Local denial of service via GUID regex backtracking
Published Mar 22, 2026 · Updated Mar 22, 2026
Regular expression denial of service in Pygments before 2.20.0 allows local users to hang syntax highlighting via crafted ADL text. The AdlLexer GUID pattern in pygments/lexers/archetype.py uses nested unbounded repetitions that catastrophically backtrack on a long hexadecimal run followed by a hyphen. Exploitation requires an application or command-line invocation to process attacker-controlled text with AdlLexer, consuming CPU and blocking the parsing thread without affecting confidentiality or integrity.
Summary
What happened
Regular expression denial of service in Pygments before 2.20.0 allows local users to hang syntax highlighting via crafted ADL text. The AdlLexer GUID pattern in pygments/lexers/archetype.py uses nested unbounded repetitions that catastrophically backtrack on a long hexadecimal run followed by a hyphen. Exploitation requires an application or command-line invocation to process attacker-controlled text with AdlLexer, consuming CPU and blocking the parsing thread without affecting confidentiality or integrity.
The record
- CVE
- CVE-2026-4539
- Published
- Mar 22, 2026
- Updated
- Mar 22, 2026
- Vendor
- Pygments
- Product
- Pygments
- Classifications
- CWE-400, CWE-1333, T1499.004
- Attack vector
- local
- Privileges
- authenticated
Timeline
How it unfolded
- Mar 22, 2026CVE publishedPublication date reported by the CVE source.
- Mar 22, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=2.19.0
- Affected versionversion=2.19.1
- Affected versionversion=2.19.2
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- AdlLexer catastrophic-backtracking proof of conceptproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
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Your stack
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