CVE-2026-48525: PyJWT: Unauthenticated DoS via unbounded Base64URL decoding of unused payload segment in b64=false detached JWS

mediumCVSS 5.3CVE-2026-48525

CSIRTS triage

vendor: PyJWT product: PyJWT Denial of service

What: PyJWT with b64=false detached JWS mode is vulnerable to unbounded Base64URL decoding of the unused payload segment, allowing unauthenticated denial of service.
Who is affected: Applications using PyJWT to validate JWS tokens with b64=false flag for payload encoding.
Urgency: Moderately urgent; unauthenticated DoS is possible via maliciously crafted JWT payloads with CVSS 5.3.
Action: Upgrade PyJWT to a patched version that bounds Base64URL decoding in detached JWS payload processing.

AI-assisted analysis generated from the source advisory — verify against the original.

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Details

Source: Microsoft Security Response Center (INTL · vendor-psirt · site)
Severity: medium — CVSS 5.3
Published: 2026-08-06
Exploitation: Not in CISA KEV at last sync

Original advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-48525

Exploitation outlook

EPSS (FIRST.org) estimates each CVE’s probability of exploitation in the next 30 days — here is the CSIRTS.com read on those numbers.

Low exploitation riskCVE-2026-485250.37% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 30% of all EPSS-scored CVEs.

Referenced CVEs

CVE	CSIRTS overview	External
CVE-2026-48525	coverage & exploitation status	NVD · CVE.org

Recent advisories for PyJWT

A cluster of recent advisories against the same product widens the attack surface — attackers routinely chain freshly published CVEs on one product, so review these together.

highCVE-2026-48526: PyJWT: Public-key JWK accepted as HMAC secret enables forged HS256 tokens when mixed families …msrc · 2026-08-06
highCVE-2025-45768: pyjwt v2.10.1 was discovered to contain weak encryption. NOTE: this is disputed by the Supplie…msrc · 2026-08-06