How severe is CVE-2021-27291?

CVE-2021-27291 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.

Is CVE-2021-27291 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 4% (89th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-27291?

CVE-2021-27291 primarily affects Pygments. In total, 5 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-27291?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-27291 published?

CVE-2021-27291 was published on 2021-03-17 and last updated on 2026-06-17.

CVE-2021-27291

CVE-2021-27291 is a high-severity vulnerability in Pygments with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-1333.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v2: 5.0
EPSS exploit prediction: 4% (89th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-1333
Affected product: Pygments
Published: 2021-03-17
Last modified: 2026-06-17

Description

In pygments 1.1+, fixed in 2.7.4, the lexers used to parse programming languages rely heavily on regular expressions. Some of the regular expressions have exponential or cubic worst-case complexity and are vulnerable to ReDoS. By crafting malicious input, an attacker can cause a denial of service.

Frequently asked questions

What is CVE-2021-27291?: In pygments 1.1+, fixed in 2.7.4, the lexers used to parse programming languages rely heavily on regular expressions. Some of the regular expressions have exponential or cubic worst-case complexity and are vulnerable to ReDoS. By crafting malicious input, an attacker can cause a denial of service.
How severe is CVE-2021-27291?: CVE-2021-27291 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability high.
Is CVE-2021-27291 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 4% (89th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-27291?: CVE-2021-27291 primarily affects Pygments. In total, 5 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-27291?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-27291 published?: CVE-2021-27291 was published on 2021-03-17 and last updated on 2026-06-17.

References

Affected products (5)

cpe:2.3:a:pygments:pygments:*:*:*:*:*:*:*:*
cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*
cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*
cpe:2.3:o:fedoraproject:fedora:32:*:*:*:*:*:*:*
cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*

More vulnerabilities in Pygments

CVE-2015-8557 — Critical (CVSS 9.0): The FontManager._get_nix_font_path function in formatters/img.py in Pygments 1.2.2 through 2.0.2 allows remote…
CVE-2021-20270 — High (CVSS 7.5): An infinite loop in SMLLexer in Pygments versions 1.5 to 2.7.3 may lead to denial of service when performing syntax…
CVE-2022-40896 — Medium (CVSS 5.5): A ReDoS issue was discovered in pygments/lexers/smithy.py in pygments through 2.15.0 via SmithyLexer.

All CVEs affecting Pygments →

Other CWE-1333 vulnerabilities

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CVE-2023-29486 — Critical (CVSS 9.8): An issue was discovered in Heimdal Thor agent versions 3.4.2 and before 3.7.0 on Windows, allows attackers to bypass…
CVE-2026-25547 — Critical (CVSS 9.2): @isaacs/brace-expansion is a hybrid CJS/ESM TypeScript fork of brace-expansion. Prior to version 5.0.1,…
CVE-2023-29487 — Critical (CVSS 9.1): An issue was discovered in Heimdal Thor agent versions 3.4.2 and before on Windows and 2.6.9 and before on macOS,…
CVE-2026-47138 — High (CVSS 8.7): Parse Server is an open source backend that can be deployed to any infrastructure that can run Node.js. Prior to…
CVE-2025-6998 — High (CVSS 8.7): ReDoS in strip_whitespaces() function in cps/string_helper.py in Calibre Web and Autocaliweb allows unauthenticated…

Browse all CWE-1333 vulnerabilities →