{"dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": {"cveId": "CVE-2026-63981", "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "state": "PUBLISHED", "assignerShortName": "Linux", "dateReserved": "2026-07-19T07:54:57.025Z", "datePublished": "2026-07-19T14:56:04.580Z", "dateUpdated": "2026-07-19T14:56:04.580Z"}, "containers": {"cna": {"providerMetadata": {"orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "shortName": "Linux", "dateUpdated": "2026-07-19T14:56:04.580Z"}, "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow\n\ntcf_mirred_act() checks sched_mirred_nest against MIRRED_NEST_LIMIT (4)\nto prevent deep recursion. However, when the action uses blockcast\n(tcfm_blockid != 0), the function returns at the tcf_blockcast() call\nBEFORE reaching the counter increment. As a result, the recursion\ncounter never advances and the limit check is entirely bypassed.\n\nWhen two devices share a TC egress block with a mirred blockcast rule,\na packet egressing on device A is mirrored to device B via blockcast;\ndevice B's egress TC re-enters tcf_mirred_act() via blockcast and\nmirrors back to A, creating an unbounded recursion loop:\n\n tcf_mirred_act -> tcf_blockcast -> tcf_mirred_to_dev -> dev_queue_xmit\n -> sch_handle_egress -> tcf_classify -> tcf_mirred_act -> (repeat)\n\nThis recursion continues until the kernel stack overflows.\n\nThe bug is reachable from an unprivileged user via\nunshare(CLONE_NEWUSER | CLONE_NEWNET): user namespaces grant\nCAP_NET_ADMIN in the new network namespace, which is sufficient to\ncreate dummy devices, attach clsact qdiscs with shared blocks, and\ninstall mirred blockcast filters.\n\n BUG: TASK stack guard page was hit at ffffc90000b7fff8\n Oops: stack guard page: 0000 [#1] SMP KASAN NOPTI\n CPU: 2 UID: 1000 PID: 169 Comm: poc Not tainted 7.0.0-rc7-next-20260410\n RIP: 0010:xas_find+0x17/0x480\n Call Trace:\n xa_find+0x17b/0x1d0\n tcf_mirred_act+0x640/0x1060\n tcf_action_exec+0x400/0x530\n basic_classify+0x128/0x1d0\n tcf_classify+0xd83/0x1150\n tc_run+0x328/0x620\n __dev_queue_xmit+0x797/0x3100\n tcf_mirred_to_dev+0x7b1/0xf70\n tcf_mirred_act+0x68a/0x1060\n [repeating ~30+ times until stack overflow]\n Kernel panic - not syncing: Fatal exception in interrupt\n\nFix this by incrementing sched_mirred_nest before calling\ntcf_blockcast() and decrementing it on return, mirroring the\nnon-blockcast path. This ensures subsequent recursive entries see the\nupdated counter and are correctly limited by MIRRED_NEST_LIMIT."}], "affected": [{"product": "Linux", "vendor": "Linux", "defaultStatus": "unaffected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": ["net/sched/act_mirred.c"], "versions": [{"version": "906736728cea480a85803c67fafb1b0e78491922", "lessThan": "25fc9352590f5ef21ebf290432bd768b336693bc", "status": "affected", "versionType": "git"}, {"version": "fe946a751d9b52b7c45ca34899723b314b79b249", "lessThan": "34457de389fb64a01fdcc71177dfebe65fd2d362", "status": "affected", "versionType": "git"}, {"version": "fe946a751d9b52b7c45ca34899723b314b79b249", "lessThan": "a005fa5d7502eefec7ee6e1c01adadc06de2f9ad", "status": "affected", "versionType": "git"}]}, {"product": "Linux", "vendor": "Linux", "defaultStatus": "affected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": ["net/sched/act_mirred.c"], "versions": [{"version": "6.19", "status": "affected"}, {"version": "0", "lessThan": "6.19", "status": "unaffected", "versionType": "semver"}, {"version": "7.0.12", "lessThanOrEqual": "7.0.*", "status": "unaffected", "versionType": "semver"}, {"version": "7.1", "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": "6.19", "versionEndExcluding": "7.0.12"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.19", "versionEndExcluding": "7.1"}]}]}], "references": [{"url": "https://git.kernel.org/stable/c/25fc9352590f5ef21ebf290432bd768b336693bc"}, {"url": "https://git.kernel.org/stable/c/34457de389fb64a01fdcc71177dfebe65fd2d362"}, {"url": "https://git.kernel.org/stable/c/a005fa5d7502eefec7ee6e1c01adadc06de2f9ad"}], "title": "net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow", "x_generator": {"engine": "bippy-1.2.0"}}}}

