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OpenSSL CVE-2026-54873

| EUVDEUVD-2026-89005 HIGH
Allocation of Resources Without Limits or Throttling (CWE-770)
2026-09-29 GHSA-8jj8-6qjx-8659
Denial Of Service OpenSSL
7.5
CVSS 3.1 · Vendor
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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

  1. Access
    Reach OpenSSL QUIC server endpoint
  2. Delivery
    Send crafted QUIC STREAM fragments
  3. Exploit
    Trigger unbounded metadata resource allocation
  4. Execution
    Exhaust server memory/CPU
  5. Impact
    Deny service to legitimate clients

Vulnerability Assessment

Weakness
CWE
CWE-770
OWASP Top 10:2025
A06:2025 - Insecure Design
ASVS 5.0
V2 - Validation and Business Logic Resource allocation limits
MITRE ATT&CK

1 technique across 1 tactic: Impact

The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up.

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Exploitation Conditions

Exploitation requires the target to be running an OpenSSL-based QUIC server using the vulnerable native QUIC stack in affected versions (OpenSSL 4.0.0-4.0.2, 3.4.0-3.4.7, 3.5.0-3.5.8, 3.6.0-3.6.4); the flaw is in QUIC STREAM fragment metadata processing, so endpoints that only perform classic TLS termination are NOT exposed. When QUIC is enabled, no authentication, user interaction, or special privileges are needed (PR:N/UI:N/AC:L) and the attack is remote (AV:N) against default QUIC configurations, with impact limited to availability (service degradation/denial, C:N/I:N). The primary limiting factor is adoption of OpenSSL's QUIC server functionality, which is comparatively rare relative to OpenSSL's overall footprint.
Impact

This is a classic high-CVSS-but-moderate-real-risk resource-exhaustion flaw. The CVSS 3.1 base of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) correctly reflects an unauthenticated, network-reachable, low-complexity denial-of-service with availability-only impact - no confidentiality or integrity exposure, consistent with CWE-770 (uncontrolled resource allocation) in the QUIC STREAM fragment metadata handling. Multi-source signals temper the raw score: SSVC from CISA records Exploitation=none and Technical Impact=partial, and there is no CISA KEV listing, no EPSS score provided, and no public exploit identified at time of analysis. The one aggravating SSVC factor is Automatable=yes, meaning that once an exploit exists it could be scripted broadly against QUIC endpoints. The dominant limiting factor is NOT the CVSS vector but the feature gate: this bug lives in OpenSSL's native QUIC stack, which is a relatively new and sparsely-adopted code path - most OpenSSL deployments terminate TLS, not QUIC, so the actually-exposed population is far smaller than OpenSSL's ubiquitous install base implies. Net: a legitimate, patch-now item for any service running OpenSSL-based QUIC servers, but low urgency for the overwhelming majority of OpenSSL users who do not enable QUIC.
Exploit ScenarioDerived

An attacker who can reach an OpenSSL-based QUIC server endpoint sends crafted QUIC STREAM frames whose fragment metadata forces the server to allocate/retain resources without proper limits, degrading or exhausting server availability. Exploitation is remote and unauthenticated (PR:N/UI:N) with low complexity, but requires the target to actually be serving QUIC via the vulnerable OpenSSL stack. No public proof-of-concept exploit code was identified at time of analysis.
This Week Schedule remediation this week. Why: HIGH severity (CVSS 7.0-8.9).
Patch available 0.5% exploitation probability (EPSS)

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AnalysisAIv1 – Oct 01, 2026

OpenSSL 3.4.x, 3.5.x, 3.6.x and 4.0.x servers that enable the native QUIC stack can be driven into denial of service through unbounded memory consumption in QUIC STREAM fragment metadata handling (CVE-2026-54873, CWE-770). Remote, unauthenticated attackers (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, 7.5) can send crafted QUIC streams that cause excessive allocation and service degradation or exhaustion; confidentiality and integrity are unaffected. Exploitation is assessed as not observed (SSVC: Exploitation none; no CISA KEV listing) and no public exploit code has been identified at time of analysis, but the attack is automatable. Risk is materially bounded by the relatively narrow deployment base of OpenSSL's QUIC server functionality - classic TLS termination endpoints are not exposed.

