CVE-2019-6293: An issue was discovered in the function mark_beginning_as_normal in nfa.c in flex 2.6.4

This month

MEDIUM 5.5DOWNGRADE

CVE-2019-6293 is a medium-severity flaw (CVSS 5.5) affecting flex. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

An issue was discovered in the function mark_beginning_as_normal in nfa.c in flex 2.6.4. There is a stack exhaustion problem caused by the mark_beginning_as_normal function making recursive calls to itself in certain scenarios involving lots of '*' characters. Remote attackers could leverage this vulnerability to cause a denial-of-service.

Why this priority

Public exploit reference

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	5.5 (Medium)
Attack vector	Local
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	1.6% (top 24%)
CISA exploited list	No
Weakness	CWE-674
Published	2019-01-15
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: westes

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

Am I affected?

Timeline

2019-01-15 00:29 UTCPublished CVE record published
2026-06-17 02:38 UTCUpdated Record last modified

westes flex DOWNGRADE

Affected: 2.6.4

Fixed in: no fixed release known

Safe to downgrade to: a release before 2.6.4

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 2.6.4 (Debian / Ubuntu)

apt-cache madison flex sudo apt install flex=<VERSION_BEFORE_2.6.4> sudo apt-mark hold flex

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list flex sudo dnf downgrade flex-<VERSION_BEFORE_2.6.4> sudo dnf versionlock add flex

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-2019-6293?

flex: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2019-6293 being exploited?

CVE-2019-6293 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-2019-6293 affect?

CVE-2019-6293 affects flex. Severity is medium (CVSS 5.5). An issue was discovered in the function mark_beginning_as_normal in nfa.c in flex 2.6.4. There is a stack exhaustion problem caused by the mark_beginning_as_normal function making recursive calls to itself in certain scenarios involving lot

How urgent is CVE-2019-6293?

Priority 43/100. Public exploit reference

References

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