VulnerabilityPublished 19 Jul 2026

CVE-2026-63886: Linux kernel · scsi: target: iscsi: Validate CHAP_R length before base64 decode

Source GCVE Database

Authoritative record:CVE-2026-63886· Critical 9.8

Not known to be exploited · CVSS 9.8 Critical · fixed in edd06675a02376ea8347dba7c29ad982ba5b36ee : patch Linux kernel as a priority.

Data as of 26 Oct 2026, 13:47 UTC

CVSS 3.1 base score

9.8Critical

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.7% chance of exploitation in 30 days – higher than 53% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in edd06675a02376ea8347dba7c29ad982ba5b36ee, c04e85799356120209b351a148ac2db888d5ffd9, bf154c657828ed05399bca5d98cf1611bb048b12 +3

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
Integrity: High
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 19 Jul 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

ghsa-f9cg-2c9v-xhmh

What to do

Upgrade to a fixed release listed under Affected software.

Affected software

Product	Affected	Fixed in
LinuxLinux	Affected ≥ 1e5733883421495908f3b90d9d807663038b4136, < edd06675a02376ea8347dba7c29ad982ba5b36ee	Fixed in edd06675a02376ea8347dba7c29ad982ba5b36ee
LinuxLinux	Affected ≥ 1e5733883421495908f3b90d9d807663038b4136, < c04e85799356120209b351a148ac2db888d5ffd9	Fixed in c04e85799356120209b351a148ac2db888d5ffd9
LinuxLinux	Affected ≥ 1e5733883421495908f3b90d9d807663038b4136, < bf154c657828ed05399bca5d98cf1611bb048b12	Fixed in bf154c657828ed05399bca5d98cf1611bb048b12
LinuxLinux	Affected ≥ 1e5733883421495908f3b90d9d807663038b4136, < 82454e6f21e56ea9a0a9de7d0ff7e1dfb83e34d6	Fixed in 82454e6f21e56ea9a0a9de7d0ff7e1dfb83e34d6
LinuxLinux	Affected ≥ 1e5733883421495908f3b90d9d807663038b4136, < 85db7391310b1304d2dc8ae3b0b12105a9567147	Fixed in 85db7391310b1304d2dc8ae3b0b12105a9567147
LinuxLinux	Affected ≥ 1e5733883421495908f3b90d9d807663038b4136, < 4a3a19c98a8207ad08bec554703d90f2c34a8cc6	Fixed in 4a3a19c98a8207ad08bec554703d90f2c34a8cc6

AI-suggested

If you cannot patch yet, avoid exposing the vulnerable iSCSI target service to untrusted networks or inputs.

Description

In the Linux kernel, the following vulnerability has been resolved: scsi: target: iscsi: Validate CHAP_R length before base64 decode 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.

Impact

This vulnerability could lead to a buffer overflow in the Linux kernel's iSCSI target CHAP authentication code. Buffer overflows in kernel code can potentially be exploited to cause denial of service or escalate privileges. However, no known exploits in the wild have been reported. The impact depends on the ability of an attacker to supply crafted CHAP_R responses to the vulnerable kernel component.

Summary

The Linux kernel's iSCSI target CHAP authentication code had a flaw where the length of the base64-encoded CHAP_R response was not validated before decoding. This allowed the base64 decoder to write beyond the allocated buffer size (client_digest), causing a buffer overflow. The vulnerability arises because the allocated buffer size is based on the digest size (e.g., 32 bytes for SHA-256), but up to 95 bytes could be decoded from the input, overflowing the buffer by up to 63 bytes. The fix applies a length check before decoding by stripping trailing '=' padding characters and rejecting inputs whose decoded length would exceed the buffer size. The HEX encoding branch already had this validation. The mutual CHAP path was not vulnerable due to stricter input length limits.

References

Community reviews

Loading reviews…