Skip to content
Noroxi
NVD / CVE Program· CNA GitHub_M

CVE-2026-63379: Libevent: HTTP Header smuggling

Noroxi assessment

Priority: Monitor (action score 25/100)

  • 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: 0.51% (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: libevent (AlmaLinux:10) 2.1.13-1.el10_2; libevent-devel (AlmaLinux:10) 2.1.13-1.el10_2; libevent-doc (AlmaLinux:10) 2.1.13-1.el10_2.

Low priority for now. Upgrade to libevent (AlmaLinux:10) 2.1.13-1.el10_2; libevent-devel (AlmaLinux:10) 2.1.13-1.el10_2; libevent-doc (AlmaLinux:10) 2.1.13-1.el10_2 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

Libevent is an event notification library. Prior to 2.1.13 and 2.2.2-alpha, libevent processes chunked HTTP trailers in http.c through evhttp_read_trailer and merges them into request headers. The fix introduces evhttp_parse_headers_impl_ and a temporary trailer header list. An unauthenticated remote attacker can place security-sensitive fields in trailers so that an upstream proxy and the libevent application interpret different effective headers, enabling header smuggling, authorization bypass, proxy-header spoofing, or cache poisoning. The fix parses trailers into a temporary header list and discards them instead of merging them into req->input_headers. This issue is fixed in versions 2.1.13 and 2.2.2-alpha.

MediumCVSS 6.3 · v4.0—No exploit Fix available
Published
Aug 20, 2026
Updated
Oct 26, 2026
EPSS
0.5% · 44th percentile

Report tools

JSON

Action score

25

Monitor

Low priority for now.

CVSS
25 / 40 · 6.3 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 0.5%

CISA SSVC decision

Exploitation
none
Automatable
yes
Technical impact
partial

Vulnrichment: CISA's decision-tree inputs.

CNA vs NVD score

NVD
—
CNA · GitHub_M
6.3
NVD has not scored this yet; the score shown is the CNA’s.

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/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.

Follow CVE-2026-63379

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

One daily email digest (06:00 UTC); connect a webhook for KEV listings and weaponized exploits instantly.

Your free account also gets you

  • Stack alerts: add the products and versions you run, get the new CVEs that affect them
  • Uptime monitoring: 1 site, instant email when it goes down
  • Add your domain: subdomains and takeover risk scanned every week (1 domain)
  • 10 scans an hour on the security tools (3 a day without an account), an API key and MCP
  • All your scans in one panel: findings, what changed in the last scan, a report per domain or per client (PDF, share link)
All free tools

Sign up with email · Have an account? Sign in

Free account, no card. Unfollow any time.

Don't want an account? Get "Exploited this week" by email every MondayDon't want an account? RSS feed for libevent records
Is your version affected? Check it

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.

We use this product, ask for help

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 (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.

