{ "dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": { "cveId": "CVE-2026-74635", "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "state": "PUBLISHED", "assignerShortName": "Linux", "dateReserved": "2026-08-15T05:44:03.922Z", "datePublished": "2026-08-22T15:32:15.280Z", "dateUpdated": "2026-10-16T19:06:04.057Z" }, "containers": { "cna": { "providerMetadata": { "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "shortName": "Linux", "dateUpdated": "2026-08-25T05:40:59.420Z" }, "descriptions": [ { "lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nfbdev: bitblit: bound-check glyph index in bit_cursor()\n\nbit_cursor() fetches the glyph under the cursor with\n\n\tc = scr_readw(vc_pos);\n\tsrc = vc_font.data + ((c & charmask) * w * height);\n\nwhere charmask is 0x1ff when vc_hi_font_mask is set. The screen buffer\nvalue comes directly from scr_readw() and may be larger than the current\nfont's glyph count.\n\nSyzkaller triggers this via vcs_write(). The Call Trace shows\nvcs_write() in vc_screen.c writing an arbitrary 16-bit value with\nwritev() to /dev/vcsa, which vcs_write_buf() in vc_screen.c stores via\nvcs_scr_writew() without checking charcount. The stored value is later\nread in bit_cursor() in bitblit.c.\n\nWhen the font is changed from a font with 512 glyphs to a font with\n256 glyphs, the screen buffer can retain characters with the high\nbit set from the previous mode, which could also produce the same\nout-of-bounds access.\n\n BUG: KASAN: global-out-of-bounds in soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70\n Read of size 16 at addr ffff800086c57970\n\n Call Trace:\n soft_cursor+0x378/0x6bc drivers/video/fbdev/core/softcursor.c:70\n bit_cursor+0xa90/0x1108 drivers/video/fbdev/core/bitblit.c:365\n fbcon_cursor+0x344/0x498 drivers/video/fbdev/core/fbcon.c:1427\n hide_cursor+0xdc/0x2d0 drivers/tty/vt/vt.c:883\n update_region+0x100/0x18c drivers/tty/vt/vt.c:669\n vcs_write+0x8ec/0xaf0 drivers/tty/vt/vc_screen.c:685\n\nbit_putcs_aligned() and bit_putcs_unaligned() already clamp the glyph\nindex to vc_font.charcount. Apply the same clamp in bit_cursor() after\nextracting the attribute and masking, before indexing fontdata.\n\nThe fix completes the bounds checking started in commit 18c4ef4e765a\n(\"fbdev: bitblit: bound-check glyph index in bit_putcs*\"), which missed\nthe cursor path.\n\nThis change should be safe because the clamp reuses the existing\ncontract from fbcon: charcount is maintained under console_lock in\ncon_font_set() and fbcon_font_set(), and hi_font_mask is cleared when\nswitching from 512 to 256 glyphs. When stale screen data with high bits\nremains after a font switch, or when vcs_write() stores an arbitrary\nvalue, clamping the index to 0 prevents the out-of-bounds read without\nchanging cursor semantics — the same fallback bit_putcs uses." } ], "metrics": [ { "cvssV3_1": { "version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H", "baseScore": 7.8, "baseSeverity": "HIGH" }, "scenarios": [ { "lang": "en", "value": "AV:L - 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.\nAC:L - 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.\nPR:L - 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.\nUI:N - 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.\nS:U - 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.\nC:H - 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.\nI:H - 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.\nA:H - 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." } ] } ], "affected": [ { "product": "Linux", "vendor": "Linux", "defaultStatus": "unaffected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "drivers/video/fbdev/core/bitblit.c" ], "versions": [ { "version": "0998a6cb232674408a03e8561dc15aa266b2f53b", "lessThan": "250159ace2dc53c1bdad267aa8da51b638748700", "status": "affected", "versionType": "git" }, { "version": "db5c9a162d2f42bcc842b76b3d935dcc050a0eec", "lessThan": "46336f476484f36145e5117e72d7b590f47433ee", "status": "affected", "versionType": "git" }, { "version": "c12003bf91fdff381c55ef54fef3e961a5af2545", "lessThan": "bf750cfeacf4e47ac72dadc7f05839696efb8576", "status": "affected", "versionType": "git" }, { "version": "9ba1a7802ca9a2590cef95b253e6526f4364477f", "lessThan": "94134d70abf9273b70499d97d0adc9185ef21091", "status": "affected", "versionType": "git" }, { "version": "901f44227072be60812fe8083e83e1533c04eed1", "lessThan": "c1e7351767dd30fc574395c82121e4c67b882da3", "status": "affected", "versionType": "git" }, { "version": "18c4ef4e765a798b47980555ed665d78b71aeadf", "lessThan": "bc9db0d879c655d5dfd8add32fd60f13e65d132c", "status": "