The vulnerability stems from insecure timing-sensitive string comparison in TempURL middleware. The commit 7546339 shows the fix replaced the vulnerable 'in' check with a constant-time comparison using 'streq_const_time'. The original code path in the call method performed a linear scan of hmac_vals with short-circuiting logic, making response times dependent on input similarity to valid signatures. This matches the CVE description of a timing side-channel attack against TempURL signatures.