EUVD-2026-64572
GHSA-gcm2-cr9p-5jc7
Severity by source
AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Local device access via tty group sets AV:L/PR:L; OOB read supports C:H and A:H but I:H is undemonstrated by the described mechanism.
Primary rating from Vendor (Linux).
CVSS VectorVendor: Linux
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Lifecycle Timeline
5DescriptionCVE.org
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.
AnalysisAI
Out-of-bounds memory read in the Linux kernel's fbdev bitblit subsystem allows a local attacker with tty group write access to /dev/vcsa to crash the kernel or disclose kernel memory by supplying an out-of-range glyph index through the virtual console screen interface. The root cause is a missing bounds check in bit_cursor() in drivers/video/fbdev/core/bitblit.c - a sibling function to bit_putcs_aligned() and bit_putcs_unaligned(), which already received the same clamp in commit 18c4ef4e765a. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Two concrete conditions must both hold for exploitation: (1) The Linux framebuffer console (fbcon) must be active - the vulnerable code in drivers/video/fbdev/core/bitblit.c is only exercised when a framebuffer-based display driver is the active console, not in VGA text mode or on headless systems. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The NVD CVSS 7.8 vector (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H) rates this as a high-severity local privilege escalation candidate, but the I:H component is not directly demonstrated by the described mechanism, which is a kernel OOB read. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | A local user with tty group membership opens /dev/vcsa and uses writev() to write a crafted 16-bit screen buffer entry containing a glyph index larger than the current font's charcount. When the kernel's framebuffer console next renders the cursor at that screen position, bit_cursor() reads the crafted index, fails to clamp it, and computes a pointer far past the end of the font data array; the subsequent 16-byte read in soft_cursor() triggers a KASAN global-out-of-bounds read, crashing the kernel or exposing adjacent kernel memory. … |
| Remediation | Upgrade to the nearest patched stable kernel version: 6.6.152, 5.10.265, 6.1.183, 6.12.104, 5.15.216, 6.18.45, 7.1.9, or 7.2 depending on the branch in use. … Detailed patch versions, workarounds, and compensating controls in full report. |
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Within 24 hours, identify all Linux systems in the environment and determine which are running affected kernel versions prior to the patched release. …
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External POC / Exploit Code
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