CVE-2026-39984
Description
A flaw was found in timestamp-authority, specifically in the timestamp-authority/v2/pkg/verification package. An attacker can exploit this issue by prepending a forged certificate to the certificate bag while the message is signed with an authorized key. This causes the library to validate the signature against one certificate but perform authorization checks against another, leading to an authorization bypass.
Statement
This vulnerability allows an attacker to bypass authorization checks by prepending a forged certificate to the certificate bag. The library validates the signature against one certificate while performing authorization checks against another.
Additionally, a user or an automated system must actively run the VerifyTimestampResponse function against the attacker-supplied payload. Due to this reason, this flaw has been rated with a moderate severity.
This issue only impacts users of the timestamp-authority/v2/pkg/verification package and does not affect the timestamp-authority service or sigstore-go.
Mitigation
To mitigate this vulnerability, users of VerifyTimestampResponse can use the TSACertificate option to explicitly specify the exact certificate they expect to be used.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 5.5 | N/A | 5.5 |
| Attack Vector | Local | N/A | Local |
| Attack Complexity | Low | N/A | Low |
| Privileges Required | None | N/A | None |
| User Interaction | Required | N/A | Required |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | None | N/A | None |
| Integrity Impact | High | N/A | High |
| Availability Impact | None | N/A | None |
Vector
Red Hat: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
cve.org: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
Understanding the Weakness (CWE)
Integrity,Authentication
Technical Impact: Bypass Protection Mechanism; Gain Privileges or Assume Identity
When a certificate is invalid or malicious, it might allow an attacker to spoof a trusted entity by interfering in the communication path between the host and client. The product might connect to a malicious host while believing it is a trusted host, or the product might be deceived into accepting spoofed data that appears to originate from a trusted host.
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