Linux: Observable Discrepancy
CVE-2025-39702 is a high-severity observable discrepancy vulnerability in Linux, with a CVSS 3.1 base score of 7.0 out of 10. It can be exploited with local access to the system by an attacker with a low-privileged account, without any action from a user. The record lists versions ff55a452d56490047f5233cc48c5d933f8586884, 3b348c9c8d2ca2c67559ffd0e258ae7e1107d4f0, 86b6d34717fe0570afce07ee79b8eeb40341f831 as the first without the flaw, so updating Linux to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.45% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update Linux to ff55a452d56490047f5233cc48c5d933f8586884 or 3b348c9c8d2ca2c67559ffd0e258ae7e1107d4f0 or 86b6d34717fe0570afce07ee79b8eeb40341f831 or 3ddd55cf19ed6cc62def5e3af10c2a9df1b861c3, whichever matches your release line, or a later version.
- Update SIMATIC CN 4100 to V5.0 or a later version.
- Update Linux Kernel to 5.15.190 or 6.1.149 or 6.6.103 or 6.12.44, whichever matches your release line, or a later version.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Linux | Linux | bf355b8d2c30a289232042cacc1cfaea4923936c up to, but not including, ff55a452d56490047f5233cc48c5d933f8586884; bf355b8d2c30a289232042cacc1cfaea4923936c up to, but not including, 3b348c9c8d2ca2c67559ffd0e258ae7e1107d4f0; bf355b8d2c30a289232042cacc1cfaea4923936c up to, but not including, 86b6d34717fe0570afce07ee79b8eeb40341f831; bf355b8d2c30a289232042cacc1cfaea4923936c up to, but not including, 3ddd55cf19ed6cc62def5e3af10c2a9df1b861c3; bf355b8d2c30a289232042cacc1cfaea4923936c up to, but not including, b3967c493799e63f648e9c7b6cb063aa2aed04e7; bf355b8d2c30a289232042cacc1cfaea4923936c up to, but not including, f7878d47560d61e3f370aca3cebb8f42a55b990a; bf355b8d2c30a289232042cacc1cfaea4923936c up to, but not including, a458b2902115b26a25d67393b12ddd57d1216aaa; 4.10 | ff55a452d56490047f5233cc48c5d933f8586884, 3b348c9c8d2ca2c67559ffd0e258ae7e1107d4f0, 86b6d34717fe0570afce07ee79b8eeb40341f831, 3ddd55cf19ed6cc62def5e3af10c2a9df1b861c3, b3967c493799e63f648e9c7b6cb063aa2aed04e7, f7878d47560d61e3f370aca3cebb8f42a55b990a |
| Siemens | SIMATIC CN 4100 | before V5.0 | V5.0 |
| Linux | Linux Kernel | 4.10 up to, but not including, 5.15.190; 5.16 up to, but not including, 6.1.149; 6.2 up to, but not including, 6.6.103; 6.7 up to, but not including, 6.12.44; 6.13 up to, but not including, 6.16.4; 6.17 | 5.15.190, 6.1.149, 6.6.103, 6.12.44, 6.16.4 |
| Debian | Debian Linux | 11.0 | Not stated |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- LocalThe attacker needs local access or must trick a user into running something.
- Attack complexity
- HighSuccess depends on conditions outside the attacker's control.
- Privileges required
- LowA basic user account is enough.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 9.8 | Critical | CNA |
| CVSS 3.1 | 7.0 | High | NVD |
Official description
In the Linux kernel, the following vulnerability has been resolved: ipv6: sr: Fix MAC comparison to be constant-time To prevent timing attacks, MACs need to be compared in constant time. Use the appropriate helper function for this.
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Observable Discrepancy
The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor.
How developers prevent this class of flaw
- Compartmentalize the system to have safe areas where trust boundaries can be unambiguously drawn. Do not allow sensitive data to go outside of the trust boundary and always be careful when interfacing with a compartment outside of the safe area. Ensure that appropriate compartmentalization is built into the system design, and the compartmentalization allows for and reinforces privilege separation functionality. Architects and designers should rely on the principle of least privilege to decide the appropriate time to use privileges and the time to drop privileges.
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
- git.kernel.org/stable/c/3b348c9c8d2ca2c67559ffd0e258ae7e1107d4f0
- git.kernel.org/stable/c/3ddd55cf19ed6cc62def5e3af10c2a9df1b861c3
- git.kernel.org/stable/c/86b6d34717fe0570afce07ee79b8eeb40341f831
- git.kernel.org/stable/c/a458b2902115b26a25d67393b12ddd57d1216aaa
- git.kernel.org/stable/c/b3967c493799e63f648e9c7b6cb063aa2aed04e7
- git.kernel.org/stable/c/f7878d47560d61e3f370aca3cebb8f42a55b990a
- git.kernel.org/stable/c/ff55a452d56490047f5233cc48c5d933f8586884
Third-party and government advisories
- lists.debian.org/debian-lts-announce/2025/10/msg00008.htmlMailing List, Third Party Advisory
Other references
Recent Observable Discrepancy vulnerabilities
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 9.8 Critical severity
- 7.8 High severity
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