CVE-2026-23409

Description

In the Linux kernel, the following vulnerability has been resolved: apparmor: fix differential encoding verification Differential encoding allows loops to be created if it is abused. To prevent this the unpack should verify that a diff-encode chain terminates. Unfortunately the differential encode verification had two bugs. 1. it conflated states that had gone through check and already been marked, with states that were currently being checked and marked. This means that loops in the current chain being verified are treated as a chain that has already been verified. 2. the order bailout on already checked states compared current chain check iterators j,k instead of using the outer loop iterator i. Meaning a step backwards in states in the current chain verification was being mistaken for moving to an already verified state. Move to a double mark scheme where already verified states get a different mark, than the current chain being kept. This enables us to also drop the backwards verification check that was the cause of the second error as any already verified state is already marked.

CVSS Details

CVSS Score

5.5

Severity

MEDIUM

CVSS Vector

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Configurations (Affected Products)

cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* - VULNERABLE

Linux Kernel (Stable branches prior to commit 0fab44285445e9012674396d5c1236a67da518e0)

Linux Kernel (Stable branches prior to commit 1439150cd3c411228b387ab5efca92199d2a659a)

Linux Kernel (Mainline versions prior to fix)

PoC / Exploit Code

⚠ For Security Research Only

The following code is for security research and authorized testing only.

python

/*
 * PoC for CVE-2026-23409
 * This code demonstrates the concept of triggering the vulnerability.
 * Actual exploitation requires a crafted AppArmor profile binary.
 */

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char **argv) {
    int fd;
    char *profile_data = "...malformed_diff_encoding_blob...";
    
    printf("[+] Attempting to load malicious AppArmor profile...\n");
    
    // In a real exploit, 'profile_data' would contain the specific
    // differential encoding structure that bypasses the verification loop check.
    // This would typically be written to a file and loaded via apparmor_parser
    // or directly interacting with the kernel interface if permissions allow.
    
    // Example: system("apparmor_parser -r malicious_profile");
    
    printf("[-] If the kernel is vulnerable, the system may hang now.\n");
    return 0;
}

References

[1] CVE.org https://www.cve.org/CVERecord?id=CVE-2026-23409
[2] NVD NIST https://nvd.nist.gov/vuln/detail/CVE-2026-23409
[3] CVE Details https://www.cvedetails.com/cve/CVE-2026-23409/
[4] VulDB https://vuldb.com/cve/CVE-2026-23409
[5] https://git.kernel.org/stable/c/0fab44285445e9012674396d5c1236a67da518e0
[6] https://git.kernel.org/stable/c/1439150cd3c411228b387ab5efca92199d2a659a
[7] https://git.kernel.org/stable/c/1ff4857fac56ac5a90ee63b24db05fa5e91a45aa
[8] https://git.kernel.org/stable/c/34fc60b125ed1d4eb002c76b0664bf0619492167
[9] https://git.kernel.org/stable/c/39440b137546a3aa383cfdabc605fb73811b6093
[10] https://git.kernel.org/stable/c/623a9d211bbbb031bb1cbdb38b23487648167f8a
[11] https://git.kernel.org/stable/c/f90e3ecd9e1ed69f1a370f866ceed1f104f3ab4a
[12] https://git.kernel.org/stable/c/ff7c73744fafe944ed9a7b2b7cf6c8d5557a3d84

Raw JSON Data

JSON

{"cve": {"id": "CVE-2026-23409", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-04-01T09:16:16.913", "lastModified": "2026-04-24T15:23:55.877", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\napparmor: fix differential encoding verification\n\nDifferential encoding allows loops to be created if it is abused. To\nprevent this the unpack should verify that a diff-encode chain\nterminates.\n\nUnfortunately the differential encode verification had two bugs.\n\n1. it conflated states that had gone through check and already been\n marked, with states that were currently being checked and marked.\n This means that loops in the current chain being verified are treated\n as a chain that has already been verified.\n\n2. the order bailout on already checked states compared current chain\n check iterators j,k instead of using the outer loop iterator i.\n Meaning a step backwards in states in the current chain verification\n was being mistaken for moving to an already verified state.\n\nMove to a double mark scheme where already verified states get a\ndifferent mark, than the current chain being kept. This enables us\nto also drop the backwards verification check that was the cause of\nthe second error as any already verified state is already marked."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H", "baseScore": 5.5, "baseSeverity": "MEDIUM", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 1.8, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-835"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "4.17.1", "versionEndExcluding": "5.10.253", "matchCriteriaId": "E395B1A0-3F3F-4299-9E96-FE513DCFE882"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.11", "versionEndExcluding": "5.15.203", "matchCriteriaId": "20DDB3E9-AABF-4107-ADB0-5362AA067045"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "5.16", "versionEndExcluding": "6.1.169", "matchCriteriaId": "DBEC0E5D-641C-4E98-A6D9-5799B10CE451"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.2", "versionEndExcluding": "6.6.130", "matchCriteriaId": "C57BB918-DF28-46B3-94F7-144176841267"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.7", "versionEndExcluding": "6.12.77", "matchCriteriaId": "B3D12E00-E42D-4056-B354-BAD4903C03A5"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.13", "versionEndExcluding": "6.18.18", "matchCriteriaId": "346AD1FB-0CE8-4D9D-8E56-5EB1A4D06199"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.19", "versionEndExcluding": "6.19.8", "matchCriteriaId": "C65A7D85-C7C6-485E-AC35-66A374C73FAC"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:4.17:-:*:*:*:*:*:*", "matchCriteriaId": "3F438846-FE97-43DC-A655-B5EF8DED552E"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc1:*:*:*:*:*:*", "matchCriteriaId": "F253B622-8837-4245-BCE5-A7BF8FC76A16"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc2:*:*:*:*:*:*", "matchCriteriaId": "4AE85AD8-4641-4E7C-A2F4-305E2CD9EE64"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc3:*:*:*:*:*:*", "matchCriteriaId": "F666C8D8-6538-46D4-B318-87610DE64C34"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc4:*:*:*:*:*:*", "matchCriteriaId": "02259FDA-961B-47BC-AE7F-93D7EC6E90C2"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc5:*:*:*:*:*:*", "matchCriteriaId": "58A9FEFF-C040-420D-8F0A-BFDAAA1DF258"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc6:*:*:*:*:*:*", "matchCriteriaId": "1D2315C0-D46F-4F85-9754-F9E5E11374A6"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:7.0:rc7:*:*:*:*:*:*", "matchCriteriaId": "512EE3A8-A590-4501-9A94-5D4B268D6138"}]}]}], "references": [{"url": "https://git.kernel.org/stable/c/0fab44285445e9012674396d5c1236a67da518e0", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/1439150cd3c411228b387ab5efca92199d2a659a", "source": "416baaa9-dc9f-4396-8d ... (truncated)