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CVE-2026-63886
scsi: target: iscsi: Validate CHAP_R length before base64 decode
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem drivers/target. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-63886 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 | fixed | 6.1.176-1 | Update the kernel and reboot. Fixed in 6.1.176-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.94-1 | Update the kernel and reboot. Fixed in 6.12.94-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
chap_server_compute_hash() allocates client_digest as kzalloc(chap->digest_size) and then, for BASE64-encoded responses, passes chap_r directly to chap_base64_decode() without checking whether the input length could produce more than digest_size bytes of output. chap_base64_decode() writes to the destination unconditionally as long as there is input to consume. With MAX_RESPONSE_LENGTH set to 128 and the "0b" prefix stripped by extract_param(), up to 127 base64 characters can reach the decoder. 127 characters decode to 95 bytes. For SHA-256 (digest_size=32) this overflows client_digest by 63 bytes; for MD5 (digest_size=16) the overflow is 79 bytes. The length check at line 344 fires after the write has already happened. The HEX branch in the same switch statement already validates the length up front. Apply the same approach to the BASE64 branch: strip trailing base64 padding characters, then reject any input whose data length exceeds DIV_ROUND_UP(digest_size * 4, 3) before calling the decoder. Stripping trailing '=' before the comparison handles both padded and unpadded encodings. chap_base64_decode() already returns early on '=', so the full original string is still passed to the decoder unchanged. The mutual CHAP path decodes CHAP_C into initiatorchg_binhex, which is kzalloc(CHAP_CHALLENGE_STR_LEN). extract_param() caps initiatorchg at CHAP_CHALLENGE_STR_LEN characters, so at most CHAP_CHALLENGE_STR_LEN-1 base64 characters reach the decoder. The maximum decoded size, DIV_ROUND_UP((CHAP_CHALLENGE_STR_LEN-1) * 3, 4), is less than CHAP_CHALLENGE_STR_LEN, so no overflow is possible there. A comment is added at the call site to document this.
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/target/iscsi/iscsi_target_auth.c
- What this is
- SCSI TARGET SUBSYSTEMMAINTAINERS section matched on drivers/target/ · linux-scsi@vger.kernel.org · Supported
- Kernel module
- iscsi_target_mod
Check it on the machine
grep -wE 'CONFIG_ISCSI_TARGET|CONFIG_TARGET_CORE' /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.
lsmod | grep -w iscsi_target_modPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sidm
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switches
- CONFIG_ISCSI_TARGET, CONFIG_TARGET_CORE
- 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
- 2026-07-19 (3 months ago)source
- CVSS
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.71%, higher than 52.7% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- drivers/target
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.0 | 6.1.176 | 82454e6f21e5 |
| 6.0 | 6.6.143 | edd06675a023 |
| 6.0 | 6.12.93 | bf154c657828 |
| 6.0 | 6.18.35 | 4a3a19c98a82 |
| 6.0 | 7.0.12 | c04e85799356 |
| 6.0 | 7.1 | 85db7391310b |
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:NAttack vector, network
- The bug is triggered by a remote iSCSI initiator sending crafted CHAP login PDUs over TCP to the in-kernel LIO iSCSI target during the network login/authentication handshake.
- AC:LAttack complexity, low
- Once an iSCSI target with CHAP is reachable, an attacker can reliably trigger the overflow by sending CHAP_R with a 0b-prefixed base64 string up to 127 characters; no race or special memory layout is required.
- PR:NPrivileges required, none
- The heap overflow occurs during the CHAP authentication handshake before the password hash comparison succeeds, so exploitation is reachable by an unauthenticated remote network attacker who knows a configured CHAP username.
- UI:NUser interaction, none
- Exploitation requires only the attacker initiating an iSCSI login connection and sending malicious CHAP PDUs; no action from a victim user is needed.
- S:UScope, unchanged
- Successful exploitation corrupts kernel heap memory within the same kernel security boundary, enabling denial of service or kernel privilege escalation, not a cross-boundary escape such as VM guest-to-host.
- C:HConfidentiality impact, high
- The out-of-bounds heap write corrupts adjacent kmalloc objects and can be leveraged for arbitrary kernel memory disclosure through heap grooming, consistent with heap overflow impact guidance.
- I:HIntegrity impact, high
- A controlled kernel heap out-of-bounds write of up to 79 bytes past a digest-sized buffer can corrupt adjacent structures and be developed into arbitrary code execution or kernel privilege escalation.
- A:HAvailability impact, high
- The overflow causes slab-out-of-bounds writes confirmed by KASAN and can crash or destabilize the kernel during login processing, constituting high availability impact.