CVE-2026-34835: Rack is a modular Ruby web server interface

This week

MEDIUM 4.8REMOTEUPGRADE

CVE-2026-34835 is a medium-severity flaw (CVSS 4.8) affecting rack. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, Rack::Request parses the Host header using an AUTHORITY regular expression that accepts characters not permitted in RFC-compliant hostnames, including /, ?, #, and @. Because req.host returns the full parsed value, applications that validate hosts using naive prefix or suffix checks can be bypassed. This can lead to host header pois

Why this priority

Public exploit reference
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	4.8 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.2% (top 91%)
CISA exploited list	No
Weakness	CWE-1286
Published	2026-04-02
Last modified	2026-07-24

Publisher (CNA): GitHub, Inc. · Vendor: rack

Last updated 2026-07-24 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-04-02 18:16 UTCPublished CVE record published
2026-07-24 21:10 UTCUpdated Record last modified

rack rack UPGRADE

Affected: >= 3.0.0 and < 3.1.21 · >= 3.2.0 and < 3.2.6

Fixed in: 3.1.21, 3.2.6

Safe to downgrade to: a release before 3.0.0

1. Check installed version

apt-cache policy rack || rpm -q rack || rack --version

2. UPGRADE to 3.1.21 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade rack

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade rack

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 3.0.0 (Debian / Ubuntu)

apt-cache madison rack sudo apt install rack=<VERSION_BEFORE_3.0.0> sudo apt-mark hold rack

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list rack sudo dnf downgrade rack-<VERSION_BEFORE_3.0.0> sudo dnf versionlock add rack

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-34835?

rack: upgrade to 3.1.21, 3.2.6. Exact commands for each OS are on this page.

Is CVE-2026-34835 being exploited?

CVE-2026-34835 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-34835 affect?

CVE-2026-34835 affects rack. Severity is medium (CVSS 4.8). Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, Rack::Request parses the Host header using an AUTHORITY regular expression that accepts characters not permitted in RFC-comp

How urgent is CVE-2026-34835?

Priority 29/100. Public exploit reference; Remotely exploitable, no login, no user action

Software pages

Ruby vulnerabilities

Related CVEs

CVE-2023-27530 HIGH 7.5 UPGRADE MITIGATE A DoS vulnerability exists in Rack <v3.0.4.2, <v2.2.6.3, <v2.1.4.3 and <v2.0.9.3 within in the Multipart MIME
CVE-2026-34827 HIGH 7.5 UPGRADE Rack is a modular Ruby web server interface
CVE-2026-34785 HIGH 7.5 UPGRADE Rack is a modular Ruby web server interface
CVE-2026-34829 HIGH 7.5 UPGRADE Rack is a modular Ruby web server interface
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References

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