PatchSiren

PatchSiren cyber security CVE debrief

CVE-2026-45999 Linux CVE debrief

A vulnerability in the Linux kernel's EROFS (Enhanced Read-Only File System) implementation could allow an attacker with a crafted filesystem image to trigger an out-of-bounds read during LZ4 decompression. The issue stems from an unsigned integer underflow in `z_erofs_lz4_handle_overlap()` when processing malformed extents that specify `m_llen < m_plen` without the `partial_decoding` flag. This causes the inplace decompression path to be incorrectly selected, where the calculation `outpages - inpages` wraps to a large unsigned value, leading to reads past the `decompressed_pages` array boundary. The vulnerability was resolved by adding an additional validation check to prevent this code path from being reached with illegal extent parameters.

Vendor
Linux
Product
Unknown
CVSS
Unknown
CISA KEV
Not listed in stored evidence
Original CVE published
2026-05-27
Original CVE updated
2026-05-27
Advisory published
2026-05-27
Advisory updated
2026-05-27

Who should care

Linux system administrators, kernel maintainers, and security teams managing systems that may mount untrusted or external EROFS filesystem images. Organizations using container or embedded Linux deployments with EROFS support should prioritize patching.

Technical summary

The EROFS filesystem driver in the Linux kernel contains an unsigned integer underflow vulnerability in the LZ4 decompression handler. When processing crafted filesystem images with malformed extent headers (specifically `!partial_decoding && m_llen < m_plen`), the code incorrectly enters an inplace decompression path that cannot handle cases where output pages are fewer than input pages. The subtraction `outpages - inpages` underflows to a large value, causing subsequent array accesses to read beyond allocated memory. The fix adds proper validation to prevent reaching this vulnerable code path with illegal extent parameters.

Defensive priority

medium

Recommended defensive actions

  • Apply kernel updates containing the referenced stable tree commits when available from your Linux distribution
  • Restrict mounting of untrusted EROFS filesystem images
  • Consider disabling EROFS support if not required in your environment
  • Monitor for kernel security advisories from your distribution regarding this CVE

Evidence notes

The vulnerability description indicates that crafted EROFS images with illegal extent parameters can trigger the vulnerable code path. A reproducible test image was provided in the original disclosure (base64-encoded gzipped blob). The fix adds validation to prevent the inplace decompression path from being taken when `outpages < inpages` would occur.

Official resources

2026-05-27