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PUT /users/v1/{username}/email validated the new email against a
catastrophic-backtracking pattern: ^([0-9a-zA-Z]([-.\w]*[0-9a-zA-Z])*@{1}...)$
The nested (X*Y)* construction over overlapping character classes lets a
crafted string with no '@' drive exponential backtracking, hanging the
worker thread (ReDoS / API4:2019 Lack of Resources & Rate Limiting).
Replace it with an equivalent-intent email pattern where every repetition
is bounded (no unbounded quantifier is nested inside another), so
backtracking work stays linear in input length, plus a hard 254-char
length cap on the input as defense in depth. Verified locally that a
crafted payload that hung the original regex for 60+ seconds at n=30 now
gets rejected in milliseconds even at 250+ chars, while legitimate email
updates (e.g. name1@example.com) still succeed with HTTP 204.
🏆 VAmPI — CTF Patch Score1 / 9 challenges patched
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Vulnerability
PUT /users/v1/{username}/emailvalidated the new email against a catastrophic-backtracking regex:The nested
(X*Y)*construction over overlapping character classes is classic ReDoS territory: a crafted input with no@(e.g. a long run of alphanumerics) forces the engine into exponential backtracking, hanging the worker thread. Locally, this pattern took 60+ seconds on a 30-character crafted payload before I gave up waiting.Fix
Replaced the regex in the
if vuln:branch ofupdate_email()(api_views/users.py) with an equivalent-intent email pattern where no unbounded quantifier is nested inside another — every repetition is capped to a small explicit bound, so backtracking work stays bounded/linear in input length regardless of content. Added a hard 254-character length cap on the input as defense in depth, since no regex-only fix should be trusted to stay cheap for arbitrarily long input.Testing (local, WSL)
vulnerable=1), seeded via/createdb, logged in asname1.'a'*30 + '!'did not return within 60 seconds.@) is now rejected with400in ~10ms.PUT /users/v1/name1/emailwith{"email": "name1@example.com"}returns204and updates the email as before. Additional valid formats (john.doe@sub.domain.co,a_b-c@x-y.io) also match correctly.