BytesStart::attributes() returns an Attributes iterator which, by default (with checks(true)), rejects a start tag that repeats an attribute name. For each attribute yielded, the iterator compared the new name against every name seen so far in the same tag using a linear scan, so a start tag with N distinct attribute names cost O(N²) byte comparisons. There was no bound on N other than the size of the buffered start tag.

Any code that parses untrusted XML and iterates a start tag's attributes with the default duplicate check enabled can be made to spend CPU time quadratic in the number of attributes on a single tag. Because the check is pure computation with no .await/I/O, an I/O-based timeout on the consumer (for example a read or request timeout) cannot interrupt it while it runs.

Measured cost of a single start tag, release build:

The cost grows with the square of the attribute count, so a start tag of a few tens of megabytes can stall a parsing thread for hours. No memory is exhausted and the parser does not crash; the effect is CPU exhaustion on the thread doing the parsing: a single crafted start tag can pin a CPU core for minutes to hours, denying service to that worker. A deployment that places a wall-clock bound on parsing, or confines it to a non-critical thread, may consider the availability impact lower.

Consumers that iterate attributes with .attributes().with checks(false) and do not use NsReader are not affected.

This was reported as reachable by a remote, unauthenticated attacker in a real-world RPKI relying party (NLnet Labs Routinator) via a crafted RRDP snapshot.xml.

Upgrade to quick-xml >= 0.41.0, where the duplicate check keeps the linear scan for start tags with a small number of attributes and switches to an O(1) hash pre-filter above a threshold, making the whole tag O(N). The reported AttrError::Duplicated positions are unchanged.

If upgrading is not possible and duplicate-name detection is not required, disable it with .attributes().with checks(false) (this does not help NsReader consumers, which have no equivalent opt-out before 0.41.0).