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CVE Record

CVE-2025-39804: lib/crypto: arm64/poly1305: Fix register corruption in no-SIMD contexts

In the Linux kernel, the following vulnerability has been resolved: lib/crypto: arm64/poly1305: Fix register corruption in no-SIMD contexts Restore the SIMD usability check that was removed by commit a59e5468a921 ("crypto: arm64/poly1305 - Add block-only interface"). This safety check is cheap and is well worth eliminating a footgun. While the Poly1305 functions should not be called when SIMD registers are unusable, if they are anyway, they should just do the right thing instead of corrupting random tasks' registers and/or computing incorrect MACs. Fixing this is also needed for poly1305_kunit to pass. Just use may_use_simd() instead of the original crypto_simd_usable(), since poly1305_kunit won't rely on crypto_simd_disabled_for_test.

HighCVSS 7.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

On ARM64 Linux systems, a Poly1305 cryptographic routine can use SIMD registers when they are unavailable. This may corrupt another task’s register state or calculate incorrect message authentication codes, potentially causing crashes, instability, or authentication failures. The supplied assessment rates it high at CVSS 7.5, but the sources do not establish code execution.

Executive priority

Treat this as an accelerated patching priority for ARM64 Linux fleets, especially production systems relying on kernel cryptography. It is not supported as an all-platform emergency because the issue is architecture-specific and no active exploitation is documented. Potential cross-task register corruption makes delayed remediation undesirable even without confirmed attacks.

Technical view

Commit a59e5468a921 removed a SIMD usability check from the ARM64 Poly1305 implementation. If Poly1305 is called in a no-SIMD context, register corruption or incorrect MAC computation can result. The referenced fixes restore the guard using may_use_simd(). The source specifically identifies ARM64 and notes that the fix is required for poly1305_kunit to pass.

Likely exposure

Exposure is limited to ARM64 Linux kernels containing the a59e5468a921 regression without a referenced fix or equivalent backport. The bundle associates the issue with Linux 6.16 and 6.17-era data, but its flattened version list does not preserve clear range boundaries. Confirm precise status with the kernel or distribution vendor.

Exploitation context

The CVE is not listed in KEV, and the supplied sources provide no evidence of active exploitation, a public exploit, or confirmed remote reachability. Although the CVSS vector uses AV:N, that alone does not prove remote triggerability. Exploitation would depend on a reachable path invoking ARM64 Poly1305 while SIMD registers are unusable.

Researcher notes

The normalized affected-version data is ambiguous: it lists 6.16, 0, 6.16.4, and 6.17 without preserved range operators. Use commit ancestry and vendor backport records for authoritative exposure decisions. No CWE is supplied. The available description establishes unsafe state corruption and incorrect MACs but does not explain a reliable external trigger or security-boundary bypass.

Mitigation direction

  • Update affected ARM64 systems to a vendor kernel containing either referenced fix or an equivalent backport.
  • Confirm downstream distribution kernels have backported the SIMD usability check; version numbers alone may be insufficient.
  • If no corrected package is available, obtain interim mitigation guidance from the kernel or distribution vendor.
  • Prioritize ARM64 systems using kernel Poly1305 functionality or exposed services that depend heavily on kernel cryptography.

Validation and detection

  • Inventory kernel versions, architectures, package revisions, and downstream patch status across Linux systems.
  • Verify patched source includes the may_use_simd() guard or an equivalent vendor backport.
  • Compare vendor build changelogs against the two referenced kernel commits.
  • Run the Poly1305 KUnit tests where supported; passing supports, but does not independently prove, remediation.
Prepared
Confidence
medium
Sources
4

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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CVE-2025-39804 mapping review

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Vulnerability profileCVE Program record
Severity
High
CVSS
7.5 (3.1)
Known Exploited
No
Published

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

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
3Timeline events
0ADP providers
3Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
7.5CVSS 3.1HighCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H3.93.6Linux

Vulnerability scoring details

Base CVSS 3.1 score

7.5High
CVSS 3.1 vector shape for CVE-2025-39804Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

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

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinuxa59e5468a921937cb7317892779c67046ad9f5cc, a59e5468a921937cb7317892779c67046ad9f5ccunaffected
LinuxLinux6.16, 0, 6.16.4, 6.17affected
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.