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CVE-2026-74635
fbdev: bitblit: bound-check glyph index in bit_cursor()
Overview
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.

Details
INFO

Published Date :

Aug. 22, 2026, 4:16 p.m.

Last Modified :

Aug. 22, 2026, 4:16 p.m.

Remotely Exploit :

No

Source :

416baaa9-dc9f-4396-8d5f-8c081fb06d67
Impact
Affected Products

The following products are affected by CVE-2026-74635 vulnerability. Even if cvefeed.io is aware of the exact versions of the products that are affected, the information is not represented in the table below.

ID Vendor Product Action
1 Linux linux_kernel
: Total Affected Vendor : 1 | Products : 1
Solution
Apply bounds checking to the glyph index in bit_cursor() to prevent out-of-bounds reads.
  • Clamp the glyph index to vc_font.charcount in bit_cursor().
  • Ensure charcount is maintained under console_lock.
  • Clear hi_font_mask when switching fonts.
References
References to Advisories, Solutions, and Tools

Here, you will find a curated list of external links that provide in-depth information, practical solutions, and valuable tools related to CVE-2026-74635.

URL Resource
https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700
https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee
https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091
https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8
https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c
https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576
https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3
https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d
CWE - Common Weakness Enumeration

While CVE identifies specific instances of vulnerabilities, CWE categorizes the common flaws or weaknesses that can lead to vulnerabilities. CVE-2026-74635 is associated with the following CWEs:

Common Attack Pattern Enumeration and Classification (CAPEC)

Common Attack Pattern Enumeration and Classification (CAPEC) stores attack patterns, which are descriptions of the common attributes and approaches employed by adversaries to exploit the CVE-2026-74635 weaknesses.

We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).

Results are limited to the first 15 repositories due to potential performance issues.

The following list is the news that have been mention CVE-2026-74635 vulnerability anywhere in the article.

Results are limited to the first 20 news articles due to potential performance issues.

The following table lists the changes that have been made to the CVE-2026-74635 vulnerability over time.

Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.

  • New CVE Received by 416baaa9-dc9f-4396-8d5f-8c081fb06d67

    Aug. 22, 2026

    Action Type Old Value New Value
    Added Affected [{'repo': 'https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git', 'vendor': 'Linux', 'product': 'Linux', 'versions': [{'status': 'affected', 'version': '0998a6cb232674408a03e8561dc15aa266b2f53b', 'lessThan': '250159ace2dc53c1bdad267aa8da51b638748700', 'versionType': 'git'}, {'status': 'affected', 'version': 'db5c9a162d2f42bcc842b76b3d935dcc050a0eec', 'lessThan': '46336f476484f36145e5117e72d7b590f47433ee', 'versionType': 'git'}, {'status': 'affected', 'version': 'c12003bf91fdff381c55ef54fef3e961a5af2545', 'lessThan': 'bf750cfeacf4e47ac72dadc7f05839696efb8576', 'versionType': 'git'}, {'status': 'affected', 'version': '9ba1a7802ca9a2590cef95b253e6526f4364477f', 'lessThan': '94134d70abf9273b70499d97d0adc9185ef21091', 'versionType': 'git'}, {'status': 'affected', 'version': '901f44227072be60812fe8083e83e1533c04eed1', 'lessThan': 'c1e7351767dd30fc574395c82121e4c67b882da3', 'versionType': 'git'}, {'status': 'affected', 'version': '18c4ef4e765a798b47980555ed665d78b71aeadf', 'lessThan': 'bc9db0d879c655d5dfd8add32fd60f13e65d132c', 'versionType': 'git'}, {'status': 'affected', 'version': '18c4ef4e765a798b47980555ed665d78b71aeadf', 'lessThan': '9ea879862e66e354e616028c31b39aa3eb6d35d8', 'versionType': 'git'}, {'status': 'affected', 'version': '18c4ef4e765a798b47980555ed665d78b71aeadf', 'lessThan': 'e033cbf3975a8465f879ebd5989dc35b04423a4d', 'versionType': 'git'}, {'status': 'affected', 'version': 'a10cede006f9614b465cf25609a8753efbfd45cc', 'versionType': 'git'}, {'status': 'affected', 'version': 'efaf89a75a29b2d179bf4fe63ca62852e93ad620', 'versionType': 'git'}, {'status': 'affected', 'version': '5.10.247', 'lessThan': '5.10.265', 'versionType': 'semver'}, {'status': 'affected', 'version': '5.15.197', 'lessThan': '5.15.216', 'versionType': 'semver'}, {'status': 'affected', 'version': '6.1.159', 'lessThan': '6.1.183', 'versionType': 'semver'}, {'status': 'affected', 'version': '6.6.117', 'lessThan': '6.6.152', 'versionType': 'semver'}, {'status': 'affected', 'version': '6.12.58', 'lessThan': '6.12.104', 'versionType': 'semver'}, {'status': 'affected', 'version': '5.4.302', 'lessThan': '5.5', 'versionType': 'semver'}, {'status': 'affected', 'version': '6.17.8', 'lessThan': '6.18', 'versionType': 'semver'}], 'programFiles': ['drivers/video/fbdev/core/bitblit.c'], 'defaultStatus': 'unaffected'}, {'repo': 'https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git', 'vendor': 'Linux', 'product': 'Linux', 'versions': [{'status': 'affected', 'version': '6.18'}, {'status': 'unaffected', 'version': '0', 'lessThan': '6.18', 'versionType': 'semver'}, {'status': 'unaffected', 'version': '5.10.265', 'versionType': 'semver', 'lessThanOrEqual': '5.10.*'}, {'status': 'unaffected', 'version': '5.15.216', 'versionType': 'semver', 'lessThanOrEqual': '5.15.*'}, {'status': 'unaffected', 'version': '6.1.183', 'versionType': 'semver', 'lessThanOrEqual': '6.1.*'}, {'status': 'unaffected', 'version': '6.6.152', 'versionType': 'semver', 'lessThanOrEqual': '6.6.*'}, {'status': 'unaffected', 'version': '6.12.104', 'versionType': 'semver', 'lessThanOrEqual': '6.12.*'}, {'status': 'unaffected', 'version': '6.18.45', 'versionType': 'semver', 'lessThanOrEqual': '6.18.*'}, {'status': 'unaffected', 'version': '7.1.9', 'versionType': 'semver', 'lessThanOrEqual': '7.1.*'}, {'status': 'unaffected', 'version': '7.2', 'versionType': 'original_commit_for_fix', 'lessThanOrEqual': '*'}], 'programFiles': ['drivers/video/fbdev/core/bitblit.c'], 'defaultStatus': 'affected'}]
    Added 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.
    Added Reference https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700
    Added Reference https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee
    Added Reference https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091
    Added Reference https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8
    Added Reference https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c
    Added Reference https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576
    Added Reference https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3
    Added Reference https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d
EPSS is a daily estimate of the probability of exploitation activity being observed over the next 30 days. Following chart shows the EPSS score history of the vulnerability.
CVSS
Vulnerability Scoring Details
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