How severe is CVE-2023-27539?

CVE-2023-27539 has a CVSS score of 5.3/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2023-27539?

CVE-2023-27539 affects the following packages: rack (RubyGems), rack (RubyGems). Ecosystems affected: RubyGems.

How do I fix CVE-2023-27539?

Update rack to 2.2.6.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-27539 is resolved across your whole dependency graph.

How do I detect CVE-2023-27539 in my RubyGems dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2023-27539 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2023-27539 actively exploited in the wild?

No public exploit code has been indexed for CVE-2023-27539 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2023-27539?

CVE-2023-27539 has an EPSS (Exploit Prediction Scoring System) score of 1.1%, placing it in the 65th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

When was CVE-2023-27539 published, and has it been updated?

CVE-2023-27539 was published on January 9, 2025 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

💎

💎 RubyGems

Not in CISA KEV

MEDIUM severity

CVE-2023-27539 — rack

MEDIUMFix: rack/rack@231ef36

CVE-2023-27539 is a medium-severity (CVSS 5.3) vulnerability in rack. A fix is available for rack — see the affected versions and patch details below.

Possible Denial of Service Vulnerability in Rack's header parsing

Also known asGHSA-c6qg-cjj8-47qp

Published

Jan 9, 2025

Updated

Oct 28, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Nov 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2023-27539.

EPSS Exploitation Probability

via FIRST.org ↗

1.1%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs65th percentile — riskier than 65% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

2 pkgs affected

💎rack💎rack

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.

Description

There is a denial of service vulnerability in the header parsing component of Rack. This vulnerability has been assigned the CVE identifier CVE-2023-27539.

Versions Affected: >= 2.0.0 Not affected: None. Fixed Versions: 2.2.6.4, 3.0.6.1

Impact

Carefully crafted input can cause header parsing in Rack to take an unexpected amount of time, possibly resulting in a denial of service attack vector. Any applications that parse headers using Rack (virtually all Rails applications) are impacted.

Workarounds

Setting Regexp.timeout in Ruby 3.2 is a possible workaround.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
💎RubyGems	`rack`	≥ 2.0.0&&< 2.2.6.4	2.2.6.4`bundle update rack --conservative`
💎RubyGems	`rack`	≥ 3.0.0&&< 3.0.6.1	3.0.6.1`bundle update rack --conservative`

Affected Products

2 products · 4 configurations

debian linuxdebian

2 versions

10.011.0

Application

rackrack

≥ 3.0.0 && < 3.0.6.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update rack to 2.2.6.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-27539 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

Workaround published by Red Hat

Setting Regexp.timeout in Ruby 3.2 is a possible workaround.

Source: Red Hat security advisory for CVE-2023-27539 (CC BY 4.0)

Product	Fixed in	Advisory
Red Hat Enterprise Linux 8	`pcs-0:0.10.15-4.el8_8.1`	RHSA-2023:3082
Red Hat Enterprise Linux 8.4 Extended Update Support	`pcs-0:0.10.8-1.el8_4.4`	RHSA-2023:1961
Red Hat Enterprise Linux 8.6 Extended Update Support	`pcs-0:0.10.12-6.el8_6.4`	RHSA-2023:3403
Red Hat Enterprise Linux 9	`pcs-0:0.11.4-7.el9_2`	RHSA-2023:2652
Red Hat Enterprise Linux 9.0 Extended Update Support	`pcs-0:0.11.1-10.el9_0.4`	RHSA-2023:1981
Red Hat Satellite 6.14 for RHEL 8	`rubygem-rack-0:2.2.7-1.el8sat`	RHSA-2023:6818
RHOL-5.6-RHEL-8	`openshift-logging/fluentd-rhel8:v1.14.6-113`	RHSA-2023:1953
RHOL-5.7-RHEL-8	`openshift-logging/fluentd-rhel8:v1.14.6-140`	RHSA-2023:3495

Frequently Asked Questions

There is a denial of service vulnerability in the header parsing component of Rack. This vulnerability has been assigned the CVE identifier CVE-2023-27539. Versions Affected: >= 2.0.0 Not affected: None. Fixed Versions: 2.2.6.4, 3.0.6.1 # Impact Carefully crafted input can cause header parsing in Rack to take an unexpected amount of time, possibly resulting in a denial of service attack vector. Any applications that parse headers using Rack (virtually all Rails applications) are impacted. # Workarounds Setting Regexp.timeout in Ruby 3.2 is a possible workaround.

O3 Security · Impact-Aware SCA

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