affected", "versionType": "git" }, { "version": "18c4ef4e765a798b47980555ed665d78b71aeadf", "lessThan": "9ea879862e66e354e616028c31b39aa3eb6d35d8", "status": "affected", "versionType": "git" }, { "version": "18c4ef4e765a798b47980555ed665d78b71aeadf", "lessThan": "e033cbf3975a8465f879ebd5989dc35b04423a4d", "status": "affected", "versionType": "git" }, { "version": "a10cede006f9614b465cf25609a8753efbfd45cc", "status": "affected", "versionType": "git" }, { "version": "efaf89a75a29b2d179bf4fe63ca62852e93ad620", "status": "affected", "versionType": "git" }, { "version": "5.10.247", "lessThan": "5.10.265", "status": "affected", "versionType": "semver" }, { "version": "5.15.197", "lessThan": "5.15.216", "status": "affected", "versionType": "semver" }, { "version": "6.1.159", "lessThan": "6.1.183", "status": "affected", "versionType": "semver" }, { "version": "6.6.117", "lessThan": "6.6.152", "status": "affected", "versionType": "semver" }, { "version": "6.12.58", "lessThan": "6.12.104", "status": "affected", "versionType": "semver" }, { "version": "5.4.302", "lessThan": "5.5", "status": "affected", "versionType": "semver" }, { "version": "6.17.8", "lessThan": "6.18", "status": "affected", "versionType": "semver" } ] }, { "product": "Linux", "vendor": "Linux", "defaultStatus": "affected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "drivers/video/fbdev/core/bitblit.c" ], "versions": [ { "version": "6.18", "status": "affected" }, { "version": "0", "lessThan": "6.18", "status": "unaffected", "versionType": "semver" }, { "version": "5.10.265", "lessThanOrEqual": "5.10.*", "status": "unaffected", "versionType": "semver" }, { "version": "5.15.216", "lessThanOrEqual": "5.15.*", "status": "unaffected", "versionType": "semver" }, { "version": "6.1.183", "lessThanOrEqual": "6.1.*", "status": "unaffected", "versionType": "semver" }, { "version": "6.6.152", "lessThanOrEqual": "6.6.*", "status": "unaffected", "versionType": "semver" }, { "version": "6.12.104", "lessThanOrEqual": "6.12.*", "status": "unaffected", "versionType": "semver" }, { "version": "6.18.45", "lessThanOrEqual": "6.18.*", "status": "unaffected", "versionType": "semver" }, { "version": "7.1.9", "lessThanOrEqual": "7.1.*", "status": "unaffected", "versionType": "semver" }, { "version": "7.2", "lessThanOrEqual": "*", "status": "unaffected", "versionType": "original_commit_for_fix" } ] } ], "cpeApplicability": [ { "nodes": [ { "operator": "OR", "negate": false, "cpeMatch": [ { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.10.247", "versionEndExcluding": "5.10.265" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.15.197", "versionEndExcluding": "5.15.216" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.1.159", "versionEndExcluding": "6.1.183" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.6.117", "versionEndExcluding": "6.6.152" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.12.58", "versionEndExcluding": "6.12.104" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.18", "versionEndExcluding": "6.18.45" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.18", "versionEndExcluding": "7.1.9" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.18", "versionEndExcluding": "7.2" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.4.302" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.17.8" } ] } ] } ], "references": [ { "url": "https://git.kernel.org/stable/c/250159ace2dc53c1bdad267aa8da51b638748700" }, { "url": "https://git.kernel.org/stable/c/46336f476484f36145e5117e72d7b590f47433ee" }, { "url": "https://git.kernel.org/stable/c/bf750cfeacf4e47ac72dadc7f05839696efb8576" }, { "url": "https://git.kernel.org/stable/c/94134d70abf9273b70499d97d0adc9185ef21091" }, { "url": "https://git.kernel.org/stable/c/c1e7351767dd30fc574395c82121e4c67b882da3" }, { "url": "https://git.kernel.org/stable/c/bc9db0d879c655d5dfd8add32fd60f13e65d132c" }, { "url": "https://git.kernel.org/stable/c/9ea879862e66e354e616028c31b39aa3eb6d35d8" }, { "url": "https://git.kernel.org/stable/c/e033cbf3975a8465f879ebd5989dc35b04423a4d" } ], "title": "fbdev: bitblit: bound-check glyph index in bit_cursor()", "x_generator": { "engine": "bippy-1.2.0" } }, "adp": [ { "metrics": [ { "other": { "type": "ssvc", "content": { "id": "CVE-2026-74635", "role": "CISA Coordinator", "options": [ { "Exploitation": "none" }, { "Automatable": "no" }, { "Technical Impact": "total" } ], "version": "2.0.3", "timestamp": "2026-10-16T19:03:10.782063Z" } } } ], "title": "CISA ADP Vulnrichment", "providerMetadata": { "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "shortName": "CISA-ADP", "dateUpdated": "2026-10-16T19:06:04.057Z" } } ] } }