Does a signed software update prove the software was safe?¶

No. A valid digital signature does not automatically prove that a software update was safe or unaltered in every meaningful sense.

Evidential caution: that signed software cannot be malicious.

What this means¶

A signed update may still be dangerous where:

the vendor’s build system was compromised;

a signing key was stolen;

malicious code entered before signing;

the update channel was abused;

the certificate was fraudulently issued;

the signed package included an unsafe component.

which certificate signed the file;

whether the signature was valid at the relevant time;

who controlled the signing process;

how the package was built;

where it was downloaded from;

whether the vendor confirmed the release;

what code executed;

what systems received it.

A signature may prove that a particular key signed the file.

Treat code signing as evidence about the signing key and package, not as a guarantee of safety, authorship or benign intent.

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What to check or do next¶

• Investigators should establish:
• Preserve the original file, signature details, certificate chain, source URL, update logs, hashes and vendor communications.

Evidential limits¶

It does not prove who used the key or whether the software was trustworthy.

Likewise, an invalid signature does not automatically prove malicious alteration; packaging or timestamp issues may provide another explanation.

The signature may also remain cryptographically valid after the certificate is revoked or the signing process is later found to be compromised. Investigators should preserve validation time, timestamping information and certificate status at the incident date. Current certificate status alone may not explain what systems trusted when the package was installed. The operational takeaway is: