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CVE-2026-74635
fbdev: bitblit: bound-check glyph index in bit_cursor()
Published 2026-08-22 (2 months ago) by the Linux kernel CNA. Subsystem drivers/video. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-74635 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixedDLA-4777-1 · 2026-09-08 · text | 6.1.187-1 | Update the kernel and reboot. DLA-4777-1 shipped the fix on 2026-09-08, in 6.1.187-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6466-1 · 2026-08-25 · text | 6.12.105-1 | Update the kernel and reboot. DSA-6466-1 shipped the fix on 2026-08-25, in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4788-1 · 2026-09-19 · text | 6.12.107-1~deb12u1 | Update the kernel and reboot. DLA-4788-1 shipped the fix on 2026-09-19, in 6.12.107-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
drivers/video/fbdev/core/bitblit.c
- What this is
- FRAMEBUFFER CONSOLEMAINTAINERS section matched on drivers/video/fbdev/core/bitblit.c · dri-devel@lists.freedesktop.org · Maintained
Check it on the machine
grep -wE 'CONFIG_FB|CONFIG_FB_CORE|CONFIG_FRAMEBUFFER_CONSOLE' /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian builds this, but not on every flavour. At least one release below builds the code on some of its kernel flavours and not on others, so the answer depends on the architecture and flavour installed.
- bookwormy
- trixie, forky, sidYnot built on s390x/s390x
- bookworm · linux-6.12y
- y
- built into the kernel image on every flavour - only a patch removes it
- Y
- built in where it is built at all; some flavours do not build it
- Build switches
- CONFIG_FB, CONFIG_FB_CORE, CONFIG_FRAMEBUFFER_CONSOLE
- Reads as
- all on means the file is compiled; any one off means it is not.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-08-22 (2 months ago)source
- CVSS
- 7.8 high
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.13%, higher than 2.3% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- drivers/video
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.10.247 | 5.10.265 | 250159ace2dc |
| 5.15.197 | 5.15.216 | 46336f476484 |
| 6.1.159 | 6.1.183 | bf750cfeacf4 |
| 6.6.117 | 6.6.152 | 94134d70abf9 |
| 6.12.58 | 6.12.104 | c1e7351767dd |
| 6.18 | 6.18.45 | bc9db0d879c6 |
| 6.18 | 7.1.9 | 9ea879862e66 |
| 6.18 | 7.2 | e033cbf3975a |
| 5.4.302 | 0 | 0 |
| 6.17.8 | 0 | 0 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4788-1 | 2026-09-19 | bookworm 6.12.107-1~deb12u1 | tracker |
| DLA-4777-1 | 2026-09-08 | bookworm 6.1.187-1 | tracker |
| DSA-6466-1 | 2026-08-25 | trixie 6.12.105-1 | tracker |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:LAttack vector, local
- Exploitation requires local write() to /dev/vcsa (or ioctl on an owned virtual console) to poison the VT screen buffer; the bug is in fbcon cursor rendering, not in any network-facing protocol handler.
- AC:LAttack complexity, low
- Syzkaller reproduces the fault reliably by writing arbitrary screen-buffer values to /dev/vcsa; the attacker fully controls the glyph index and can repeat the trigger on every console refresh without races or victim-specific timing.
- PR:LPrivileges required, low
- Triggering needs only a local account with write access to /dev/vcsa* (typically membership in the tty group) or a session owning the target console for KDFONTOP/font-switch paths; no CAP_SYS_ADMIN or init-namespace root is required.
- UI:NUser interaction, none
- Once the attacker has the needed local device access, exploitation is fully self-contained and does not require any additional action from another user or administrator.
- S:UScope, unchanged
- Impact is confined to kernel framebuffer-console memory within the same kernel security domain; it does not cross VM, container, or IOMMU boundaries to affect a separate authority.
- C:HConfidentiality impact, high
- An out-of-bounds read past the font glyph table copies adjacent kernel memory (global or heap font data) into cursor buffers and framebuffer blit paths, disclosing more than a few bytes per trigger.
- I:HIntegrity impact, high
- The out-of-bounds glyph pointer feeds update_attr() and soft_cursor() memcpy paths that copy attacker-influenced kernel memory into kmalloc cursor buffers and framebuffer state, constituting kernel memory corruption with integrity impact.
- A:HAvailability impact, high
- KASAN reports a kernel BUG on the out-of-bounds read in soft_cursor(), and repeated malicious /dev/vcsa writes force console refresh through hide_cursor(), enabling kernel oops/panic and console denial of service.