CVE-2025-70887: 
Python vulnerability analysis and mitigation

CVE-2025-70887 is a privilege escalation vulnerability in ralphje Signify, a Python library for Authenticode signature verification. The flaw exists in the signed_data.py and context.py components and stems from insufficient enforcement of X.509 Key Usage extension constraints during code-signing certificate validation. All versions of Signify before 0.9.2 are affected. The vulnerability was reported by researchers Hanqing Zhao and Zi-Quan You on December 15, 2025, and publicly disclosed in March 2026. It carries a CVSS v3.1 base score of 8.8 (High) (Red Hat Advisory, Feedly).

The root cause is classified as CWE-269 (Improper Privilege Management). Signify failed to verify two critical aspects of the X.509 Key Usage extension: (1) it did not require the Key Usage extension to be present in signing certificates, and (2) it did not require the extension to be marked as critical. As a result, certificates not intended for code signing — such as those issued for identity or authentication purposes — could successfully pass Authenticode signature verification. The fix, introduced in commit 64f21c0, added a strict_validation parameter (defaulting to True) to the verify() method in signed_data.py, which enforces the presence of the digital_signature key usage and the code_signing extended key usage. The context.py component was updated to support an optional_ku flag controlling whether key usage validation is skipped when the extension is absent (GitHub Issue #60, Patch Commit).

Successful exploitation allows a remote attacker with low-level privileges to pass maliciously crafted or misappropriated certificates through Signify's verification logic, effectively bypassing code-signing trust controls. This could enable an attacker to have unauthorized or improperly authorized code accepted as legitimately signed, leading to high impacts on confidentiality, integrity, and availability of systems relying on Signify for signature validation. In software supply chain contexts, this could facilitate the distribution of malicious binaries that appear to carry valid signatures (Red Hat Advisory, Feedly).

There is no public proof-of-concept exploit and no evidence of in-the-wild exploitation at this time. The EPSS score is approximately 0.052%, indicating a low probability of exploitation in the near term. The vulnerability is not listed in CISA's Known Exploited Vulnerabilities (KEV) catalog. Exploitation requires low privileges and no user interaction, but an attacker would need to control or obtain a certificate that lacks proper Key Usage constraints and use it in a context where Signify performs signature verification (Feedly).

1. Obtain a non-code-signing certificate: Acquire or generate an X.509 certificate that lacks the Key Usage extension or does not have it marked as critical, and which is not explicitly scoped for code signing (e.g., an identity or authentication certificate from a trusted CA).
2. Sign a binary: Use the obtained certificate to sign a PE binary or other Authenticode-compatible file using a signing tool.
3. Submit to a vulnerable Signify instance: Present the signed binary to a system or application using Signify (versions before 0.9.2) for signature verification.
4. Bypass verification: Because Signify does not enforce the presence or criticality of the Key Usage extension in pre-0.9.2 versions, the verification succeeds despite the certificate not being authorized for code signing.
5. Achieve objective: The attacker's binary is accepted as legitimately signed, potentially enabling execution in environments that enforce code-signing policies (GitHub Issue #60, Patch Commit).

Upgrade ralphje Signify to version 0.9.2 or later, which enforces strict Key Usage and Extended Key Usage validation by default via the new strict_validation=True parameter in the verify() method (Patch Commit). For systems that cannot be immediately patched, restrict network access to services relying on Signify for signature validation and audit certificates being accepted by the verifier. Note that a related issue was also fixed in osslsigncode 2.11, which added keyUsage digitalSignature validation for signer certificates (osslsigncode 2.11 Release).

The vulnerability was discovered and reported by academic researchers Hanqing Zhao and Zi-Quan You as part of an empirical study on the code-signing ecosystem, and they simultaneously reported the same class of weakness to the osslsigncode project (GitHub Issue #60, osslsigncode Issue #475). The osslsigncode maintainer credited the researchers in the 2.11 release notes. Red Hat tracked the CVE in their security advisory system (Red Hat Advisory).