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NVD / CVE Program· CNA EEF

CVE-2026-65624: Cowboy HTTP/1.1 max_headers Bypass via Duplicate Header Names Enables Memory Exhaustion

Noroxi assessment

Priority: Monitor (action score 27/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 0.38% (FIRST EPSS).
  • Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
  • According to CISA the attack is automatable: targets can be found by mass scanning.
  • Fixed version: ninenines cowboy 2.18.0, 3a34d8c1cfd94326466aa16a9017236691dc9c55; azl3 rabbitmq-server 3.13.7-8 on Azure Linux 3.0 3.13.7-9; azl3 rabbitmq-server 3.13.7-9 on Azure Linux 3.0 3.13.7-9.

Low priority for now. Upgrade to ninenines cowboy 2.18.0, 3a34d8c1cfd94326466aa16a9017236691dc9c55; azl3 rabbitmq-server 3.13.7-8 on Azure Linux 3.0 3.13.7-9; azl3 rabbitmq-server 3.13.7-9 on Azure Linux 3.0 3.13.7-9 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

Allocation of Resources Without Limits or Throttling vulnerability in ninenines cowboy allows an unauthenticated remote attacker to exhaust connection process memory over HTTP/1.1. The HTTP/1.1 handler in cowboy_http enforces the max_headers limit by counting the number of distinct header names in a map (maps:size(Headers)). When a request contains multiple header lines with the same name, the values are concatenated into a single ever-growing binary stored under that one map key (", " for regular headers, "; " for cookies), so the map size stays at one and the max_headers cap (default 100) is never reached. Because no accumulator bounds the total number of header lines or the total byte size of the header block (only per-line max_header_name_length and max_header_value_length apply), an unauthenticated client can send an arbitrary number of header lines with the same name and grow the connection process's binary memory to arbitrary size within the request window. The impact per connection is bounded by request_timeout (default 5 seconds, not reset by header data), and by max_heap_size when set (the offending connection process is killed once its heap grows past the limit). When max_heap_size is left at the default (unset), sustained abuse can drive the Erlang VM into out-of-memory conditions. This issue affects cowboy from 2.0.0-pre.4 before 2.18.0.

MediumCVSS 6.9 · v4.0—No exploit Fix available
Published
Jul 28, 2026
Updated
Oct 24, 2026
EPSS
0.3% · 32th percentile

Report tools

JSON

Action score

27

Monitor

Low priority for now.

CVSS
27 / 40 · 6.9 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.3%

CISA SSVC decision

Exploitation
none
Automatable
yes
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · EEF
6.9
NVD has not scored this yet; the score shown is the CNA’s.

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

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When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

—

Versions reported by the vendor

Affected version ranges reported by the assigning authority (EEF). Independent of NVD's CPE analysis and usually ahead of it.

  • ninenines cowboy

    • 2.0.0-pre.4 and later · before 2.18.0affected · semver
    • 309780a9fda145c262a47ac7811ffd50a0271c5b and later · before 3a34d8c1cfd94326466aa16a9017236691dc9c55affected · git

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Debian:14erlang-cowboybefore 2.18.0+dfsg-12.18.0+dfsg-1
Hexcowboyfrom 2.0.0-pre.4 · before 2.18.02.18.0
Red Hat:hummingbird:1rabbitmq-server4.2before 0:4.2.9-0.2.hum10:4.2.9-0.2.hum1
Red Hat:hummingbird:1rabbitmq-server4.3before 0:4.3.4-0.2.hum10:4.3.4-0.2.hum1
Red Hat:hummingbird:1rabbitmq-server4.3before 0:4.3.5-0.1.hum10:4.3.5-0.1.hum1

Same product

ninenines: all records

Other highest-scoring records for the same primary product.

  • CVE-2026-8466Unbounded buffer accumulation in multipart header parsing causes denial of service in cowboy
    32Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
ninenines cowboy2.18.0Vendor (CNA)
ninenines cowboy3a34d8c1cfd94326466aa16a9017236691dc9c55Vendor (CNA)
azl3 rabbitmq-server 3.13.7-8 on Azure Linux 3.03.13.7-9Microsoft (MSRC)
azl3 rabbitmq-server 3.13.7-9 on Azure Linux 3.03.13.7-9Microsoft (MSRC)
debian:erlang-cowboy2.18.0+dfsg-1 · Debian:14Package registry (OSV)
Hex:cowboy2.18.0Package registry (OSV)
red hat:rabbitmq-server4.20:4.2.9-0.2.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:rabbitmq-server4.30:4.3.4-0.2.hum1 · Red Hat:hummingbird:1Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

  • CVE-2026-846676 days apartUnbounded buffer accumulation in multipart header parsing causes denial of service in cowboy
    32Monitor

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • No account or password is required.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
Integrity
Availability
Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
X

Weakness class (CWE)

CWE-770 · Allocation of Resources Without Limits or Throttling

CVSS vector

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Change log

  1. Fix✗ → ✓

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References

Show all references (2 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records