CVE-2026-98493 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
net: ipconfig: bound DHCP option construction
ic_dhcp_init_options() appends the hostname (option 12), vendor-class (option 60) and client-ID (option 61) options into the fixed 312-byte bootp_pkt.exten[] buffer. Only the client-ID branch checked the
remaining space; the hostname and vendor-class writes were unbounded.
A 64-byte hostname together with the maximum 252-byte dhcpclass= identifier needs 18 + (2 + 64) + (2 + 252) = 338 of the 312 available bytes even before the terminating END marker, so the vendor-class memcpy runs past the end of exten[]. With CONFIG_FORTIFY_SOURCE this is
reported as a field-spanning write and, when the kernel is booted with panic_on_warn=1, aborts boot with a panic.
Route the optional options through a common helper that makes sure the option, its 2-byte header and the END marker all fit and drops an option that would not. Configurations with short options keep sending exactly the same bytes as before.
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Analysis
by VulDB Data Team • 10/19/2026
The Linux kernel vulnerability identified in the net: ipconfig subsystem involves a buffer overflow within the DHCP client implementation, specifically affecting the construction of bootp packet extension fields. The function ic_dhcp_init_options is responsible for appending various Dynamic Host Configuration Protocol options into a fixed-size 312-byte array named exten[] located within the bootp_pkt structure. This process includes adding critical identification data such as the hostname corresponding to option twelve, the vendor-class identifier for option sixty, and the client-ID for option sixty-one. While the code correctly implemented bounds checking for the client-ID branch, it failed to apply similar safeguards to the writes involving the hostname and vendor-class identifiers. This inconsistency creates a significant security gap where input data exceeding available buffer space can overwrite adjacent memory regions without detection by standard runtime checks in many configurations.
The technical flaw manifests when specific combinations of configuration parameters are used during network initialization. Specifically, providing a sixty-four-byte hostname alongside a maximum-length two hundred fifty-two byte vendor-class identifier results in a total required size that exceeds the allocated buffer capacity even before accounting for the mandatory END marker. The calculation reveals that these options require three hundred thirty-eight bytes, which surpasses the three hundred twelve bytes available in exten[]. Consequently, the memcpy operation responsible for writing the vendor-class data executes past the end of the array boundary. This constitutes a classic heap or stack buffer overflow depending on memory layout, leading to potential corruption of adjacent kernel structures. In environments where CONFIG_FORTIFY_SOURCE is enabled, this out-of-bounds write triggers a field-spanning write error detection mechanism. If the system is further configured with panic_on_warn set to one, the vulnerability causes an immediate kernel panic during boot, effectively resulting in a denial of service condition that prevents the operating system from starting successfully.
From a classification perspective, this issue aligns with CWE-120 Buffer Copy without Checking Size of Input and CWE-787 Out-of-bounds Write within the Common Weakness Enumeration framework. The attack vector is primarily local or network-based depending on whether an attacker can influence DHCP server responses or client configuration parameters prior to boot. In terms of MITRE ATT&CK, this vulnerability relates to techniques involving memory corruption that could potentially lead to arbitrary code execution if exploited in a non-panic scenario, although the immediate impact described here is availability disruption through kernel panic. The lack of input validation on critical network initialization paths highlights a deficiency in defensive coding practices for handling variable-length protocol fields within constrained buffer structures.
The resolution involves refactoring the option construction logic to route all optional parameters through a unified helper function that performs rigorous bounds checking before any memory copy operations occur. This helper ensures that each option, including its two-byte header and the final END marker, fits entirely within the remaining space of the exten[] buffer. If an option would cause an overflow, it is safely dropped rather than written partially or fully out of bounds. This approach maintains backward compatibility for configurations using short options while eliminating the risk associated with oversized inputs. By enforcing strict size validation at the point of data insertion into fixed-size buffers, the patch mitigates both information disclosure and potential privilege escalation risks that could arise from memory corruption in this critical network initialization path.