Vulnerability Details CVE-2026-4867
Impact:
A bad regular expression is generated any time you have three or more parameters within a single segment, separated by something that is not a period (.). For example, /:a-:b-:c or /:a-:b-:c-:d. The backtrack protection added in path-to-regexp@0.1.12 only prevents ambiguity for two parameters. With three or more, the generated lookahead does not block single separator characters, so capture groups overlap and cause catastrophic backtracking.
Patches:
Upgrade to path-to-regexp@0.1.13
Custom regex patterns in route definitions (e.g., /:a-:b([^-/]+)-:c([^-/]+)) are not affected because they override the default capture group.
Workarounds:
All versions can be patched by providing a custom regular expression for parameters after the first in a single segment. As long as the custom regular expression does not match the text before the parameter, you will be safe. For example, change /:a-:b-:c to /:a-:b([^-/]+)-:c([^-/]+).
If paths cannot be rewritten and versions cannot be upgraded, another alternative is to limit the URL length.
Exploit prediction scoring system (EPSS) score
EPSS Score 0.0
EPSS Ranking 5.0%
CVSS Severity
CVSS v3 Score 7.5
References
Products affected by CVE-2026-4867
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cpe:2.3:a:pillarjs:path-to-regexp:0.0.1
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cpe:2.3:a:pillarjs:path-to-regexp:0.0.2
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.0
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.1
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.10
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.11
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.12
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.2
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.3
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.4
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.5
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.6
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.7
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.8
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cpe:2.3:a:pillarjs:path-to-regexp:0.1.9