RemediationAI

Upgrade to a fixed OpenSSL release: 4.0.3 for the 4.0.x line, and the corresponding maintenance releases 3.4.8, 3.5.9, and 3.6.5 for the older branches (the highest fixed version on each branch per the EUVD affected-version ranges; 4.0.3 is the explicitly named release at https://github.com/openssl/openssl/releases/tag/openssl-4.0.3, with the vendor advisory at https://openssl-library.org/news/secadv/20260929.txt). Vendor-released patch: 4.0.3 (and branch equivalents). Where immediate upgrade is not possible, the most effective compensating control is to disable OpenSSL's native QUIC/HTTP-3 listener entirely - do not bind a QUIC UDP port, or build/configure the stack without QUIC support - which removes the attack surface at the cost of losing HTTP/3 and any QUIC-based client functionality; note this is a hard trade-off, not a partial mitigation. If QUIC must stay enabled, front the endpoint with infrastructure that enforces per-connection and per-peer limits before traffic reaches OpenSSL: cap concurrent QUIC connections and streams per source, cap inbound UDP payload and datagram rates, and apply UDP rate limiting on the QUIC port to throttle the automated trigger. Because SSVC rates this as Automatable: yes, also monitor for elevated memory growth and QUIC connection churn to detect attempted exhaustion early, and inventory embedded or statically linked OpenSSL copies so the fix is applied everywhere the vulnerable versions are present, not only in the system package.

Affected VersionsENISA

OpenSSL 4.0.0 <4.0.3
OpenSSL 3.4.0 <3.4.8
OpenSSL 3.5.0 <3.5.9
OpenSSL 3.6.0 <3.6.5

Affected Packages

euvd openssl fix: 3.4.8, 3.5.9, 3.6.5, 4.0.3

Source: OSV (Open Source Vulnerabilities)

Affects your stack

Affects 1 package in euvd

Checking this CVE against your own stacks and servers is part of the Basic plan and up.

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Technical ContextAI · show details

OpenSSL's native QUIC implementation (QUIC-TLS plus the multi-stream transport shipped in the 3.2/3.4+ line and the modernized stack in 4.0) parses STREAM frames carrying fragmented stream data. The flaw is an instance of CWE-770, Allocation of Resources Without Limits or Throttling: fragment metadata associated with QUIC STREAM frames is not adequately bounded, so a peer can force the server to retain and allocate ever-growing per-fragment state without backpressure. CWE-770 is the same root-cause class as several sibling issues patched in the same OpenSSL 4.0.3 release (for example CVE-2026-42772, O(n^2) QUIC fragment reassembly CPU exhaustion, and CVE-2026-35189, excessive allocation in relative CRLDP processing), which points to a broad hardening effort around allocation limits across the codebase. Because the defect lives in QUIC transport/fragment handling rather than in TLS record processing or X.509 parsing, only endpoints that actually terminate QUIC sessions are affected; code paths used for classic TLS or DTLS connections do not reach the vulnerable logic. The CVSS vector AV:N/AC:L/PR:N/UI:N with C:N/I:N/A:H confirms a remotely reachable, unauthenticated, default-configuration availability-only impact, and CISA's SSVC evaluation (Automatable: yes, Technical Impact: partial) indicates an attacker could script the trigger at scale without gaining control of the process.

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Severity by source

Vendor (CNA) PRIMARY 7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
vuln.today AI 7.5 HIGH

Remote unauthenticated QUIC endpoint, low complexity, no interaction; uncontrolled resource allocation yields availability-only impact with no confidentiality or integrity effect.

3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
SUSE 5.3 MEDIUM
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Red Hat 5.3 MEDIUM

Primary rating from Vendor (CNA).

