CVE-2026-98969 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: em_text: reject unterminated algo before autoload
This is a follow-up to commit 9c572a83037a ("net/sched: fix potential stack infoleak in em_text_dump()"), which zero-initialised the dump-side struct. Sashiko pointed at a pre-existing bug that exposed the change-side input-validation gap open.
em_text_change() casts the raw netlink attribute payload to struct tcf_em_text and passes conf->algo, a 16-byte array with no enforced NUL terminator, to textsearch_prepare(). On the TS_AUTOLOAD retry textsearch_prepare() calls request_module("ts_%s", algo); vsnprintf() walks %s until it finds a NUL, so an algo[] with no NUL reads past the
array into the adjacent struct fields and payload and feeds those bytes to the usermode helper command line. This is an out-of-bounds read and a kernel memory disclosure.
Reject the attribute when no NUL exists within TC_EM_TEXT_ALGOSIZ bytes, matching the check xt_string has had since 3ab720881b6e.
Conditions to recreate the bug: - CAP_NET_ADMIN on the target netns (namespace-local via unshare -Urn is enough) - install clsact on an interface, then add a basic filter with one text ematch whose algo[] is 16 bytes with no NUL (e.g. "A"*16) and
pattern_len at least 1 - the add fails with -ENOENT and the kernel invokes modprobe with a module name made of "ts_" plus the 16 bytes and the adjacent payload
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Analysis
by VulDB Data Team • 10/24/2026
The Linux kernel’s network traffic control subsystem contains a critical input validation flaw within the em_text classifier extension, specifically in the textsearch_prepare function path. This vulnerability arises from an insufficient check for string termination when processing user-supplied algorithm identifiers via netlink attributes. The core issue lies in how the kernel handles the algo field of struct tcf_em_text during configuration changes initiated by userspace applications with network administration privileges. When em_text_change processes incoming data, it casts raw netlink payload directly into this structure without enforcing strict boundary checks on string length or termination characters. This oversight allows attackers to supply algorithm names that lack a null terminator, triggering out-of-bounds memory reads and potential kernel information disclosure through module loading mechanisms.
The technical mechanism of exploitation involves the interaction between em_text_change and textsearch_prepare functions. When configuring an extended match rule for traffic classification, userspace applications pass configuration data containing an algo array intended to specify which string matching algorithm should be employed. The code incorrectly assumes this 16-byte array will always contain a properly terminated C-string. In reality, if the application provides exactly sixteen non-null bytes followed by arbitrary payload content, textsearch_prepare proceeds without detecting the missing terminator. During TS_AUTOLOAD retry logic, the function attempts to dynamically load the requested algorithm module using request_module with format string ts_%s where s is derived from algo via vsnprintf operations that continue reading past array boundaries until encountering a null byte in adjacent memory regions including potentially sensitive kernel structures or user-controlled payload data.
This vulnerability enables several dangerous attack vectors beyond simple information disclosure. By carefully crafting the input pattern, an attacker can cause the kernel to attempt loading non-existent modules with names constructed from leaked kernel memory contents combined with controlled payload bytes. This creates opportunities for denial of service through excessive module request failures and potential privilege escalation if subsequent operations leverage these malformed requests against other subsystems. The impact extends beyond immediate data exposure as repeated exploitation attempts could degrade system performance or trigger security mechanisms designed to prevent unauthorized module loading, effectively creating a persistent availability threat vector within network processing paths that handle classified traffic flows.
The operational impact encompasses both confidentiality and integrity concerns for systems relying on Linux kernel networking capabilities. Systems configured with complex traffic control rules involving text-based packet classification become vulnerable when managed by processes possessing CAP_NET_ADMIN privileges in any network namespace context. Attackers can exploit this through local access requiring only unprivileged user accounts capable of creating isolated namespaces, making remote exploitation unlikely but local privilege escalation feasible for authenticated users who gain administrative capabilities within containerized or virtualized environments where such controls are commonly deployed.
Mitigation strategies must address both immediate remediation and long-term architectural improvements to prevent similar input validation failures across the kernel networking stack. The primary defense involves implementing strict length verification before processing any string-based configuration parameters passed through netlink interfaces, ensuring all algorithm identifiers include proper null termination within their defined size limits. Kernel developers should enforce consistent validation patterns matching those established in related subsystems like xt_string which already implements appropriate boundary checks for similar functionality. System administrators can reduce exposure by restricting CAP_NET_ADMIN capabilities to trusted processes and monitoring module loading events through audit frameworks that detect unusual request_module invocations with suspiciously constructed names containing non-standard characters or excessive lengths indicative of memory corruption attempts rather than legitimate algorithm specifications.