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CVE-2026-89720
ubifs: fix out-of-bounds read in signature length check
Published 2026-09-11 (37 days ago) by the Linux kernel CNA. Subsystem fs/ubifs. Debian urgency: not yet assigned.
Debian does not build this code. The per-release build detail is under "Where it lives" below.
What to do, per Debian release
Debian's own status for CVE-2026-89720 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-4841-1 · 2026-10-17 · text | 6.1.190-1 | Update the kernel and reboot. DLA-4841-1 shipped the fix on 2026-10-17, in 6.1.190-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6528-1 · 2026-09-29 · text | 6.12.111-1 | Update the kernel and reboot. DSA-6528-1 shipped the fix on 2026-09-29, in 6.12.111-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.2.6-1 | Update the kernel and reboot. Fixed in 7.2.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4817-1 · 2026-10-04 · text | 6.12.111-1~deb12u1 | Update the kernel and reboot. DLA-4817-1 shipped the fix on 2026-10-04, in 6.12.111-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
ubifs_sb_verify_signature() bounds the on-disk ubifs_sig_node->len field before handing the signature payload to verify_pkcs7_signature(), but the check has the wrong sign: if (le32_to_cpu(signode->len) > snod->len + sizeof(struct ubifs_sig_node)) The signature bytes start sizeof(struct ubifs_sig_node) (UBIFS_SIG_NODE_SZ, 64 bytes) into the node, so the payload is at most snod->len - sizeof(struct ubifs_sig_node) bytes long. Adding the header size instead of subtracting it accepts a declared length up to 2 * UBIFS_SIG_NODE_SZ larger than the node actually holds -- past the end of c->sbuf, which is vmalloc(c->leb_size). verify_pkcs7_signature() -> pkcs7_parse_message() -> asn1_ber_decoder() is then handed that inflated length and reads beyond the allocation while walking the DER headers. The node length comes straight from the mounted image, so a crafted signed UBIFS image reaches this via ubifs_read_superblock() before the signature is cryptographically checked. snod->len is guaranteed to be >= UBIFS_SIG_NODE_SZ by the node scanner (c->ranges[UBIFS_SIG_NODE].min_len == UBIFS_SIG_NODE_SZ), so the corrected subtraction cannot underflow. Legitimately signed images are unaffected: a correct superblock never declares a signature longer than the node it is embedded in.
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.
fs/ubifs/auth.c
- What this is
- UBI FILE SYSTEM (UBIFS)MAINTAINERS section matched on fs/ubifs/ · linux-mtd@lists.infradead.org · Supported
- Kernel module
- none to look for. The file would have gone into ubifs, and Debian does not compile it into that module, so finding ubifs in lsmod would say nothing about this CVE.
Check it on the machine
grep -wE 'CONFIG_UBIFS_FS|CONFIG_UBIFS_FS_AUTHENTICATION' /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 does not build this code. In 5 releases below, every kernel flavour Debian publishes has the switch off, so the vulnerable file is compiled into no kernel binary Debian ships.
- bookworm, trixie, forky, sid, bookworm · linux-6.12n
- n
- not enabled on any flavour - this release does not build the code
- Build switches
- CONFIG_UBIFS_FS, CONFIG_UBIFS_FS_AUTHENTICATION
- 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
- 2026-09-11 (37 days ago)source
- CVSS
- 7.7 high
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:Hsource recompute it - Attack vector
- local (AV:L)source
- EPSS
- 0.18%, higher than 7.6% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- fs/ubifs
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.3 | 5.10.270 | ab7405bd8633 |
| 5.3 | 5.15.221 | 11abc34698cb |
| 5.3 | 6.1.188 | 8cc3da72cf57 |
| 5.3 | 6.6.157 | 37a9d25a563f |
| 5.3 | 6.12.109 | f76b79d6e42a |
| 5.3 | 6.18.50 | a1dc246f98bb |
| 5.3 | 7.2.4 | 83e1aa9f5f90 |
| 5.3 | 7.3 | 95d27c1708bb |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4841-1 | 2026-10-17 | bookworm 6.1.190-1 | tracker |
| DLA-4817-1 | 2026-10-04 | bookworm 6.12.111-1~deb12u1 | tracker |
| DSA-6528-1 | 2026-09-29 | trixie 6.12.111-1 | 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
- The OOB read is reached only while mounting a local UBIFS/UBI volume (mount → ubifs_fill_super → mount_ubifs → ubifs_read_superblock → authenticate_sb_node → ubifs_sb_verify_signature). The inflated ubifs_sig_node->len comes from on-disk flash, not ksmbd, nfsd, or any received-packet path.
- AC:LAttack complexity, low
- The attacker fully controls the on-disk SIG node size and signode->len. The inverted check accepts a payload up to snod->len+64; with a node filling the LEB that deterministically reads 128 bytes past vmalloc'd c->sbuf in asn1_ber_decoder(), with no race or uncontrolled layout.
- PR:NPrivileges required, none
- Supplying the crafted authenticated image needs no account on the victim. ubifs_fs_type lacks FS_USERNS_MOUNT, so the CAP_SYS_ADMIN mount (boot-time auth_key mount on embedded, automotive, and IoT devices that use offline-signed UBIFS) is performed by the platform, matching the malicious-filesystem model.
- UI:NUser interaction, none
- Offline-signed UBIFS is verified automatically at mount; boot/fstab auto-mount with auth_key runs ubifs_sb_verify_signature in ubifs_read_superblock before any file access, so planting or presenting the crafted volume is sufficient with no separate victim click or open.
- S:UScope, unchanged
- The out-of-bounds read and any oops remain inside the mounting host kernel. This is a same-authority memory-safety issue, not a VM escape, IOMMU bypass, or other cross-boundary impact.
- C:HConfidentiality impact, high
- verify_pkcs7_signature() → pkcs7_parse_message() → asn1_ber_decoder() walks an attacker-inflated length of up to 128 bytes past c->sbuf (vmalloc of the LEB). That is an out-of-bounds kernel read of adjacent vmalloc memory, which is Confidentiality High rather than a few-byte leak.
- I:NIntegrity impact, none
- The defect is an out-of-bounds read of PKCS#7/ASN.1 headers; the decoder copies those bytes only into newly allocated parse objects sized from the claimed length and does not write past any allocation or provide an arbitrary-write or control-flow primitive.
- A:HAvailability impact, high
- Reading past the vmalloc LEB buffer into an unmapped vmalloc guard page oopses or panics the kernel during mount, and a crafted image can retrigger that crash on every boot or mount of the volume.