  • libevent libevent

    • < 2.1.13, >= 2.2.0-alpha, < 2.2.2-alphaaffected

Package-level exposure

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

EcosystemPackageAffected rangeFix
AlmaLinux:10libeventbefore 2.1.13-1.el10_22.1.13-1.el10_2
AlmaLinux:10libevent-develbefore 2.1.13-1.el10_22.1.13-1.el10_2
AlmaLinux:10libevent-docbefore 2.1.13-1.el10_22.1.13-1.el10_2
AlmaLinux:9libeventbefore 2.1.13-1.el9_82.1.13-1.el9_8
AlmaLinux:9libevent-develbefore 2.1.13-1.el9_82.1.13-1.el9_8
AlmaLinux:9libevent-docbefore 2.1.13-1.el9_82.1.13-1.el9_8
Debian:12libeventbefore 2.1.12-stable-8+deb12u12.1.12-stable-8+deb12u1
Debian:13libeventbefore 2.1.13-stable-1~deb13u12.1.13-stable-1~deb13u1
Debian:14libeventbefore 2.1.13-stable-12.1.13-stable-1
openSUSE:Tumbleweedlibeventbefore 2.1.13-1.12.1.13-1.1
Red Hat:enterprise_linux:10.2libeventbefore 0:2.1.13-1.el10_20:2.1.13-1.el10_2
Red Hat:enterprise_linux:10.2libevent-debuginfobefore 0:2.1.13-1.el10_20:2.1.13-1.el10_2
Red Hat:enterprise_linux:10.2libevent-debugsourcebefore 0:2.1.13-1.el10_20:2.1.13-1.el10_2
Red Hat:enterprise_linux:10.2libevent-develbefore 0:2.1.13-1.el10_20:2.1.13-1.el10_2
Red Hat:enterprise_linux:10.2libevent-docbefore 0:2.1.13-1.el10_20:2.1.13-1.el10_2
Red Hat:enterprise_linux:9::appstreamlibevent-debuginfobefore 0:2.1.13-1.el9_80:2.1.13-1.el9_8
Red Hat:enterprise_linux:9::appstreamlibevent-debugsourcebefore 0:2.1.13-1.el9_80:2.1.13-1.el9_8
Red Hat:enterprise_linux:9::appstreamlibevent-develbefore 0:2.1.13-1.el9_80:2.1.13-1.el9_8
Red Hat:enterprise_linux:9::appstreamlibevent-docbefore 0:2.1.13-1.el9_80:2.1.13-1.el9_8
Red Hat:enterprise_linux:9::baseoslibeventbefore 0:2.1.13-1.el9_80:2.1.13-1.el9_8
Red Hat:hummingbird:1libeventbefore 0:2.1.12-19.1.hum10:2.1.12-19.1.hum1
Red Hat:hummingbird:1libevent-develbefore 0:2.1.12-19.1.hum10:2.1.12-19.1.hum1
Red Hat:hummingbird:1libevent-docbefore 0:2.1.12-19.1.hum10:2.1.12-19.1.hum1
Rocky Linux:10libeventbefore 0:2.1.13-1.el10_20:2.1.13-1.el10_2

+1

Same product

libevent: all records

Other highest-scoring records for the same primary product.

  • CVE-2026-63382libevent evhttp: Multiple HTTP Parser Bugs Enable Request Smuggling
    37Monitor
  • CVE-2026-63385Libevent: HTTP header handling bugs create risk of access control bypass.
    36Monitor
  • CVE-2026-63384Libevent: `evtag_unmarshal_header()` decodes a wire `uint32` length into a signed `int` return value.
    34Monitor

Published before and after this one in the same product

  • Later →CVE-2026-63380Aug 20, 2026 · Libevent: Null Pointer Dereference in `evws_new_session`
    22Monitor
  • Later →CVE-2026-63381Aug 20, 2026 · Libevent: Dangling Pointer in `evbuffer_add_buffer_reference`
    23Monitor
  • Later →CVE-2026-63383Aug 20, 2026 · Libevent: decode_tag_internal() can lead to out-of-bounds read
    34Monitor

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
almalinux:libevent2.1.13-1.el10_2 · AlmaLinux:10Package registry (OSV)
almalinux:libevent-devel2.1.13-1.el10_2 · AlmaLinux:10Package registry (OSV)
almalinux:libevent-doc2.1.13-1.el10_2 · AlmaLinux:10Package registry (OSV)
debian:libevent2.1.12-stable-8+deb12u1 · Debian:12Package registry (OSV)
opensuse:libevent2.1.13-1.1 · openSUSE:TumbleweedPackage registry (OSV)
red hat:libevent0:2.1.12-19.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:libevent-debuginfo0:2.1.13-1.el10_2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:libevent-debugsource0:2.1.13-1.el10_2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:libevent-devel0:2.1.12-19.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
red hat:libevent-devel0:2.1.13-1.el10_2 · Red Hat:enterprise_linux:10.2Package registry (OSV)
red hat:libevent-doc0:2.1.12-19.1.hum1 · Red Hat:hummingbird:1Package registry (OSV)
rocky linux:libevent0:2.1.13-1.el10_2 · Rocky Linux:10Package 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.

No credits in the CNA record.

Variant candidates

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

  • CVE-2026-633820 days apartlibevent evhttp: Multiple HTTP Parser Bugs Enable Request Smuggling
    37Monitor
  • CVE-2026-633850 days apartLibevent: HTTP header handling bugs create risk of access control bypass.
    36Monitor

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-444 · Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

CVSS vector

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/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.

ATT&CK techniques

—

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (4 more)

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

All records