines has VerificationPolicy regex pattern bypass via substring matching

hey guys,

triage contract
this is a first-screen summary; deterministic proof is in the proof bundle (canonical.log/control.log/witness.txt).

summary
trusted resources verification policies match a resource source string (refSource.URI) against spec.resources[].pattern using regexp.MatchString. in go, regexp.MatchString reports a match if the pattern matches anywhere in the string, so common unanchored patterns (including examples in tekton documentation) can be bypassed by attacker-controlled source strings that contain the trusted pattern as a substring. this can cause an unintended policy match and change which verification mode/keys apply.

pins
- repo: https://github.com/tektoncd/pipeline
- commit: 0133513db03dadb3cb08801d6b0330badcb63830
- callsite: pkg/trustedresources/verify.go:118-137 (getMatchedPolicies)

severity
MEDIUM (provisional CVSS 5.3–6.5) (signing request tampering)

repro (canonical)
- command: unzip -q -o poc.zip -d poc && cd poc/poc-F-TEKTON-REGEX-001 && make canonical
- expected: cap not reached; canonical does not emit the vulnerability markers.
- actual: cap reached; canonical emits the vulnerability markers.
- canonical markers (mandatory): [CALLSITE_HIT] + [PROOF_MARKER]

negative control
- command: unzip -q -o poc.zip -d poc && cd poc/poc-F-TEKTON-REGEX-001 && make control
- expected: cap not reached under the same harness; control emits the control marker and does not emit the vulnerability markers.
- control markers (mandatory): [CALLSITE_HIT] + [NC_MARKER]

fix
consider making matching safe-by-default by requiring full-string matches (or validating patterns and documenting substring semantics clearly). one option is to anchor patterns before matching (e.g., wrap pattern as ^(?:pattern)$ when not already anchored), or to provide a separate field for exact match vs regex match.
fix accepted when: under the same harness, canonical still hits [CALLSITE_HIT] but does not emit [PROOF_MARKER].

proof bundle pointers
- bundle: poc.zip
- bundle convention: zip extracts under a single top-level folder (poc-F-TEKTON-REGEX-001/) to avoid collisions
- contains: canonical.log, control.log, witness.txt
- extracted paths: after extraction, see ./poc/poc-F-TEKTON-REGEX-001/canonical.log, ./poc/poc-F-TEKTON-REGEX-001/control.log, ./poc/poc-F-TEKTON-REGEX-001/witness.txt
- verify: compare shasum -a 256 for canonical.log/control.log/fix.patch/test source against witness.txt
- supported-mode note: if your supported integration uses verified https app-links/universal links only, provide the supported tag/branch and we can retest on that pin.

poc.zip

impact
an attacker can craft a trusted resources source string that embeds a trusted substring and still matches an unanchored verificationpolicy spec.resources[].pattern, even if the policy is intended to constrain matches to a specific trusted source. this occurs because regexp.MatchString succeeds on substring matches, so patterns like https://github.com/tektoncd/catalog.git match attacker-controlled sources such as https://evil.com/?x=https://github.com/tektoncd/catalog.git.

affected: deployments using trusted resources verification with unanchored verificationpolicy patterns, where an attacker can influence the refSource.URI value used for policy matching.

not affected: deployments that anchor all patterns (^...$) or otherwise enforce full-string matching; deployments where attackers cannot influence refSource.URI.

steps to reproduce

unzip -q -o poc.zip -d /tmp/poc-tekton-regex-001
cd /tmp/poc-tekton-regex-001/poc-F-TEKTON-REGEX-001
bash ./run.sh canonical | tee /tmp/tekton-regex-001-canonical.log
bash ./run.sh control | tee /tmp/tekton-regex-001-control.log
grep -n '\\[PROOF_MARKER\\]' /tmp/tekton-regex-001-canonical.log && grep -n '\\[NC_MARKER\\]' /tmp/tekton-regex-001-control.log && ! grep -n '\\[PROOF_MARKER\\]' /tmp/tekton-regex-001-control.log

suggested patch options:
- make matching safe-by-default by anchoring patterns before matching (or by validating and rejecting unanchored patterns).
- document the substring semantics explicitly and update documentation examples to include anchors.

workarounds
anchor verificationpolicy resource patterns so they must match the full source string. example:

• ^https://github.com/tektoncd/catalog\\.git$

best,
oleh