{ "dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": { "cveId": "CVE-2024-49940", "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "state": "PUBLISHED", "assignerShortName": "Linux", "dateReserved": "2024-10-21T12:17:06.043Z", "datePublished": "2024-10-21T18:01:59.668Z", "dateUpdated": "2026-08-05T11:40:35.425Z" }, "containers": { "cna": { "providerMetadata": { "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "shortName": "Linux", "dateUpdated": "2026-08-05T11:40:35.425Z" }, "descriptions": [ { "lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nl2tp: prevent possible tunnel refcount underflow\n\nWhen a session is created, it sets a backpointer to its tunnel. When\nthe session refcount drops to 0, l2tp_session_free drops the tunnel\nrefcount if session->tunnel is non-NULL. However, session->tunnel is\nset in l2tp_session_create, before the tunnel refcount is incremented\nby l2tp_session_register, which leaves a small window where\nsession->tunnel is non-NULL when the tunnel refcount hasn't been\nbumped.\n\nMoving the assignment to l2tp_session_register is trivial but\nl2tp_session_create calls l2tp_session_set_header_len which uses\nsession->tunnel to get the tunnel's encap. Add an encap arg to\nl2tp_session_set_header_len to avoid using session->tunnel.\n\nIf l2tpv3 sessions have colliding IDs, it is possible for\nl2tp_v3_session_get to race with l2tp_session_register and fetch a\nsession which doesn't yet have session->tunnel set. Add a check for\nthis case." } ], "metrics": [ { "cvssV3_1": { "version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H", "baseScore": 7.8, "baseSeverity": "HIGH" }, "scenarios": [ { "lang": "en", "value": "AV:L - The refcount underflow is triggered by local session creation — either the L2TP generic-netlink `L2TP_CMD_SESSION_CREATE` command or a `connect()` on an `AF_PPPOX`/`PX_PROTO_OL2TP` socket. A remote peer cannot itself drive `l2tp_session_create()`/`l2tp_session_register()`, though remote packets in `l2tp_udp_encap_recv()` are what subsequently dereference the prematurely freed tunnel.\nAC:L - The attacker fully controls both sides: they choose the session ID, so they can deterministically force `l2tp_session_register()` to fail (`-EEXIST` on a colliding L2TPv3 ID, or `-ENODEV`), and they can run concurrent create/teardown threads to widen the window between `l2tp_session_create()` and the tunnel refcount bump. No condition outside attacker influence is required, and L2TP is a stock distro module.\nPR:L - `pppol2tp_create()`/`pppox_create()` perform no capability check, so a plain unprivileged local user can reach `l2tp_session_create()` and `l2tp_session_register()` through `connect()`. The netlink path is `GENL_UNS_ADMIN_PERM` (CAP_NET_ADMIN against the netns user_ns), which any unprivileged user obtains via `unshare -Urn`.\nUI:N - The attacker performs the entire sequence — tunnel setup, session creation with a colliding ID, and session teardown — from their own process. No victim action or cooperating user is involved.\nS:U - The underflow and the resulting use-after-free are confined to the kernel's own l2tp objects within the same security authority; no VM, IOMMU, or sandbox boundary is crossed.\nC:H - The dropped-too-early tunnel reference frees `struct l2tp_tunnel` while `session->tunnel` and the datapath still reference it, giving a use-after-free on a kmalloc'd object that can be reclaimed with attacker-sprayed data and then read back (e.g. `tunnel->sock`, `tunnel->encap`, `tunnel->version` used in `l2tp_recv_common()`/`l2tp_xmit_skb()`), enabling kernel memory disclosure.\nI:H - A use-after-free on the tunnel object lets an attacker reoccupy the freed slab and control fields such as `tunnel->sock`, which the transmit and receive paths dereference and pass to socket operations, yielding a write/control-flow-hijack primitive. Additionally the extra `l2tp_tunnel_dec_refcount()` corrupts kernel reference-counting state itself.