Quick answers
- Is CVE-2026-79591 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-79591 being exploited in the wild.
- How severe is CVE-2026-79591?
- High severity, with a CVSS score of 7.8, published 10 September 2026.
- Which products and versions are affected?
- Libxls 1.6.3.
- Is there a fix for CVE-2026-79591?
- A patch was published on 2026-08-21: Fix merged in PR #164 validating font indices when generating CSS Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is CVE-2026-79591 attacked?
- It is attacked over the network, in a software library.
- Am I affected by CVE-2026-79591?
- Check whether you run Libxls at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
Technical details
A heap-buffer-overflow and use-after-free vulnerability exists in the xls_getCSS() function due to insufficient validation of a file-controlled font index. The xf->font field, copied directly from the BIFF XF record without bounds checking, is used as an index into the font array. An out-of-range or zero value can cause out-of-bounds reads, including underflow when xf->font is 0. The vulnerability is triggered via the public API chain xls_open_buffer() → xls_summaryInfo() → xls_getCSS() → xls_parseWorkSheet() when processing a malicious XLS file. An attacker can achieve information disclosure or denial of service. A patch validating font indices was merged on 2026-08-21.
Affected products
- libxls libxls 1.6.3
Timeline
- 2026-09-10: disclosed
- 2026-08-21: patched: Fix merged in PR #164 validating font indices when generating CSS