# CVE-2026-39984: Sigstore Timestamp Authority has Improper Certificate Validation in verifier > Attackers can bypass timestamp authority validation by forging certificates in the PKCS#7 bag, affecting Go applications using the verification package. - **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Local or same network only - **Victim action needed:** A user has to click or open something - **Fix available:** Yes - **Published:** 14 Apr 2026 - **Updated:** 16 Apr 2026 - **Also known as:** CLEANSTART-2026-DK67034, CLEANSTART-2026-KJ61978, CLEANSTART-2026-LI04504, CLEANSTART-2026-NS94125, CLEANSTART-2026-SK27128, DEBIAN-CVE-2026-39984, GHSA-xm5m-wgh2-rrg3, GO-2026-5763, MINI-2384-758m-p59f, MINI-24qg-xp7x-5ppc, MINI-2m69-gwj8-8gj5, MINI-2mqw-63cg-rqf9, MINI-2qhv-5h89-rg2m, MINI-3j3p-w7hf-cp54, MINI-3ppw-rwhx-wrc7, MINI-46gj-8wjv-2p72, MINI-4xpc-367p-j2x5, MINI-54rc-jxc7-f375, MINI-694q-wq88-78qj, MINI-6xr2-8463-3wrx, MINI-78jh-m5r5-2wj2, MINI-799p-7545-q7p5, MINI-7g28-6qff-7jm6, MINI-7g5c-w83j-c74c, MINI-7rhw-fx98-7r3v, MINI-7vcc-q36m-h957, MINI-85hx-w2h2-297f, MINI-89hr-v6f6-3954, MINI-8g5j-gcr8-9c2w, MINI-9gmr-7439-jj4f, MINI-9jcr-h76w-jf5h, MINI-9v94-wp2j-7r4g, MINI-cj2p-3qhp-95x3, MINI-hm85-47j3-f394, MINI-j8x2-ghrv-6xq7, MINI-jm99-79vh-9vg8, MINI-jxvh-frv7-gqpf, MINI-p2pp-cm64-m257, MINI-pc63-fhqm-mvrf, MINI-pxwv-hf2v-76g7, MINI-qmpv-rgvw-mf3q, MINI-rq83-cp6x-9vf2, MINI-v2fh-2h8g-c3w6, MINI-v2h5-h4mx-hjcj, MINI-x5jp-w42f-6mf9, MINI-x85v-x273-q54x, UBUNTU-CVE-2026-39984 ## What is CVE-2026-39984? CVE-2026-39984 is a medium-severity authentication bypass vulnerability in sigstore timestamp-authority, published on 14 Apr 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link. ### Official description Sigstore Timestamp Authority is a service for issuing RFC 3161 timestamps. Versions 2.0.5 and below contain an authorization bypass vulnerability in the VerifyTimestampResponse function. VerifyTimestampResponse correctly verifies the certificate chain signature, but the TSA-specific constraint checks in VerifyLeafCert uses the first non-CA certificate from the PKCS#7 certificate bag instead of the leaf certificate from the verified chain. An attacker can exploit this by prepending a forged certificate to the certificate bag while the message is signed with an authorized key, causing the library to validate the signature against one certificate but perform authorization checks against another. This vulnerability only affects users of the timestamp-authority/v2/pkg/verification package and does not affect the timestamp-authority service itself or sigstore-go. The issue has been fixed in version 2.0.6. Weakness: CWE-295 (Improper Certificate Validation) ## Which versions are affected by CVE-2026-39984? 4 open source packages ship an affected version, including github.com/sigstore/timestamp-authority/v2 and github.com/sigstore/timestamp-authority. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `github.com/sigstore/timestamp-authority/v2` | Go | All versions | 2.0.6 | | `github.com/sigstore/timestamp-authority/v2` | Go | All versions | 2.0.6 | | `github.com/sigstore/timestamp-authority/v2` | Go | All versions | 2.0.6 | | `github.com/sigstore/timestamp-authority` | Go | All versions | None | Products: Linuxfoundation Sigstore Timestamp Authority. ## How do I fix CVE-2026-39984? Upgrade github.com/sigstore/timestamp-authority/v2 to 2.0.6 or later. ## Is CVE-2026-39984 being exploited? There is no known exploitation of CVE-2026-39984 in the wild and no public exploit code. ## Other vulnerabilities in sigstore timestamp-authority - [CVE-2025-66564](https://insights.sbomify.com/CVE-2025-66564/): Sigstore Timestamp Authority allocates excessive memory during request parsing (high 7.5) - [CVE-2026-49835](https://insights.sbomify.com/CVE-2026-49835/): Sigstore Timestamp Authority: OOM due to unbounded metric label cardinality (medium 5.9) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-39984 and fix it if it is affected. What it is: CVE-2026-39984 is a medium-severity authentication bypass vulnerability in sigstore timestamp-authority, published on 14 Apr 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link. Exploitation: There is no known exploitation of CVE-2026-39984 in the wild and no public exploit code. Affected packages and fixed versions: - github.com/sigstore/timestamp-authority/v2 (Go): affected all versions; fixed in 2.0.6 - github.com/sigstore/timestamp-authority/v2 (Go): affected all versions; fixed in 2.0.6 - github.com/sigstore/timestamp-authority/v2 (Go): affected all versions; fixed in 2.0.6 - github.com/sigstore/timestamp-authority (Go): affected all versions; no fix listed Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-39984/ (Markdown: https://insights.sbomify.com/CVE-2026-39984/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39984 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39984 - OSV: https://osv.dev/vulnerability/CVE-2026-39984 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-39984/ Summaries are generated and may contain mistakes; the official description above is the record.