CVSS VectorVendor

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

Lifecycle Timeline

3
CVSS changed
Sep 30, 2026 - 22:08 NVD
7.5 (None) 7.5 (HIGH)
CVE Published
Sep 29, 2026 - 13:53 github-releases
HIGH 7.5
CVE Published
Sep 29, 2026 - 13:53 github-releases
UNKNOWN (no severity yet)

Description PRE-NVD

Disclosed via GitHub release of openssl/openssl. NVD scoring and full description are pending.

Executive ViewAI

Why It Matters

OpenSSL's native QUIC server stack (versions 3.4.x, 3.5.x, 3.6.x, and 4.0.x) contains a high-severity flaw that allows unauthenticated remote attackers to exhaust server memory, causing denial of service. Organizations that do not use OpenSSL's QUIC server functionality are not exposed, so overall business risk is limited to a narrow set of QUIC-based services. Leadership should ensure affected teams prioritize patching and verify whether QUIC is enabled in their environments.

Business Impact: Medium
Compensating Controls

Apply vendor patch.

Threat Intelligence Overview

EPSS Score
0.5%
Top 62% exploitable
Prevalence
Ubiquitous
OpenSSL underpins TLS across virtually all server environments
Data Quality
8 sources
Confidence: High
EUVD 🇯🇵JVN NVD CVE.org EPSS Vulnrichment MSRC Vendor GitHub VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity HIGH
CISA KEV No
Public POC No
EPSS 0.5%
Patch available Yes
Active threat No
Deciding rule
HIGH severity (CVSS 7.0-8.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision
Scheduled
Exploitation
None
System Exposure· derived
Open
Automatable
Yes
Human Impact
Medium
Technical Impact
Partial

Priority Score

38
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +38
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Denial of Service MEDIUM

Denial of Service attacks render applications or systems unavailable by overwhelming resources or triggering failure conditions.

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A buffer overrun can be triggered in X.509 certificate verification, specifically in name constraint checking. Rated hig

Same weakness CWE-770 – Allocation of Resources Without Limits or Throttling

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CVE-2017-8779 HIGH POC
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CVE-2017-5850 HIGH POC
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CVE-2022-32206 MEDIUM POC
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CVE-2023-24998 HIGH
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Same technique Denial Of Service

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CVE-2015-5119 CRITICAL POC
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CVE-2011-2462 CRITICAL POC
9.8
CVE-2011-0611 HIGH POC
8.8
CVE-2012-1889 HIGH POC
8.8
CVE-2012-0507 CRITICAL POC
9.8
CVE-2014-1761 HIGH POC
7.8
CVE-2015-5122 CRITICAL POC
9.8
CVE-2011-0609 HIGH POC
7.8

Vendor StatusVendor

SUSE

Severity: Moderate
Product Status
SLES15-SP5-CHOST-BYOS-SAP-CCloud Not-Affected
SLES15-SP5-CHOST-BYOS-SAP-CCloud Not-Affected
SLES15-SP6-CHOST-BYOS Not-Affected
SLES15-SP6-CHOST-BYOS Not-Affected
SLES15-SP6-CHOST-BYOS-Aliyun Not-Affected

References

Advisories & patches
ADVISORY https://access.redhat.com/errata/RHSA-2026:74162 ADVISORY https://access.redhat.com/errata/RHSA-2026:74166 ADVISORY https://github.com/openssl/openssl/releases/tag/openssl-4.0.3 ADVISORY https://nvd.nist.gov/vuln/detail/CVE-2026-54873 ADVISORY https://seclists.org/oss-sec/2026/q3/1001 ADVISORY https://ubuntu.com/security/notices/USN-8861-1 ADVISORY https://vuldb.com/vuln/411433 REF https://github.com/openssl/openssl/commit/1f643b8bc735487b500a1f68a7fb3a22d5e38e23 REF https://github.com/openssl/openssl/commit/279e7ee1392af98785746788168749491c74bd53 REF https://github.com/openssl/openssl/commit/3ea6213e050e938ecbbf8c4eff32bec2736780eb REF https://github.com/openssl/openssl/commit/7127fb10888b49711c63128a09e524c0d2d5d0b2 REF https://openssl-library.org/news/secadv/20260929.txt

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