VulnerabilityPublished 13 Aug 2026Updated 29 Oct 2026

CVE-2026-28490: Safety · Authlib Vulnerable to JWE RSA1_5 Bleichenbacher Padding Oracle

Affects safetySource GCVE Database

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Not known to be exploited · CVSS 8.3 High : upgrade to 3.7.0_6 soon.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 4.0 base score

8.3High

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 3.7.0_6

CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:L/VA:N/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: High
Attack Requirements: Present
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: High
Vulnerable system integrity: Low
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: The successful attack depends on the evasion or circumvention of security-enhancing techniques in place that would otherwise hinder the attack. These include: Evasion of exploit mitigation techniques, for example, circumvention of address space randomization (ASLR) or data execution prevention (DEP) must be performed for the attack to be successful; Obtaining target-specific secrets. The attacker must gather some target-specific secret before the attack can be successful. A secret is any piece of information that cannot be obtained through any amount of reconnaissance. To obtain the secret the attacker must perform additional attacks or break otherwise secure measures (e.g. knowledge of a secret key may be needed to break a crypto channel). This operation must be performed for each attacked target.
Attack Requirements: Present: The successful attack depends on the presence of specific deployment and execution conditions of the vulnerable system that enable the attack. These include: a race condition must be won to successfully exploit the vulnerability (the successfulness of the attack is conditioned on execution conditions that are not under full control of the attacker, or the attack may need to be launched multiple times against a single target before being successful); the attacker must inject themselves into the logical network path between the target and the resource requested by the victim (e.g. vulnerabilities requiring an on-path attacker).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: High: There is a total loss of confidentiality, resulting in all information within the Vulnerable System being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
Vulnerable system integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact to the Vulnerable System.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 13 Aug 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade safety to 3.7.0_6.

Affected software

Product	Affected	Fixed in
Homebrewsafety`pkg:brew/safety`Other threats	Affected >=3.2.4 <3.7.0_6	Fixed in 3.7.0_6

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

## 1. Executive Summary A cryptographic padding oracle vulnerability was identified in the Authlib Python library concerning the implementation of the JSON Web Encryption (JWE) `RSA1_5` key management algorithm. Authlib registers `RSA1_5` in its default algorithm registry without requiring explicit opt-in, and actively destroys the constant-time Bleichenbacher mitigation that the underlying `cryptography` library implements correctly. When `cryptography` encounters an invalid PKCS#1 v1.5 padding, it returns a randomized byte string instead of raising an exception — the correct behavior per RFC 3218 §2.3.2. Authlib ignores this contract and raises `ValueError('Invalid "cek" length')` immediately after decryption, before reaching AES-GCM tag validation. This creates a clean, reliable **Exception Oracle**: - **Invalid padding** → `cryptography` returns random bytes → Authlib length check fails → `ValueError: Invalid "cek" length` - **Valid padding, wrong MAC** → decryption succeeds → length check passes → AES-GCM fails → `InvalidTag` **This oracle is active by default in every Authlib installation without any special configuration by the developer or the attacker.** The three most widely used Python web frameworks — Flask, Django, and FastAPI — all expose distinguishable HTTP responses for these two exception classes in their default configurations, requiring no additional setup to exploit. **Empirically confirmed on authlib 1.6.8 + cryptography 46.0.5:** ``` [PADDING INVALIDO] ValueError: Invalid "cek" length [PADDING VALIDO/MAC] InvalidTag ``` --- ## 2. Technical Details & Root Cause ### 2.1 Vulnerable Code **File:** `authlib/jose/rfc7518/jwe_algs.py` ```python def unwrap(self, enc_alg, ek, headers, key): op_key = key.get_op_key("unwrapKey") # cryptography implements Bleichenbacher mitigation here: # on invalid padding it returns random bytes instead of raising. # Empirically confirmed: returns 84 bytes for a 2048-bit key. cek = op_key.decrypt(ek, self.padding) # VULNERABILITY: This length check destroys the mitigation. # cryptography returned 84 random bytes. len(84) * 8 = 672 != 128 (A128GCM CEK_SIZE). # Authlib raises a distinct ValueError before AES-GCM is ever reached. if len(cek) * 8 != enc_alg.CEK_SIZE: raise ValueError('Invalid "cek" length') # <- ORACLE TRIGGER return cek ``` ### 2.2 Root Cause — Active Mitigation Destruction `cryptography` 46.0.5 implements the Bleichenbacher mitigation correctly at the library level. When PKCS#1 v1.5 padding validation fails, it does not raise an exception. Instead it returns a randomized byte string (empirically observed: 84 bytes for a 2048-bit RSA key). The caller is expected to pass this fake key to the symmetric decryptor, where MAC/tag validation will fail in constant time — producing an error indistinguishable from a MAC failure on a valid padding. Authlib does not honor this contract. The length check on the following line detects that 84 bytes != 16 bytes (128-bit CEK for A128GCM) and raises `ValueError('Invalid "cek" length')` immediately. This exception propagates before AES-GCM is ever reached, creating two execution paths with observable differences: ``` Path A — invalid PKCS#1 v1.5 padding: op_key.decrypt() -> 84 random bytes (cryptography mitigation active) len(84) * 8 = 672 != 128 (CEK_SIZE for A128GCM) raise ValueError('Invalid "cek" length') <- specific exception, fast path Path B — valid padding, wrong symmetric key: op_key.decrypt() -> 16 correct bytes len(16) * 8 = 128 == 128 -> length check passes AES-GCM tag validation -> mismatch raise InvalidTag <- different exception class, slow path ``` The single line `raise ValueError('Invalid "cek" length')` is the complete root cause. Removing the raise and replacing it with a silent random CEK fallback eliminates both the exception oracle and any residual timing difference. ### 2.3 Empirical Confirmation **All results obtained on authlib 1.6.8 / cryptography 46.0.5 / Linux x86_64 running the attached PoC (`poc_bleichenbacher.py`):** ``` TEST 1 - cryptography behavior on invalid padding: cryptography retorno bytes: len=84 NOTA: esta version implementa mitigacion de random bytes TEST 2 - Exception Oracle: [ORACLE] Caso A (padding invalido): ValueError: Invalid "cek" length [OK] Caso B (padding valido/MAC malo): InvalidTag TEST 3 - Timing (50 iterations): Padding invalido (ValueError) mean=1.500ms stdev=1.111ms Padding valido (InvalidTag) mean=1.787ms stdev=0.978ms Delta: 0.287ms TEST 4 - RSA1_5 in default registry: [ORACLE] RSA1_5 activo por defecto (no opt-in required) TEST 5 - Fix validation: [OK] Both paths return correct-length CEK after patch [OK] Exception type identical in both paths -> oracle eliminated ``` **Note on timing:** The 0.287ms delta is within the noise margin (stdev ~1ms across 50 iterati

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