Bug 2467298 (CVE-2026-33079)

Summary: CVE-2026-33079 mistune: Mistune: Regular Expression Denial of Service (ReDoS) via crafted Markdown input
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: high Docs Contact:
Priority: high    
Version: unspecifiedCC: anthomas, ehelms, ggainey, jkoehler, jpasqual, juwatts, lphiri, mhulan, nmoumoul, osousa, pcreech, rchan, rjohnson, smallamp, tmalecek
Target Milestone: ---Keywords: Security
Target Release: ---   
Hardware: All   
OS: Linux   
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A flaw was found in Mistune, a Markdown parser. This vulnerability, known as Regular Expression Denial of Service (ReDoS), exists in the `LINK_TITLE_RE` regular expression. A remote attacker can exploit this by providing specially crafted Markdown input, which causes the regular expression engine to engage in catastrophic backtracking. This leads to significant CPU consumption, making applications that use Mistune unresponsive and resulting in a Denial of Service.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description OSIDB Bzimport 2026-05-06 18:01:37 UTC 
In versions 3.0.0a1 through 3.2.0 of Mistune, there is a ReDoS (Regular Expression Denial of Service) vulnerability in `LINK_TITLE_RE` that allows an attacker who can supply Markdown for parsing to cause denial of service. The regular expression used for parsing link titles contains overlapping alternatives that can trigger catastrophic backtracking. In both the double-quoted and single-quoted branches, a backslash followed by punctuation can be matched either as an escaped punctuation sequence or as two ordinary characters, creating an ambiguous pattern inside a repeated group. If an attacker supplies Markdown containing repeated ! sequences with no closing quote, the regex engine explores an exponential number of backtracking paths. This is reachable through normal Markdown parsing of inline links and block link reference definitions. A small crafted input can therefore cause significant CPU consumption and make applications using Mistune unresponsive.