Spreadsheet::ParseExcel: code execution flaw under attack

What is affected

Spreadsheet::ParseExcel is mostly run by companies, schools and public bodies, not on home computers. You are unlikely to have it yourself, but organisations that hold your data may. Breaches that start with flaws like this one are how personal data ends up leaked.

Spreadsheet::ParseExcel contains a remote code execution vulnerability due to passing unvalidated input from a file into a string-type “eval”. Specifically, the issue stems from the evaluation of Number format strings within the Excel parsing logic.

What an attacker can do

This is a code execution flaw. It lets an attacker run their own commands or programs on the affected system. In practice that usually means taking it over: installing malware, stealing data or using it to reach other devices on the same network.

How it is reached

Needs access to the device or a file opened on it

Access the attacker needs

No account needed

Does the victim have to do something?

Yes, the victim must open or click something

What to do

1. At home: nothing to install. Keep your own devices updated and use a different password for every service, so that a breach at one organisation does not open your other accounts.
2. If you administer Spreadsheet::ParseExcel at work: apply the vendor's fix or mitigation now and review logs for signs of compromise, because the flaw was being exploited before it was listed.
3. If the product is past its support date and no fix exists, take it off the internet or retire it.

Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.

Vendor advisories and fixes

• metacpan.orgmetacpan.org/dist/Spreadsheet-ParseExcel
• barracuda.comwww.barracuda.com/company/legal/esg-vulnerability

How urgent is it

CISA added this vulnerability to its Known Exploited Vulnerabilities catalogue on January 2, 2024, which means there is reliable evidence of attacks in the wild. US federal agencies must fix it by January 23, 2024, a deadline that has already passed. That deadline does not bind anyone else, but it shows how seriously the agency rates it.

The EPSS model estimates a 19% probability that this flaw will be exploited somewhere in the next 30 days. That is higher than 97% of all scored vulnerabilities.

Its CVSS severity score is 7.8 out of 10 (high), as recorded in the US National Vulnerability Database.

Technical description

Spreadsheet::ParseExcel version 0.65 is a Perl module used for parsing Excel files. Spreadsheet::ParseExcel is vulnerable to an arbitrary code execution (ACE) vulnerability due to passing unvalidated input from a file into a string-type “eval”. Specifically, the issue stems from the evaluation of Number format strings (not to be confused with printf-style format strings) within the Excel parsing logic.