CVE-2026-39984 is a medium-severity security vulnerability in github.com/sigstore/timestamp-authority/v2 (go), affecting versions <= 2.0.5. It is fixed in 2.0.6.
Authorization bypass via certificate bag manipulation in sigstore/timestamp-authority verifier An authorization bypass vulnerability exists in sigstore/timestamp-authority verifier (timestamp-authority/v2/pkg/verification): VerifyTimestampResponse function correctly verifies the certificate chain but when the TSA specific constraints are verified in VerifyLeafCert, the first non-CA certificate from the PKCS#7 certificate bag is used instead of the leaf certificate from the certificate chain. An attacker can exploit this by prepending a forged certificate to the certificate bag while the message is signed with an authorized key. The library validates the signature using the one certificate but performs authorization checks on the another, allowing an attacker to bypass some authorization controls. This vulnerability does not apply to timestamp-authority service, only to users of timestamp-authority/v2/pkg/verification package. This vulnerability does not apply to sigstore-go even though it is a user of timestamp-authority/v2/pkg/verification: Providing TSACertificate option to VerifyTimestampResponse fully mitigates the issue. Patches The issue will be fixed in timestamp-authority 2.0.6 Workarounds Users of VerifyTimestampResponse can use the TSACertificate option to specify the exact certificate they expect to be used: this fully mitigates the issue. References This issue was found after reading CVE-2026-33753 / GHSA-3xxc-pwj6-jgrj (originally reported by @Jaynornj and @Pr00fOf3xpl0it)
CVE-2026-39984 has a CVSS score of 5.5 (Medium). The vector is requires local access, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment.
A fixed version is available (2.0.6). Upgrading removes the vulnerable code path.
go
github.com/sigstore/timestamp-authority/v2 (<= 2.0.5)github.com/sigstore/timestamp-authority/v2 → 2.0.6 (go)Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Already deployed Kodem? See CVE-2026-39984 in your environment →Upgrade github.com/sigstore/timestamp-authority/v2 to 2.0.6 or later to resolve this vulnerability.
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CVE-2026-39984 is a medium-severity security vulnerability in github.com/sigstore/timestamp-authority/v2 (go), affecting versions <= 2.0.5. It is fixed in 2.0.6.
CVE-2026-39984 has a CVSS score of 5.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
github.com/sigstore/timestamp-authority/v2 (go) versions <= 2.0.5 is affected.
Yes. CVE-2026-39984 is fixed in 2.0.6. Upgrade to this version or later.
Whether CVE-2026-39984 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
Upgrade github.com/sigstore/timestamp-authority/v2 to 2.0.6 or later.