[CVE-2026-64306][MODERATE 7.0] crypto: drbg - Fix returning success on failure in CTR_DRBG

[CVE-2026-64306][MODERATE 7.0] crypto: drbg - Fix returning success on failure in CTR_DRBG

[AL-KERNEL]

CVE: CVE-2026-64306
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: crypto: drbg - Fix returning success on failure in CTR_DRBG
Commit: 074db6db03a0aaa78f05ca9d4838053713796665
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=074db6db03a0aaa78f05ca9d4838053713796665
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64306
Analysis date: Sat, 25 Jul 2026 10:28:46 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
CTR_DRBG could incorrectly return success after drbg_ctr_update() failed, allowing callers to consume uninitialized or stale RNG output as valid random data with possible confidentiality or integrity impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64306 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64306

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE 7.0
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64306	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N';CWE-393;CWE-252;*CWE-457;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '6.3';DESCR 'CTR_DRBG could incorrectly report success when drbg_ctr_update failed while processing additional input, leaving the requested RNG output buffer uninitialized or containing stale data. A caller may then consume invalid random output as if it was generated successfully, which can weaken cryptographic keys, nonces, or other security sensitive values depending on the consumer. For the CVSS the PR:L is used for the paranoid score because reliable triggering is expected to require a local process or access to a local crypto RNG interface rather than a remote network packet. The issue is not directly network reachable. Impact is not a crash only issue. It may affect confidentiality or integrity if stale buffer data is exposed or if weak random output is used in security decisions.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-64306 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: cde001e4c3c3 YES  NO NO unknown 	MAYBE	DANGER SIMPLEFIX LINUS KPANIC 	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process may reach DRBG generation paths through local crypto consumers or user reachable crypto interfaces, depending on configuration.
* Confidentiality impact plausible: +1. Returning success with uninitialized or stale RNG output can expose previous buffer contents or cause consumers to use non-random data as secret material.
* Integrity impact plausible: +1. Weak or invalid random output can affect keys, nonces, signatures, tokens, or other security decisions.
* Broad/default/common subsystem: +1. DRBG is a core crypto RNG component and can be used by multiple kernel or userspace-facing crypto consumers.
* Hard or unreliable condition: -1. The issue requires drbg_ctr_update() to fail while additional input is present, which is not an ordinary fast path trigger.

Paranoid delta:

* Local unprivileged high-control crypto API concern: +1. If AF_ALG or another local crypto RNG interface exposes controllable generation requests with additional input, a local user may have more practical influence over the affected path.

No memory corruption, UAF, OOB write, arbitrary write, or page-cache corruption primitive is shown by the patch.

## 3. Reachability analysis

The bug is local, not directly network reachable. The most realistic trigger requires a local caller or a kernel crypto consumer that invokes CTR_DRBG generation with additional input and then encounters a failure in drbg_ctr_update(). Namespaces do not obviously create a new privilege boundary here, but containerized or sandboxed workloads may still reach crypto APIs if exposed by the host configuration. The affected component is security sensitive because callers trust a zero return value as successful RNG generation.

Call-site confidence: medium. The changed function and error behavior are visible, but concrete external callers and exact failure paths are not included in the provided patch.

## 4. Severity interpretation

This is not an ordinary crash-only Moderate issue. It behaves like an actionable Moderate crypto correctness flaw because it can cause consumers to accept uninitialized or stale output as valid random data. Theoretical impact includes confidentiality or integrity degradation, especially if the output is used for keys or nonces. Realistic exploitation is less certain because the attacker must influence or coincide with a failing update path, and the patch does not show a direct LPE or memory corruption primitive. It is therefore below a strong Important candidate, but should not be auto-closed without manual review.

## 5. One-sentence report phrase

CTR_DRBG could incorrectly return success after drbg_ctr_update() failed, allowing callers to consume uninitialized or stale RNG output as valid random data with possible confidentiality or integrity impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. This is not memory corruption, but crypto RNG output correctness affects security-sensitive consumers and the real impact depends on reachable callers and whether local users can trigger the additional-input failure path.

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: crypto: drbg - Fix returning success on failure in CTR_DRBG
Commit: 074db6db03a0aaa78f05ca9d4838053713796665
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=074db6db03a0aaa78f05ca9d4838053713796665

Commit description:

drbg_ctr_generate() sometimes returns success when it fails, leaving the
output buffer uninitialized.  Fix it.

Fixes: cde001e ("crypto: rng - RNGs must return 0 in success case")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  crypto/drbg.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=074db6db03a0aaa78f05ca9d4838053713796665

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DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-64306 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64306

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
  The Best / paranoid CVSS score: 6.3

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 4
  Paranoid ActionableScore: 5

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).