SKELETONKEY/modules/fragnesia_cve_2026_46300/MODULE.md

fragnesia — CVE-2026-46300

🟡 PRIMITIVE / ported. Faithful port of the public V12 PoC into the skeletonkey_module interface. Not yet validated end-to-end on a vulnerable-kernel VM — see Verification status below.

Summary

Fragnesia ("Fragment Amnesia") is an XFRM ESP-in-TCP local privilege escalation. skb_try_coalesce() fails to propagate the SKBFL_SHARED_FRAG marker when moving paged fragments between socket buffers — so the kernel forgets that a fragment is externally backed by page-cache pages spliced in from a file. The ESP-in-TCP receive path then decrypts in place, corrupting the page cache of a read-only file.

Fragnesia is a latent bug exposed by the Dirty Frag fix: the candidate patch cites the Dirty Frag remediation (f4c50a4034e6) as a commit it "fixes". It is the same page-cache-write bug class as Copy Fail / Dirty Frag, reached through a different code path.

Primitive

1. Build a 256-entry AES-GCM keystream-byte table via AF_ALG ecb(aes) — for any wanted output byte, this yields the ESP IV whose keystream byte XORs the current byte to the target.
2. Enter a mapped user namespace + network namespace, bring loopback up, and install an XFRM ESP-in-TCP state (rfc4106(gcm(aes)), TCP_ENCAP_ESPINTCP).
3. A receiver accepts a loopback TCP connection and flips it to the espintcp ULP; a sender splice()s page-cache pages of the target file into that TCP stream behind a crafted ESP prefix.
4. The coalesce bug makes the kernel decrypt the spliced page-cache pages in place — one chosen byte per trigger.

The exploit rewrites the first 192 bytes of a setuid-root binary (/usr/bin/su and friends) with an ET_DYN ELF that drops privileges to 0 and execves /bin/sh.

Operations

Op	Behaviour
--scan	Checks unprivileged-userns availability + a readable setuid carrier ≥ 4096 bytes. With --active, runs the full ESP-in-TCP primitive against a disposable /tmp file and reports VULNERABLE/OK empirically.
--exploit … --i-know	Forks a child that places the payload into the carrier's page cache and execs it for a root shell. --no-shell stops after the page-cache write.
--cleanup	Evicts the carrier from the page cache. The on-disk binary is never written.
--detect-rules	Emits embedded auditd + sigma rules.

Preconditions

• Unprivileged user namespaces enabled. On Ubuntu, AppArmor blocks this by default — sysctl kernel.apparmor_restrict_unprivileged_userns=0 (or chain a separate bypass). This is the scoping question the old _stubs/fragnesia_TBD raised; the module ships and reports PRECOND_FAIL cleanly when the userns gate is closed.
• CONFIG_INET_ESPINTCP built into the kernel.
• A readable setuid-root binary ≥ 4096 bytes as the payload carrier.
• x86_64 (the embedded ELF payload is x86_64 shellcode).

Port notes

The upstream PoC renders a full-screen ANSI "smash frame" TUI (draw_smash_frame + terminal scroll-region escapes). That is not ported — it cannot coexist with a shared multi-module dispatcher. Progress is logged with [*]/[+]/[-] prefixes, gated on --json. The exploit mechanism itself is reproduced faithfully.

Verification status

This module is a faithful port of https://github.com/v12-security/pocs/tree/main/fragnesia, compiled into the SKELETONKEY module interface. It has not been validated end-to-end against a known-vulnerable kernel inside the SKELETONKEY CI matrix.

detect() deliberately does not return a kernel-version-based patched/vulnerable verdict: the CVE-2026-46300 fix commit is not yet pinned here. Instead:

• preconditions missing → PRECOND_FAIL
• preconditions present, no --active → TEST_ERROR so --auto does not fire it blind
• --active → empirical VULNERABLE / OK via the /tmp sentinel probe

Before promoting to 🟢: pin the fix commit + branch-backport thresholds, add a kernel_range, and validate on a vulnerable VM.