CVE-2026-63886

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.

Weakness

The product writes data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer.

• Be wary that a language’s interface to native code may still be subject to overflows, even if the language itself is theoretically safe.

• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• Examples include the Safe C String Library (SafeStr) by Messier and Viega [REF-57], and the Strsafe.h library from Microsoft [REF-56]. These libraries provide safer versions of overflow-prone string-handling functions.

• Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking.

• D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail.

• Consider adhering to the following rules when allocating and managing an application’s memory:

• Run or compile the software using features or extensions that randomly arrange the positions of a program’s executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code.

• Examples include Address Space Layout Randomization (ASLR) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64]. Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as “rebasing” (for Windows) and “prelinking” (for Linux) [REF-1332] using randomly generated addresses. ASLR for libraries cannot be used in conjunction with prelink since it would require relocating the libraries at run-time, defeating the whole purpose of prelinking.

• For more information on these techniques see D3-SAOR (Segment Address Offset Randomization) from D3FEND [REF-1335].

• Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX [REF-60] [REF-61]. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment.

• For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND [REF-1336].

References

• https://git.kernel.org/stable/c/4a3a19c98a8207ad08bec554703d90f2c34a8cc6
• https://git.kernel.org/stable/c/82454e6f21e56ea9a0a9de7d0ff7e1dfb83e34d6
• https://git.kernel.org/stable/c/85db7391310b1304d2dc8ae3b0b12105a9567147
• https://git.kernel.org/stable/c/bf154c657828ed05399bca5d98cf1611bb048b12
• https://git.kernel.org/stable/c/c04e85799356120209b351a148ac2db888d5ffd9
• https://git.kernel.org/stable/c/edd06675a02376ea8347dba7c29ad982ba5b36ee