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{}

{"bugzilla": {"description": "kernel: net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow", "id": "2502417", "url": "https://bugzilla.redhat.com/show_bug.cgi?id=2502417"}, "csaw": false, "cvss3": {"cvss3_base_score": "7.0", "cvss3_scoring_vector": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H", "status": "draft"}, "cwe": "CWE-770", "details": ["In the Linux kernel, the following vulnerability has been resolved:\nnet/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow\ntcf_mirred_act() checks sched_mirred_nest against MIRRED_NEST_LIMIT (4)\nto prevent deep recursion. However, when the action uses blockcast\n(tcfm_blockid != 0), the function returns at the tcf_blockcast() call\nBEFORE reaching the counter increment. As a result, the recursion\ncounter never advances and the limit check is entirely bypassed.\nWhen two devices share a TC egress block with a mirred blockcast rule,\na packet egressing on device A is mirrored to device B via blockcast;\ndevice B's egress TC re-enters tcf_mirred_act() via blockcast and\nmirrors back to A, creating an unbounded recursion loop:\ntcf_mirred_act -> tcf_blockcast -> tcf_mirred_to_dev -> dev_queue_xmit\n-> sch_handle_egress -> tcf_classify -> tcf_mirred_act -> (repeat)\nThis recursion continues until the kernel stack overflows.\nThe bug is reachable from an unprivileged user via\nunshare(CLONE_NEWUSER | CLONE_NEWNET): user namespaces grant\nCAP_NET_ADMIN in the new network namespace, which is sufficient to\ncreate dummy devices, attach clsact qdiscs with shared blocks, and\ninstall mirred blockcast filters.\nBUG: TASK stack guard page was hit at ffffc90000b7fff8\nOops: stack guard page: 0000 [#1] SMP KASAN NOPTI\nCPU: 2 UID: 1000 PID: 169 Comm: poc Not tainted 7.0.0-rc7-next-20260410\nRIP: 0010:xas_find+0x17/0x480\nCall Trace:\nxa_find+0x17b/0x1d0\ntcf_mirred_act+0x640/0x1060\ntcf_action_exec+0x400/0x530\nbasic_classify+0x128/0x1d0\ntcf_classify+0xd83/0x1150\ntc_run+0x328/0x620\n__dev_queue_xmit+0x797/0x3100\ntcf_mirred_to_dev+0x7b1/0xf70\ntcf_mirred_act+0x68a/0x1060\n[repeating ~30+ times until stack overflow]\nKernel panic - not syncing: Fatal exception in interrupt\nFix this by incrementing sched_mirred_nest before calling\ntcf_blockcast() and decrementing it on return, mirroring the\nnon-blockcast path. This ensures subsequent recursive entries see the\nupdated counter and are correctly limited by MIRRED_NEST_LIMIT.", "A flaw was found in the Linux kernel's `net/sched: act_mirred` component. An unprivileged local user, by leveraging user namespaces to gain network administration capabilities, can create a specific network configuration that triggers an unbounded recursion. This recursion bypasses a depth limit check, leading to a kernel stack overflow. Successful exploitation of this vulnerability results in a kernel panic, effectively causing a Denial of Service (DoS) on the system."], "name": "CVE-2026-63981", "package_state": [{"cpe": "cpe:/a:redhat:hummingbird:1", "fix_state": "Not affected", "impact": "important", "package_name": "erlang27", "product_name": "Red Hat Hardened Images"}], "public_date": "2026-07-19T00:00:00Z", "references": ["https://www.cve.org/CVERecord?id=CVE-2026-63981\nhttps://nvd.nist.gov/vuln/detail/CVE-2026-63981\nhttps://lore.kernel.org/linux-cve-announce/2026071902-CVE-2026-63981-1d04@gregkh/T"], "threat_severity": "Moderate"}

{"affected": [{"configurations": [{"platform": null, "status": "affected", "versions": {"scheme": "code_commit", "value": "[906736728cea480a85803c67fafb1b0e78491922,25fc9352590f5ef21ebf290432bd768b336693bc)"}}, {"platform": null, "status": "affected", "versions": {"scheme": "code_commit", "value": "[fe946a751d9b52b7c45ca34899723b314b79b249,34457de389fb64a01fdcc71177dfebe65fd2d362)"}}, {"platform": null, "status": "affected", "versions": {"scheme": "code_commit", "value": "[fe946a751d9b52b7c45ca34899723b314b79b249,a005fa5d7502eefec7ee6e1c01adadc06de2f9ad)"}}], "enrichment": {"confidence": 99.0, "confidence_source": "inferred", "scores": [{"score": 99.0, "source": "inferred"}, {"score": 100.0, "source": "matching"}]}, "original": {"product": "Linux", "source": "cna", "vendor": "Linux"}, "product": "linux_kernel", "vendor": "linux"}, {"configurations": [{"platform": null, "status": "affected", "versions": {"scheme": "generic", "value": "6.19"}}, {"platform": null, "status": "unaffected", "versions": {"scheme": "generic", "value": "[0,6.19)"}}, {"platform": null, "status": "unaffected", "versions": {"scheme": "generic", "value": "[7.0.12,7.0.*]"}}, {"platform": null, "status": "unaffected", "versions": {"scheme": "generic", "value": "[7.1,*]"}}], "enrichment": {"confidence": 99.0, "confidence_source": "inferred", "scores": [{"score": 99.0, "source": "inferred"}, {"score": 100.0, "source": "matching"}]}, "original": {"product": "Linux", "source": "cna", "vendor": "Linux"}, "product": "linux_kernel", "vendor": "linux"}], "created": "2026-07-21T16:00:08.544947+00:00", "updated": "2026-07-22T18:15:03.252843+00:00", "vendors": ["linux", "linux$PRODUCT$linux_kernel"], "weaknesses": ["CWE-674", "CWE-734"]}