\nA:H - Premature tunnel free reliably produces an oops/panic when the still-live session or a received L2TP packet dereferences the dangling `session->tunnel`, and the `refcount_t` underflow itself raises a WARN that panics on `panic_on_warn` systems." } ] } ], "affected": [ { "product": "Linux", "vendor": "Linux", "defaultStatus": "unaffected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "net/l2tp/l2tp_core.c", "net/l2tp/l2tp_core.h", "net/l2tp/l2tp_netlink.c", "net/l2tp/l2tp_ppp.c" ], "versions": [ { "version": "3953ae7b218df4d1e544b98a393666f9ae58a78c", "lessThan": "f7415e60c25a6108cd7955a20b2e66b6251ffe02", "status": "affected", "versionType": "git" }, { "version": "3953ae7b218df4d1e544b98a393666f9ae58a78c", "lessThan": "24256415d18695b46da06c93135f5b51c548b950", "status": "affected", "versionType": "git" }, { "version": "b102bfc2a90d14f342580285782a9a51c74f7369", "status": "affected", "versionType": "git" }, { "version": "10c15ddabbcf888922adbdd44ca3fecf6eab19d9", "status": "affected", "versionType": "git" }, { "version": "8d1c650d452c53fcb3f02a7b1d772741639f89a4", "status": "affected", "versionType": "git" }, { "version": "12b5fb58ac993c24210cf8cbc72d407d3a4e6490", "status": "affected", "versionType": "git" }, { "version": "aef37401b467a0b1a9517c69924a1d66937e0789", "status": "affected", "versionType": "git" }, { "version": "3.2.99", "lessThan": "3.3", "status": "affected", "versionType": "semver" }, { "version": "3.16.54", "lessThan": "3.17", "status": "affected", "versionType": "semver" }, { "version": "4.4.225", "lessThan": "4.5", "status": "affected", "versionType": "semver" }, { "version": "4.9.225", "lessThan": "4.10", "status": "affected", "versionType": "semver" }, { "version": "4.14.182", "lessThan": "4.15", "status": "affected", "versionType": "semver" } ] }, { "product": "Linux", "vendor": "Linux", "defaultStatus": "affected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "net/l2tp/l2tp_core.c", "net/l2tp/l2tp_core.h", "net/l2tp/l2tp_netlink.c", "net/l2tp/l2tp_ppp.c" ], "versions": [ { "version": "4.15", "status": "affected" }, { "version": "0", "lessThan": "4.15", "status": "unaffected", "versionType": "semver" }, { "version": "6.11.3", "lessThanOrEqual": "6.11.*", "status": "unaffected", "versionType": "semver" }, { "version": "6.12", "lessThanOrEqual": "*", "status": "unaffected", "versionType": "original_commit_for_fix" } ] } ], "cpeApplicability": [ { "nodes": [ { "operator": "OR", "negate": false, "cpeMatch": [ { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "4.15", "versionEndExcluding": "6.11.3" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "4.15", "versionEndExcluding": "6.12" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "3.2.99" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "3.16.54" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "4.4.225" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "4.9.225" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "4.14.182" } ] } ] } ], "references": [ { "url": "https://git.kernel.org/stable/c/f7415e60c25a6108cd7955a20b2e66b6251ffe02" }, { "url": "https://git.kernel.org/stable/c/24256415d18695b46da06c93135f5b51c548b950" } ], "title": "l2tp: prevent possible tunnel refcount underflow", "x_generator": { "engine": "bippy-1.2.0" } }, "adp": [ { "metrics": [ { "other": { "type": "ssvc", "content": { "id": "CVE-2024-49940", "role": "CISA Coordinator", "options": [ { "Exploitation": "none" }, { "Automatable": "no" }, { "Technical Impact": "partial" } ], "version": "2.0.3", "timestamp": "2024-10-22T13:37:52.827630Z" } } } ], "title": "CISA ADP Vulnrichment", "providerMetadata": { "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0", "shortName": "CISA-ADP", "dateUpdated": "2024-10-22T13:38:50.656Z" } } ] } }