CVE-2021-33642: When a file is processed, an infinite loop occurs in next_inline() of the more_curly() function.

Routine

MEDIUM 5.5UPGRADE

CVE-2021-33642 is a medium-severity flaw (CVSS 5.5) affecting byacc. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

When a file is processed, an infinite loop occurs in next_inline() of the more_curly() function.

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	5.5 (Medium)
Attack vector	Local
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.2% (top 87%)
CISA exploited list	No
Weakness	CWE-835
Published	2023-01-20
Last modified	2026-06-17

Publisher (CNA): openEuler · Vendor: openeuler

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2023-01-20 21:15 UTCPublished CVE record published
2026-06-17 03:54 UTCUpdated Record last modified

openeuler byacc UPGRADE

Affected: < 1.9.20200330 · >= 2.0 and < 2.0.20210808

Fixed in: 1.9.20200330, 2.0.20210808

Safe to downgrade to: a release before 2.0

1. Check installed version

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

2. UPGRADE to 1.9.20200330 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade byacc

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 2.0 (Debian / Ubuntu)

apt-cache madison byacc sudo apt install byacc=<VERSION_BEFORE_2.0> sudo apt-mark hold byacc

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

dnf --showduplicates list byacc sudo dnf downgrade byacc-<VERSION_BEFORE_2.0> sudo dnf versionlock add byacc

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-2021-33642?

byacc: upgrade to 1.9.20200330, 2.0.20210808. Exact commands for each OS are on this page.

Is CVE-2021-33642 being exploited?

CVE-2021-33642 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-2021-33642 affect?

CVE-2021-33642 affects byacc. Severity is medium (CVSS 5.5). When a file is processed, an infinite loop occurs in next_inline() of the more_curly() function.

How urgent is CVE-2021-33642?

Priority 22/100. No elevated risk signals yet.

Related CVEs

CVE-2021-33641 HIGH 7.8 UPGRADE When processing files, malloc stores the data of the current line

References

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