CVE-2021-27836: 
NixOS vulnerability analysis and mitigation

A vulnerability was discovered in libxls version 1.6.2, specifically in the xls_getWorkSheet function in xls.c. The vulnerability was assigned CVE-2021-27836 and was disclosed in early 2021. This security flaw affects the libxls library, which is used for reading binary Excel files in the old OLE format (MITRE CVE, Debian Tracker).

The vulnerability stems from a NULL pointer dereference in the xlsparseWorkSheet() function when attempting to access a NULL pointer returned by xlsgetWorkSheet(). The issue occurs when either the num parameter is less than 0 or greater than pWB->sheets.count in the xls_getWorkSheet function. This condition can be triggered when processing XLS files that contain no BoundSheet8 records, causing pWB->sheets.count to remain 0 (GitHub Issue).

When exploited, this vulnerability allows attackers to cause a denial of service condition through a specially crafted XLS file. The impact primarily affects applications that use the libxls library for processing Excel files (MITRE CVE).

The vulnerability has been patched in subsequent releases. Fedora has released security updates for versions 33, 34, and 35 (libxls-1.6.2-5) to address this issue. Users are advised to update to the patched versions (Fedora Update).