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+```toml
+[advisory]
+id = "RUSTSEC-0000-0000"
+package = "quick-xml"
+date = "2026-06-29"
+url = "https://github.com/tafia/quick-xml/issues/969"
+references = [
+    "https://github.com/tafia/quick-xml/pull/971",
+    "https://github.com/tafia/quick-xml/commit/07f3db8343cf152f5bc3483ef5b3164582489bea",
+]
+categories = ["denial-of-service"]
+keywords = ["xml", "parser", "dos", "algorithmic-complexity", "quadratic"]
+cvss = "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
+
+[versions]
+patched = [">= 0.41.0"]
+```
+
+# Quadratic run time when checking a start tag for duplicate attribute names
+
+`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.
+
+## Impact
+
+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:
+
+| Attributes on one tag | Time |
+|---|---|
+| 80,000  | ~6 s   |
+| 800,000 | ~10 min |
+
+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.
+
+## Affected code paths
+
+* `BytesStart::attributes()` / `Attributes` iterated with checks enabled (the
+  default), and `BytesStart::try_get_attribute`.
+* `NsReader`, which resolves namespaces by iterating a tag's attributes and so
+  reaches the same check internally.
+
+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`.
+
+## Remediation
+
+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).